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Invasions: the trail behind, the path ahead, and a test of a disturbing idea
Journal of Ecology
... Davis et al., 2000;Shea & Chesson, 2002;Mitchell et al., 2006;Catford et al., 2009;Blackburn et al., 2011;. Nonetheless, generalizations are still scarce and contradictory results have been found for many hypotheses (Moles et al., 2012;. ...
... Even highly invasive species sometimes end up declining, a phenomenon called boom-bust dynamics (Strayer et al., 2017). However, although we have gained a good understanding of the invasion process, satisfactory explanations of invasions are still insufficient and predicting which species will become invasive and which habitat are going to be invaded remains a challenge, therefore limiting our ability to implement efficient management strategies (Moles et al., 2012;. For a long time, these questions (invasiveness and invasibility) were addressed separately, but it is now acknowledged that they should be considered together, for instance through the use of methods from community ecology Gallien & Carboni, 2017). ...
... Finally, it is also important to account for the fact that invasions, their pathways, their dynamic manifestations in space and time, and their underlying processes are evolving under the combined forcing of the other components of anthropogenic global change, such as landuse or climate change (Theoharides & Dukes, 2007;Walther et al., 2009;Bradley et al., 2010;Pyšek et al., 2010;Moles et al., 2012;Colautti & Barrett, 2013;Kueffer, 2017). ...
Although less mediatized than deforestation or climate change, biological invasions remain problematic for the conservation of many ecosystems and the maintenance of various socio-economical activities.
Understanding the way invasive non-native species spread in their introduced range is enabling a better apprehension of their impacts, the possibility to predict their expansion, and the development of better management strategies. A problem is that a species invasion dynamics are actually composed of a hierarchy of processes occurring simultaneously at various spatiotemporal scales and which are controlled by drivers that are time- or context-dependent. To gain more profound insights into these dynamics, one has to study the patterns they create and their underlying processes at all relevant scales.
Occurring on the five continents and highly invasive on at least two of them, Asian knotweeds (Reynoutria spp.) are acknowledged as being among the most invasive species in the world. Gifted with a large environmental tolerance and high abilities for vegetative multiplication, these giant herbaceous plants can quickly form large clonal monocultures that exclude the other plant species, modify biogeochemical cycles, and affect various anthropic activities. Target of management campaigns for decades, knotweeds display an insolent resilience to the great despair of many stakeholders. Despite having been extensively studied, many aspects of their dynamics are still elusive. Consequently, in this thesis, we have tried to highlight what the drivers of the spatiotemporal dynamics of knotweeds across scales are, and how their management could benefit from a better understanding of these dynamics?
To answer these questions, we first focused our attention on the clonal dynamics of knotweeds and on their variations when they experience differing conditions in terms of light availability and disturbance (repeated mowing). We have experimentally shown that although shade or mowing affects the vigour and the development of clones, it does not prevent their establishment or their growth. In fact, knotweeds seem able to adopt different growth strategies to cope with environmental heterogeneity, suggesting some management avenues. In a second study, we tried to identify the drivers that controlled the expansion of knotweed stands along an elevational gradient. If we showed that the lateral expansion of stands is mostly constrained by their size (and thus, possibly their age) and the vicinity of roads and rivers, we also brought evidences that knotweeds could have the potential to invade mountain regions. Then, to help for the acquisition of large-scale datasets on knotweeds’ distribution, we developed a procedure to accurately detect and map knotweeds using satellite or drone imagery. Our results were quite encouraging and could be useful to both the basic research and to the detection of newly invaded areas, enabling earlier control operations and more efficient management.
Finally, we tried to draw a full picture of the current knowledge on the invasion dynamics of knotweeds by reviewing the literature on the movement of these plants across spatial and temporal scales, to discuss and expand the reach of the insights gained in the various chapters of this thesis.
... A invasão biológica de áreas naturais por espécies exóticas representa um dos principais problemas ambientais da atualidade, uma vez que suas consequências afetam negativamente a biodiversidade, o funcionamento dos ecossistemas, além de causar prejuízos econômicos (Charles & Dukes 2008). Neste sentido, estudos sobre processo de invasão biológica têm sido o foco de várias pesquisas nas últimas décadas, tendo como propósito caracterizar a autoecologia de espécies invasoras e identificar fatores que levam as comunidades biológicas a serem mais ou menos susceptíveis à invasão (Moles et al. 2012). Em uma ampla revisão Veja material suplementar em <https: //doi.org/10.6084/m9.figshare.4980680.v2> ...
... Em uma ampla revisão Veja material suplementar em <https: //doi.org/10.6084/m9.figshare.4980680.v2> bibliográfica, Moles et al. (2012) destacaram que espécies invasoras apresentam características que representam vantagem competiva em relação às nativas, como elevada taxa de crescimento demográfico e de sobrevivência, boa resistência à herbivoria e elevada plasticidade de atributos funcionais. Moles et al. (2012) também relataram que ambientes com maior diversidade de espécies podem ser mais resistentes à invasão, pelo fato de existir nestas condições uma maior competição interespecífica. ...
... bibliográfica, Moles et al. (2012) destacaram que espécies invasoras apresentam características que representam vantagem competiva em relação às nativas, como elevada taxa de crescimento demográfico e de sobrevivência, boa resistência à herbivoria e elevada plasticidade de atributos funcionais. Moles et al. (2012) também relataram que ambientes com maior diversidade de espécies podem ser mais resistentes à invasão, pelo fato de existir nestas condições uma maior competição interespecífica. No entanto, outros padrões também podem ser observados, pois, como os mesmos autores ressaltaram, a relação entre as características da comunidade residente e a sua susceptibilidade à invasão é de elevada complexidade, tendo em vista que a mesma é condicionada por diversos fatores ecológicos (e.g., processos neutros, distúrbios e produtividade). ...
Resumo Objetivou-se inferir sobre os mecanismos de invasão (oportunismo x competição) de Ligustrum lucidum em um fragmento de floresta com araucária em Lages, Santa Catarina. O fragmento foi inventariado nos anos de 2012 e 2015, por meio de parcelas, onde foram avaliados os indivíduos arbóreos com DAP maior ou igual a 5,0 cm. As parcelas foram classificadas em invadidas (I) e não invadidas (NI) por L. lucidum e foram calculadas as taxas de dinâmica para a comunidade e populações. A influência de L. lucidum sobre as taxas da comunidade foi investigada por meio de regressões logísticas e o padrão de coexistência das espécies nos setores I e NI pelo índice de c-score. No fragmento, foram observadas maiores taxas de recrutamento do que mortalidade e maior ganho do que perda em área basal. A comunidade apresentou maior recrutamento nos locais com maior abundância de L. lucidum. Enquanto nas parcelas NI observou-se a segregação espacial das espécies, nas parcelas I as mesmas ocorreram de forma aleatória. Conclui-se que L. lucidum está aumentando sua importância relativa na comunidade, estando associado a parcelas desestruturadas em relação ao padrão de co-ocorrência das espécies, o que sugere seu caráter oportunista.
... As the conventional disturbance hypothesis proposes, disturbance might create empty niches that enhance the invasibility of recipient habitats by providing opportunities for the invasion of exotic plants (Hobbs and Huenneke, 1992;Jauni et al., 2015). However, the idea embedded in the disturbance hypothesis tends to be controversial, as the role of disturbance in the invasion of exotic species is ambiguous (Jauni et al., 2015;Lockwood et al., 2007;Moles et al., 2012). Other factors, such as the species identity, the disturbance properties (e.g., type, intensity and frequency) and the invasion phase (e.g., residence time of exotic invaders in recipient habitats), affect how disturbance influences the exotic plant invaders; thus, the exotic species are likely to respond differently to different disturbances at different temporal scales (Buckley et al., 2007;Davis and Pelsor, 2001;Jauni et al., 2015;Moles et al., 2012). ...
... However, the idea embedded in the disturbance hypothesis tends to be controversial, as the role of disturbance in the invasion of exotic species is ambiguous (Jauni et al., 2015;Lockwood et al., 2007;Moles et al., 2012). Other factors, such as the species identity, the disturbance properties (e.g., type, intensity and frequency) and the invasion phase (e.g., residence time of exotic invaders in recipient habitats), affect how disturbance influences the exotic plant invaders; thus, the exotic species are likely to respond differently to different disturbances at different temporal scales (Buckley et al., 2007;Davis and Pelsor, 2001;Jauni et al., 2015;Moles et al., 2012). Certain types of disturbance may contribute to the invasiveness (Blumenthal, 2005;Davis et al., 2000;Lake and Leishman, 2004;Mallon et al., 2015;Mata et al., 2013;Melbourne et al., 2007;Shea and Chesson, 2002). ...
... Similarly, exogenous nutrient inputs could improve resource availability in invaded habitats and enhance the performance of exotic plant invaders (Blumenthal, 2005;Davis et al., 2000;Lake and Leishman, 2004;Mallon et al., 2015;Mata et al., 2013). On the other hand, if the disturbance is strong enough to overwhelm the tolerance limits of certain plants or if several types of disturbance are endogenously harmful to plant performance (e.g., grazing or herbivory on plants and inundation of some riparian hygrophytes), the disturbance may induce stress on both invasive and non-invasive plants (Jauni et al., 2015;Moles et al., 2012). Thus, whether a plant invader can tolerate or resist the stress caused by a specific disturbance is likely more crucial for invasion success during the late stages of the invasion process (Chen et al., 2013;Zhang et al., 2015). ...
... Several research groups have attempted to use this large body of knowledge to create a conceptual framework or synthesis for invasion biology that can at least provide a basis for the design of studies that can fill knowledge gaps and critically address long-held assumptions based on limited information (Catford et al. 2009;Davis et al. 2000;Funk et al. 2008;Sher and Hyatt 1999;Theoharides and Dukes 2007). These syntheses often have the specific intent of describing the concepts of invasion biology under the more general rubric of ecology, with the idea that the underlying processes regulating community assembly in native ecosystems should be the same as those that mediate invasions (Davis et al. 2005;Facon et al. 2006;Gurevitch et al. 2011;MacDougall et al. 2009;Moles et al. 2012;Shea and Chesson 2002). More recently, there has been recognition of the complexity of the invasion process and the low likelihood that broad generalizations applicable across classes of organisms (e.g., plants, pathogens, insects, earthworms, vertebrates) will emerge (Catford et al. 2012a;Heger et al. 2013;Jeschke et al. 2012;Jeschke 2014;Kueffer et al. 2013). ...
... Post hoc level of invadedness is therefore not a reliable indicator of ecosystem invasibility. For example, Moles et al. (2012) found in a meta-analysis that current and past disturbance regimes per se had low ability to predict the degree of plant invasion, accounting for <10% of the variation in invadedness. They used a correlative approach to relate the degree of invadedness in a large number of different ecosystems to reported levels of disturbance in those ecosystems. ...
Much of the literature dealing with the biology and management of invasive species has focused on the damaging ecological and economic consequences of invasions (see Chaps. 10.1007/978-3-030-45367-1_2 , 10.1007/978-3-030-45367-1_3 , and 10.1007/978-3-030-45367-1_14 of this volume for review). In this chapter, we shift the focus to the causes of invasion, with the goal of proactively limiting or preventing invasions rather than reacting to them once they have occurred. Preventing the introduction of invasive species is one key element in this proactive approach (Chap. 10.1007/978-3-030-45367-1_6 , this volume). Here, we specifically focus on ecosystem attributes that affect whether or not an ecosystem is vulnerable to invasion, that is, the features that affect its invasibility (Lonsdale 1999), with particular emphasis on the role of natural and anthropogenic disturbance.
... Biological invasions threaten native species biodiversity on a global scale (Simberloff, 2000;Vilà et al., 2011;Pysek et al., 2012), and yet the mechanisms that facilitate invasion remain poorly understood (Levine et al., 2003;Pauchard and Shea, 2006;Melbourne et al., 2007;Moles et al., 2012). Differences in physiological and morphological traits associated with carbon economy [i.e. the leaf economic spectrum (LES)] are thought to promote invasiveness in plants, such that invasive species can acquire and utilize resources faster than natives (Leishman et al., 2007;Ordonez et al., 2010;Peñuelas et al., 2010b). ...
... Differences in physiological and morphological traits associated with carbon economy [i.e. the leaf economic spectrum (LES)] are thought to promote invasiveness in plants, such that invasive species can acquire and utilize resources faster than natives (Leishman et al., 2007;Ordonez et al., 2010;Peñuelas et al., 2010b). An acquisitive resource use strategy among invasives, coupled with fast growth and high reproductive output, is thought to drive the displacement of native species throughout their geographic ranges (Richardson and Pyšek, 2006;Moles et al., 2012). However, within any given species' geographic range, traits often vary in response to resource gradients such as rainfall, temperature, soil nutrients and light availability (Lavorel and Garnier, 2002;Reich and Oleksyn, 2004;Wright et al., 2005;Ordoñez et al., 2009;Atkin et al., 2015). ...
Background and Aims
Displacement of native plant species by non-native invaders may result from differences in their carbon economy, yet little is known regarding how variation in leaf traits influences native-invader dynamics across climate gradients. In Hawaii, one of the most heavily invaded biodiversity hotspots in the world, strong spatial variation in climate results from the complex topography, which underlies variation in traits that likely drives shifts in species interactions.
Methods
Using one of the most comprehensive trait datasets for Hawaii to date (91 species and four islands), we determined the extent and sources of variation (climate, species, species origin) in leaf traits, and used mixed models to examine differences between natives and non-native invasives.
Key Results
We detected significant differences in trait means, such that invasives were more resource-acquisitive than natives over most of the climate gradients. However, we also detected trait convergence and a rank reversal (natives more resource-acquisitive than invasives) in a subset of conditions. There was significant intraspecific variation (ITV) in leaf traits of natives and invasives, although invasives expressed significantly greater ITV than natives in water loss and photosynthesis. Species accounted for more trait variation than did climate for invasives, while the reverse was true for natives. Incorporating this climate-driven trait variation significantly improved the fit of models that compared natives and invasives. Lastly, in invasives, ITV was most strongly explained by spatial heterogeneity in moisture whereas solar energy explains more ITV in natives.
Conclusions
Our results indicate that trait expression and ITV vary significantly between natives and invasives, and that this is mediated by climate. These findings suggest that although natives and invasives are functionally similar at the regional scale, invader success at local scales is contingent on climate.
... Previous work on the ecosystem impacts of invasive plants has focused on their capacity to alter soil nutrient dynamics (Ehrenfeld 2003;Corbin and D'Antonio 2004;Liao et al. 2008, Castro-Díez et al. 2014Suseela et al. 2016;Bray et al. 2017). Although theories regarding the invasibility of a site often link plant invasions to disturbance (Davis et al. 2000;Moles et al. 2012;Lockwood et al. 2013), altered forest disturbance regimes can also affect soil nutrient availability (Johnson and Curtis 2001;Keiser et al. 2016;Sokol et al. 2017). As such, invasive plants could be ''passengers'' of altered disturbance regimes or ''drivers'' of changes in ecological processes (MacDougall and Turkington 2005). ...
... Disturbance is often cited as a key driver of species invasions (Moles et al. 2012;Lockwood et al. 2013). Yet it remains unclear whether urbanization and anthropogenic disturbance increase the invasibility of a site directly through reduced biotic competition (Elton 1958) and elevated abiotic resource availability (Davis et al. 2000) or indirectly by increasing humanmediated dispersal of non-native propagules (Colautti et al. 2006). ...
Lianas (woody vines) are a leading concern in urban forest management given their ability to reduce the growth rate and survival of trees. Efforts to control invasive lianas are widespread, yet forests are often reinvaded following management. Changes in soil nutrient availability could help explain why some forests are susceptible to reinvasion. Specifically, invasive lianas could be “drivers” of altered nutrient dynamics that persist in the soil and favor their re-establishment, or they could be “passengers” of conditions created by prior site disturbances. We test these alternative models by analyzing soils across gradients of liana cover and forest disturbance in mature hardwood forests in New York City. Overall, total non-native liana cover was linked to elevated soil pH, reduced labile carbon, and altered net potential nitrification—three conditions indicative of increased soil nutrient availability. However, the effects of lianas varied substantially by species. Non-native lianas had a positive, negative, neutral, or non-linear effect on a single soil variable depending on the species. Aggregation of total non-native liana cover can therefore obscure the direction and magnitude of species-level effects. We also found positive relationships between forest disturbance, non-native liana cover, and soil pH, but we found no relationships between disturbance and net nitrification or labile carbon. These results suggest that non-native lianas are passengers to elevated levels of disturbance and soil pH but drivers of altered carbon and nitrogen dynamics. As such, management efforts aimed at reducing soil nutrient availability and removing invasive lianas may be required to limit reinvasion and promote ecosystem recovery.
... Non-native species can overcome abiotic resistance if they are equipped with traits well-suited for the invaded range habitat (Lee and Gelembiuk 2008; Schlaepfer et al. 2010;Warren II et al. 2018b) or if they can rapidly adjust ('niche shift') to the novel habitat (Hill et al. 2013;Moran and Alexander 2014). Once established, non-native species may co-exist with native species if the invader requires novel resources, essentially occupying an 'empty niche,' resulting in low interspecific competition (Stachowicz and Tilman 2005); for example, non-native species often thrive in anthropogenically altered habitat that is relatively inhospitable for native species (King and Tschinkel 2008;Moles et al. 2012). Where non-native species come in conflict with native species for shared resources, the non-native species often prevail because they bring superior competitive abilities and are released from the burden of negative species interactions, such as predation, herbivory, disease and territoriality (Callaway and Ridenour 2004;Liu and Stiling 2006;Warren II et al. 2018a). ...
... We found no evidence of biotic resistance to B. chinensis invasion, its success likely based (at least in part) on its own intraspecific lack of biotic resistance in the Southeastern U.S. With invasion, B. chinensis appears to eliminate the dominant native ants (Aphaenogaster; with which it shares considerable resource overlap) in approximately 10 years. At the same time, B. chinensis' inability to persist at higher elevations in the Southern Appalachian Mountains and its inflexible and relatively poor ability to tolerate cold temperatures, suggests its primary invasion may be limited to the Southeastern U.S. Without biotic resistance, vulnerable native species may only remain viable if they can tolerate abiotic conditions beyond the invasive species' tolerance (King and Tschinkel 2008;Moles et al. 2012). For B. chinensis invasion, Aphaenogaster species may only remain unaffected in eastern U.S. deciduous forests at high elevations and latitudes beyond the invasive ant's cold tolerance. ...
A successful invasion of novel habitat requires that non-native organisms overcome native abiotic and biotic resistance. Non-native species can overcome abiotic resistance if they arrive with traits well-suited for the invaded habitat or if they can rapidly acclimate or adapt. Non-native species may co-exist with native species if they require novel, underused resources or if they can out-compete similar native species. We investigated abiotic and biotic resistance to the progression of a Brachyponera chinensis invasion in the southeastern U.S. relative to the dominant native woodland ant (Aphaenogaster). We used observational data from long-term plots along the elevation gradient of the Southern Appalachian Mountain escarpment to investigate the patterns of B. chinensis invasion, and we used physiological thermal tolerance, aggression assays and stable isotope analysis to determine whether abiotic or biotic factors explained B. chinensis invasion. We found that B. chinensis exhibited an inflexible and relatively poor ability to tolerate cold temperatures, which corresponded with limited success at higher elevations in the Southern Appalachian Mountains. Though we found native ant resistance to B. chinensis invasion, it paled in comparison to the invasive ant’s ability to form huge, cooperating supercolonies that eventually eliminated the native ant. Without biotic resistance, susceptible native species may only be protected if they can tolerate abiotic conditions that the invasive species cannot. For Aphaenogaster species, high elevations and northern latitudes beyond B. chinensis’ cold tolerance may be their only refuge.
... However, despite considerable research effort, our understanding of the factors that allow certain species to dominate novel communities has not led to high predictive certainty. Although certain factors have been shown to be important in particular instances, these appear to be idiosyncratic among systems (Hawkes 2007, Moles 2012. It may be that we need to reframe this question as, "How often is a particular mechanism important?" ...
... (cf. Moles 2012). In this paper, we evaluate the role of plant-soil feedbacks in determining differences in distribution and performance of native and invasive tree seedlings. ...
The Janzen-Connell effect is often generated by interactions between trees and soil microbes while the enemy release hypothesis states that invasive species are released from regulation by natural enemies. Thus, the strength of the Janzen-Connell effect could differ between native vs. nonnative plants. We tested this hypothesis with congeneric pairs of invasive and native tree species in Europe: boxelder (Acer negundo) vs. Norway maple (A. platanoides), and Northern red oak (Quercus rubra) vs. pedunculate oak (Q. robur). We conducted greenhouse experiments using soil sterilization treatments, field experiments on distance-dependent germination, and field surveys of early life stages of the focal species. Greenhouse and field experiments demonstrated patterns consistent with Janzen-Connell effect that is most likely caused by negative distance-dependence in seed germination and stem growth which was found in both genera of the native, but not the invasive trees. Soil sterilization experiments suggested that these effects are driven by interactions with soil biota. Field surveys revealed the Janzen-Connell pattern in the distribution of seedlings and saplings of the native, but not the invasive Acer species. Our findings indicate that weakened Janzen-Connell effect might contribute to successful invasions of certain nonnative plant species.
... Determining which plant traits are associated with successful invasions has been a core theme in ecology over the past 50 years (e.g. Baker 1965; Moles et al. 2011;Jauni et al. 2014). However, we know relatively little about the traits of native plants that successfully colonise novel habitats. ...
... Next, we asked whether higher historic disturbance frequencies in nearby natural ecosystems are associated with higher rates of colonisation of novel habitats by native species. Similarities between the historic disturbance frequency of a region and the disturbance frequency in the novel habitat could allow native species to be more suitable colonisers (Moles et al. 2011;Hejda et al. 2014). However, relatively few studies have compared species' historic and novel distributions, and the existing studies have usually focused on single species (e.g. ...
There is an enormous body of literature on plant invasions, including many investigations of the types of introduced species that are most likely to invade natural ecosystems. In this study we turn invasion biology upside down, and ask what sort of native species colonise novel anthropogenic habitats such as roadside lawns, infrequently tended road shoulders, railway embankments and fire trails. We quantified species richness and cover in roadside lawns and infrequently tended road shoulders in five regions of New South Wales, Australia. The native vegetation in these regions included sclerophyll forest, fertile and infertile Eucalypt‐dominated woodlands, rainforest, and semi‐arid woodland. We performed a complementary survey of sites spanning five disturbance levels within the region containing sclerophyll forest vegetation. Although many non‐native species were present in disturbed, novel habitats, a total of 136 native species were also found. Most of these native species were in sites with low levels of disturbance (fire trails and railway embankments), but 35 native species were found to colonise roadside lawns, our most highly‐disturbed vegetation type. There was a significant negative relationship between the disturbance level in novel habitats and the number and cover of native species. Native species that colonised novel habitats were disproportionately likely be generalist species whose natural habitat includes both high and low light and high and low disturbance conditions. The native species colonising novel habitats also tended to have traits associated with a fast life‐history, including short stature and small seeds. A surprisingly high number of native plant species are colonising novel, anthropogenic habitats. Our findings highlight the potential importance of urban ecosystems for conservation and restoration biology.
... Disturbance often promotes the invasibility of plant communities (Hobbs and Huenneke 1992) by increasing resource flux or decreasing resource uptake by native plants (Davis et al. 2000;Sher and Hyatt 1999). However, not all studies have shown a positive relationship between disturbance and invasion (see reviews by Colautti et al. 2006;Moles et al. 2012). Sher and Hyatt (1999) and Moles et al. (2012) concluded that for most nonnative species, change in disturbance regimes, rather than disturbance itself, was a better predictor of invasion. ...
... However, not all studies have shown a positive relationship between disturbance and invasion (see reviews by Colautti et al. 2006;Moles et al. 2012). Sher and Hyatt (1999) and Moles et al. (2012) concluded that for most nonnative species, change in disturbance regimes, rather than disturbance itself, was a better predictor of invasion. ...
A critical question in invasion biology involves the relative importance of propagule rain and community invasibility. For plant invasions, invasibility is often related to disturbance, but few studies of forest invaders have simultaneously investigated both canopy and ground-level disturbance. We investigated the relative importance of seed rain, canopy disturbance, and soil disturbance in a mature forest in Maryland on the recruitment of four invasive species: wine raspberry ( Rubus phoenicolasius Maxim.), Japanese barberry ( Berberis thunbergii DC), multiflora rose ( Rosa multiflora Thunb.), and Japanese stiltgrass [ Microstegium vimineum (Trin.) A. Camus]. Using complete censuses of a 9-ha plot at two points in time (2011–12 and 2014), we mapped new recruits, and related their locations to canopy and soil disturbance, as well as to a seed rain index based on locations of reproducing plants and seed-dispersal kernels.
We found that propagule rain, as measured by the seed rain index, was a significant predictor of recruitment for B. thunbergii , R. phoenicolasius , and M. vimineum . For R. multiflora , seed sources were not located, precluding assessment of propagule rain, but recruitment was linked to canopy disturbance, as was recruitment of M. vimineum . However, because reproduction of R. phoenicolasius and, in some years, of B. thunbergii is higher in treefall gaps, these gaps experience higher propagule rain, with the result that recruitment is indirectly associated with these gaps. Ground-layer disturbance was an important predictor of recruitment only for B. thunbergii . Our findings reveal that the importance of propagule rain is the most consistent driver of recruitment, but canopy or ground-layer disturbance promotes recruitment of some invasive plant species.
... The ruderal strategy is widely shared among nonnative plants (Baker 1974), providing a general explanation for the commonly observed positive effects of disturbance on invasions (Mack et al. 2000;Hierro et al. 2006Hierro et al. , 2011Jauni et al. 2015, but see Moles et al. 2012). Here, we conducted extensive field samplings to assess the effects of different types of disturbance on the relative abundance, richness, and diversity of native and non-native ruderals in a system where the ruderal strategy is shared by a large number of native and non-native species. ...
... Comparing the response of native and non-native species with ruderal strategy to disturbance can provide fundamental insight into the role of disturbance in plant invasion (Moles et al. 2012;Jauni et al. 2015;Seabloom et al. 2015). According to a recent meta-analysis that did not grouped plant species into life-history strategies, disturbance, particularly human disturbance and grazing, strongly increases the diversity and abundance of non-natives, but disturbance does not affect those of natives (Jauni et al. 2015). ...
The ruderal strategy is widely shared among non-native plants, providing a general explanation for the commonly observed positive effects of disturbance on invasions. How native ruderals respond to disturbance and how their abundance compares to that of non-native ruderals remains, however, poorly understood. Similarly, little is known about the role that disturbance type plays in the coexistence between native and non-native ruderals. We proposed that natural disturbance favors native over non-native ruderals, whereas novel anthropogenic disturbance favors non-natives over natives. To assess our general hypothesis, we conducted extensive field samplings in which we measured relative abundance, richness, and diversity of native and non-native ruderals in sites with natural and anthropogenic disturbance in central Argentina, a system where the ruderal strategy is common to a large number of native and non-native species. We found that natives dominated ruderal communities growing in recently burned grasslands, whereas non-natives dominated in roadsides. Additionally, the richness and diversity of native ruderal species were much greater than those of non-natives in sites with fire and in sites with grazing, but species richness and diversity did not differ between groups in roadsides. Because vegetation evolved with fire in our system and, in contrast, the construction and maintenance of roads is recent in it, these results support our hypothesis. Our work indicates that the ruderal strategy does not seem to suffice to explain why disturbance facilitates invasions. According to our data, species origin interacts with disturbance type to determine dominance in communities with coexisting native and non-native ruderals.
... У багатьох наукових джерелах європейських країн та США питання інтродукції рослин розглядається переважно в сенсі аналізу глобальних загроз біологічному різноманіттю, зокрема через вторгнення інвазійних видів. Аналіз поширення таких видів дає можливість дослідити біологію популяцій, а також генетичні процеси, які в них відбуваються, що має важливе значення для розвитку синтетичної теорії еволюції, оскільки саме популяція є одиницею еволюції [38][39][40][41]. ...
Distinctive features of ex situ plant populations (outside natural habitats) are manifested during the formation and development of groups of individuals mainly in the process of their introduction and acclimatization. The experience and practice of using terms for ex situ plant populations has been analyzed. Information about artificially created plant groups is generalized. The results of the analysis concerning the definition of the term "ex situ plant population" and its use in relation to introduced plants are presented. The list of definitions for the following terms are given: "initial population", "introductive population", "introductive coenotic population", "introduced population", "cultivated populations", "agro population", "cultural population", "hybrid population", "artificial population", "experimental population", "spontaneous population". It has been stated that modeling, forming and copying methods are used to form ex situ plant populations. It has been noted that ex situ plant populations can be formed and developed independently or artificially formed. The scheme of distribution of ex situ plant populations by the degree of genetic integrity, ability to reproduce, duration of existence, size and stage of development, place and style of formation of the initial population is proposed. It has been established that groups of genetically homogeneous individuals of artificial plant groups that freely cross each other and for a long time reproduce offsprings can be characterized as populations. Such populations are the result of purposeful human activity on the introduction and cultivation of plants, and are largely dependent on anthropic influence. The study of their structure and dynamics is a prerequisite for predicting and evaluating the success of introductions and preventing threats of spontaneous propagation and uncontrolled, unwanted naturalization of plants under new growth conditions.
... En effet, la suppression d'une espèce invasive ne suffit pas toujours à favoriser le retour spontané de la végétation native. Au contraire, cela peut parfois favoriser l'arrivée d'autres espèces invasives (Gaetner et al. 2012 (Catford et al. 2009;Hooper, Dukes 2010;Moles et al. 2012). ...
Les méthodes traditionnelles de gestion des renouées asiatiques (Fallopia spp.), plantes invasives parmi les plus menaçantes au monde, ont des résultats mitigés et coûteux. Certains gestionnaires s’orientent donc vers des solutions alternatives comme le génie végétal. Ordinairement utilisé contre l’érosion des berges de cours d’eau, il permet également d’installer un couvert végétal rapidement et durablement. Un des enjeux pour le contrôle des renouées asiatiques est donc d’optimiser ces méthodes afin de reconstituer des communautés végétales capables de les réguler. Ce travail de thèse tente de répondre à cet enjeu en apportant une meilleure compréhension du rôle des interactions biotiques dans la régulation des renouées asiatiques. Dans ce projet, nous nous sommes intéressés aux populations déjà installées de renouées asiatiques et donc aux facteurs qui interviennent en tant que régulateur de leur performance. Les renouées asiatiques étant plus rares en forêts structurées et étant connues comme des espèces pionnières dans leur aire d’origine, l’hypothèse est que la compétition, en particulier pour la lumière, joue un rôle central dans la régulation de leur performance. L’objectif du travail de thèse a été de vérifier cette hypothèse, en évaluant le rôle de la lumière dans la performance des renouées asiatiques (chapitre 1), en étudiant le rôle des interactions biotiques de type compétition (chapitre 2) et en évaluant l’effet inhibiteur de la renouée du Japon sur des boutures de Salicaceae en fonction de la lumière disponible (chapitre 3). Le premier chapitre met en évidence, par des mesures in situ, l’importance de la quantité de lumière sur la production de biomasse aérienne des renouées asiatiques, comparativement à d’autres facteurs liés aux perturbations ou aux conditions édaphiques (partie 1). Une expérimentation en conditions semi-contrôlées, dans laquelle des renouées asiatiques ont été cultivées et soumises à un gradient de lumière, complète ces résultats (partie 2). Elle met en évidence leur plasticité phénotypique ainsi que l’existence de différents seuils de lumière pour la production de biomasse, l’allocation des ressources ou leurs effets compétitifs. Le deuxième chapitre s’intéresse plus spécifiquement aux interactions compétitives et en particulier à la compétition pour la lumière. La première partie démontre, par l’évaluation in situ de la performance des renouées asiatiques le long de gradients environnementaux, le rôle prépondérant de la structure fonctionnelle de la communauté végétale. La deuxième partie s’appuie sur une expérimentation en microcosme et montre les effets compétitifs respectifs du saule des vanniers (espèce prometteuse pour le contrôle des renouées asiatiques) et de la renouée du Japon. Enfin, le dernier chapitre de la thèse s’intéresse plus spécifiquement aux effets allélopathiques de la renouée du Japon sur des boutures de différentes espèces de Salicaceae. Une première analyse met en évidence des effets phytotoxiques différentiels entre les espèces de Salicaceae (partie 1). La deuxième analyse répète cette expérimentation mais dans des conditions de lumières limitées, mettant ainsi en évidence les compromis de ressources chez la renouée du Japon qui privilégie l’acquisition de la lumière au détriment de la production de composés inhibiteurs. Cette thèse, située à l’interface de l’écologie des invasions, de l’écologie des communautés et de l’écologie fonctionnelle, est centrée sur un complexe d’espèces hautement invasives, les renouées asiatiques. Elle propose des éclairages sur leur écologie permettant une meilleure compréhension de ses capacités invasives dans la perspective d’améliorer leur gestion.
... Although many support the idea that successful invaders perform differently than native species (Lai et al., 2015), measures of how invasive species might grow differently than native plants in response to disturbances such as nutrient enrichment are often not tested experimentally across a wide ranging set of sites in the same biome like grasslands (Leifso et al., 2012;Moles et al., 2012;MacDougall et al., 2014;Buckley & Catford, 2016;Funk et al., 2017). Our study demonstrates that dominant non-native graminoids generally grow leaves with higher nutrient concentrations than co-occurring dominant native graminoids, revealing important physiological differences between these groups. ...
Aim
Nutrient enrichment is associated with plant invasions and biodiversity loss. Functional trait advantages may predict the ascendancy of invasive plants following nutrient enrichment but this is rarely tested. Here, we investigate (a) whether dominant native and non‐native plants differ in important morphological and physiological leaf traits, (b) how their traits respond to nutrient addition, and (c) whether responses are consistent across functional groups.
Location
Australia, Europe, North America and South Africa.
Time period
2007–2014.
Major taxa studied
Graminoids and forbs.
Methods
We focused on two types of leaf traits connected to resource acquisition: morphological features relating to light‐foraging surfaces and investment in tissue (specific leaf area, SLA) and physiological features relating to internal leaf chemistry as the basis for producing and utilizing photosynthate. We measured these traits on 503 leaves from 151 dominant species across 27 grasslands on four continents. We used an identical nutrient addition treatment of nitrogen (N), phosphorus (P) and potassium (K) at all sites. Sites represented a broad range of grasslands that varied widely in climatic and edaphic conditions.
Results
We found evidence that non‐native graminoids invest in leaves with higher nutrient concentrations than native graminoids, particularly at sites where native and non‐native species both dominate. We found little evidence that native and non‐native forbs differed in the measured leaf traits. These results were consistent in natural soil fertility levels and nutrient‐enriched conditions, with dominant species responding similarly to nutrient addition regardless of whether they were native or non‐native.
Main conclusions
Our work identifies the inherent physiological trait advantages that can be used to predict non‐native graminoid establishment, potentially because of higher efficiency at taking up crucial nutrients into their leaves. Most importantly, these inherent advantages are already present at natural soil fertility levels and are maintained following nutrient enrichment.
... These results are in accordance with the previously observed low competitive ability of S. inaequidens. Furthermore, disturbance is considered to be one of the most important factors that support invasion success in general (Alpert et al., 2000;Catford et al., 2009;Moles et al., 2012) as well as in mountain ecosystems (Seipel et al., 2012;Lembrechts et al., 2016;Haider et al., 2018). We had furthermore expected a more pronounced positive effect of disturbance in more productive communities with stronger competition (i.e. at in lowland communities / at the low-elevation site). ...
Worldwide, biodiversity is under pressure from anthropogenic environmental change. In concert, environmental change and modifications of dispersal pathways induce immense and rapid alterations to the geographic distributions of species including range retractions, expansions, shifts, and the invasion into novel regions. Here, rare or declining species vs. invasive species represent the opposite ends of a gradient of demographic success. This habilitation thesis assembles conceptual and empirical research on five phenomena that shape the eco-evolutionary dynamics of natural plant populations under global change: 1) anthropogenic alterations of biotic and abiotic environments, 2) demographic and genetic bottlenecks 3) inbreeding depression and inbreeding x environment interactions, 4) interspecific hybridization between previously isolated taxa, 5) and ultimately altered evolutionary trajectories. In summary, the research assembled in this thesis contributes to a more general understanding of the important role of eco-evolutionary processes for the demographic success and thus management implications of declining and invasive species.
... declining population sizes of other species and causing local extinctions (Moles et al. 2012;Pyšek et al. 2017). ...
Climate change, urbanization, biological invasions and interspecific hybridization are major current threats to biodiversity. The objective of this thesis was to assess their interactive effects in Tetramorium ant species from the Rhône-Saône valley in France. Using a broad array of tools from several fields of biology, I worked on almost two thousand colonies from five Tetramorium species including T. immigrans and T. caespitum. Both climate and urbanization limited the ranges of these species, and urbanization impacted their distributions simultaneously at several spatial scales. The occurrence probabilities of T. immigrans depended on the interaction between climatic and urban factors. Several introductions from external sources seemed to have given rise to four genetically distinct populations of T. immigrans, making it likely that it is not native to the northernmost urban areas. Hybridization and introgression between T. immigrans and T. caespitum was frequent in zones of sympatry, and the detection of nuclear DNA backcrosses and mitochondrial-nuclear DNA discordance between the two species suggested that hybridization lead to fertile offspring (i.e. introgression). Cuticular hydrocarbon profiles were clearly differentiated between the two species and were correlated to heightened interspecific aggression against heterospecifics. Both species presented a monogyne polyandrous mating system, in which hybrid queens but no hybrid fathers contributed to hybrid offspring. Few studies on interspecific hybridization have addressed global change or biological invasions. These concepts are infrequently studied together, and their interactive effects are overlooked in the current literature. Taking these interrelationships into account and exploring multiple scales are essential to better understand the processes that generate patterns of genetic exchange
... Firstly, rfPermute package (Archer, 2016) was used to calculate the variation of 12 environmental proxies for dominant phylotypes. For a given soil bacteria, when the random forest model can explain > 30% of variation in soil bacterial distribution, then the soil bacterial environmental preferences can be identified (Moles et al., 2012). Furthermore, semi-partial correlation (Spearman) was determined using ppcor package (Kim, 2015) to identify the unique contribution of each proxy in explaining the distribution of a given phylotype (> 30% of variation explained) (Warner, 2012). ...
Soils harbour highly diverse bacterial communities that are essential for nutrient cycling and carbon storage. However, this diversity impedes the efforts to characterize individual taxa and their ecological attributes. In particular, comprehensive assessments of soil bacterial communities in drylands in China are limited. In this study, soil samples were collected across 12 deserts, covering four climatic regions (dry sub‐humid, semi‐arid, arid and hyper‐arid) in northern China to examine the dominant phylotypes, and their environmental preferences and functional traits. Only 3.2% of the bacterial phylotypes (913 phylotypes) were dominant. These dominant phylotypes can be grouped into six major ecological clusters based on high pH, high elevation, low aridity index, high aridity index, low mean annual temperature and low soil water content, each with multiple phyla. Based on the association between diversity indexes for dominant and subdominant phylotypes, these dominant bacteria might be the drivers or indicators of key soil processes across desert soils in northern China. Genes involved in carbon fixation and decomposition and nitrogen metabolism varied across desert soils, suggesting functional differences. These findings provide deeper insights into soil bacterial communities, which are critical for forecasting the ecological consequences of ongoing global environmental change and for a better mechanistic understanding of the biogeochemical cycle.
Highlights
3.2% of the bacterial phylotypes were dominant across 12 deserts in northern China.
The dominant phylotypes were grouped into six major ecological clusters.
Carbon fixation, decomposition and nitrogen metabolism genes varied across desert soils.
Some northern China desert soils might undergo nitrogen and carbon loss in the near future.
... That is, sampling different portions of the same animal might inspire very different descriptions based on what part of the animal is touched: a tusk like a spear, or a trunk like a snake, or a tail like a rope. Indeed, despite support for individual theories in species invasion, contradictory evidence suggests that the results often are contingent upon species and system (Broennimann et al. 2007;Daehler 2001;Diez et al. 2008;Felker-Quinn et al. 2013;Fridley et al. 2007;Gallagher et al. 2010;Kennedy et al. 2002;Levine et al. 2004;Liu et al. 2006;Lonsdale 1999;Moles et al. 2012;Ordonez et al. 2010). Hence, individual parts of the animal are well explained, but not the animal itself. ...
Aims: A plethora of theories explain species invasion, yet when tested in isolation, support or falsification becomes contingent on study species, system and approach. Our objective was to examine community-level species invasion as a function of multiple competing hypotheses. Methods: We used data from >3500 woodland plant species in 2750 plots in 49 national parks in eastern US deciduous forests to test multiple competing theories of species invasion: competition, empty niche, propagule pressure and latitude matching. We also tested interactions with residence time to account for non-native species naturalization and spread since arrival. Important Findings: The non-native herbs generally thrived at latitudes similar to those from which they originated, but not necessarily where they were originally introduced to the eastern US. Overall, we found that each hypothesis explained at least some aspect of woodland plant species invasion, but examining them simultaneously allowed assessment of their relative strengths and interactions. Our results suggested that residence time is a strong predictor of non-native woodland plant success, particularly as it interacts with other mechanisms of invasion, such as competition (abundance of native woodland plants), climate matching (similar invaded latitude as home range), propagule pressure (distance to putative seed sources) and empty niche (relatedness to native plants). We found that initial barriers, such as distance from propagule source or suboptimal habitat, were overcome, as was resistance from native relatives. However, the biggest challenge for the non-native woodland plants appeared to be time, as they declined after ∼1 to 2 centuries.
... Kemunculan dan ancaman dari spesies alien invasif menarik perhatian dalam hubungannya dengan kegiatan restorasi (Daehler 2003 (Cronk and Fuller 2014, Moles et al. 2012, Osunkoya et al. 2005. Spesies asli seperti pakis atau alang-alang (Imperata cylindrica) dapat juga menjadi masalah di area terganggu di mana kegiatan restorasi dilakukan (lihat studi kasus di bawah). ...
This is the Indonesian language version of "Reforesting for the climate of tomorrow: Recommendations for strengthening orangutan conservation and climate change resilience in Kutai National Park, Indonesia"
See
https://www.iucn.org/news/species/201902/iucn-study-identifies-tree-species-climate-resilient-reforestation
... include species known to be invasive in some regions of the world. Such species can be found in natural closed forest, but they are generally favoured by disturbance (Cronk and Fuller 2014, Moles et al. 2012, Osunkoya et al. 2005. Native species such as ferns or alang-alang grass (Imperata cylindrica) can be equally problematic in disturbed areas where restoration activities occur (see case studies below). ...
Recommendations for strengthening orangutan conservation and climate change resilience in Kutai National Park, Indonesia.
In Indonesia, Kutai National Park is home to what is likely to be East Kalimantan’s largest population of the Critically Endangered eastern subspecies of the Bornean Orangutan, Pongo pygmaeus morio. It also hosts an astounding diversity of other species including ~80 mammal, 369 bird and 1287 plant species. The park plays an important role in regulating water supply to neighbouring towns, attracts tourism and its forests serve as a valuable carbon sink. Yet East Kalimantan faces many challenges in maintaining and protecting biodiversity from threats, particularly from population expansion into the protected area with associated hunting and forest clearing for agriculture, fire, and coal
mining. More recently, climate change has been identified as an emerging threat, with both observed and projected changes indicating with high confidence that higher temperatures are to be expected. These are likely to exacerbate drought
conditions, which enable wildfires and lead to a range of other negative impacts on the species of Kutai National Park. To date, however, few initiatives have attempted to assess the vulnerability of the region’s biodiversity to climate change, nor to develop strategies to minimise negative impacts.
Forest restoration, also referred to as reforestation, presents a valuable opportunity to restore biodiversity and function to degraded areas that were once forested. Reforestation initiatives are being carried out in Kutai National Park, ranging from protection to enrichment planting in areas that were previously burnt but are now recovering. While several of these programmes have successfully planted large numbers of seedlings, little attention has been placed on restoring species richness, ecological function or selecting species that are of value for orangutan survival. In addition, most fail to consider climate change and hence that selected species must be able to establish and survive in the warmer and drier climatic conditions of the future. There is a clear and pressing need to update Kutai National Park’s existing restoration practices to ensure forest integrity, provide opportunities for threatened species, and guide consideration of how to build climate change resilience. By doing so, the forests that orangutans need to survive into the future are more likely to persist.
To meet the need for guidance on climate change resilient reforestation practices, we collaborated with park authorities and other experts to identify the tree species that are most vulnerable to climate change and those likely to be most climate change resilient. The importance of orangutans in Kutai National Park’s conservation objectives led us to expand our scope to identify those tree species that are valuable resources for them, and this extended further to addressing the need for identification of those that are ecologically and commercially important; those that are iconic (have tourist potential); those
that are most representative of primary forest; those resilient to fire; as well as those that are locally threatened. To assess climate change vulnerability and resilience, we examined the biological characteristics or traits of species that are associated
with their sensitivity and/or adaptive capacity to the anticipated climate changes and the resulting altered fire regimes. We examine restoration case studies, remind readers of restoration best practice, and present sets of tree species from a set of ~250 considered in the analysis that are likely to be suited to various restoration targets for Kutai National Park, e.g. with a focus on habitat restoration for orangutan; or a focus on conservation of rare and useful species. Given the fire prone nature of the area, two species stand out due to their resilience to fire events: Borassodendron borneense, and
Eusideroxylon zwageri: known locally as Bendang and Ulin respectively. The following species emerged as most important food plants for Orangutan: Dracontomelon dao, Merremia mammosa, Kleinhovia hospita, Alangium hirsutum, Dillenia reticulata, Callicarpa pentandra, and Ficus obpyramidata. Species that are most likely to be climate change resilient were dominated by pioneer or invasive species.
It emerged from workshops held in Bontang, Indonesia, that supply of seedlings for restoration projects is a challenge.
Special provision must also be made for the collection of seedlings for masting species, as these events provide a rare
opportunity to source otherwise rare stock for key species such as those of the Dipterocarpaceae. This family in particular emerged as vulnerable to climate change, but also one that is regionally important. Furthermore, the success of any restoration project lies in addressing the issues that lead to deforestation in the first place. These issues need to be addressed and long term monitoring needs to be in place to ensure the success of all restoration projects.
The intended audiences of this work include: orangutan researchers, government, mining companies, nurseries and other companies that are seeking guidance on habitat restoration for climate change resilience in East Kalimantan, as well as those wishing to support biodiversity conservation and/or restoration in the region.
... However, our full set of results show that such conclusions are true only under certain conditions. Second, in observational studies of invasions, which largely focus on exotic species, invading species are typically found in modified ecosystems (Catford et al. 2011;Moles et al. 2012) or disturbed sites characterized by high resource availability; under these conditions, the importance of resource conservation and longevity are secondary to high growth rates and short generation times (Kuster et al. 2008;Moravcov a et al. 2015). In our study savannah, resource availability was low and disturbance matched historical forms and levels. ...
Much uncertainty remains about traits linked with successful invasion – the establishment and spread of non‐resident species into existing communities. Using a 20‐year experiment, where 50 non‐resident (but mostly native) grassland plant species were sown into savannah plots, we ask how traits linked with invasion depend on invasion stage (establishment, spread), indicator of invasion success (occupancy, relative abundance), time, environmental conditions, propagule rain, and traits of invaders and invaded communities. Trait data for 164 taxa showed that invader occupancy was primarily associated with traits of invaders, traits of recipient communities, and invader‐community interactions. Invader abundance was more strongly associated with community traits (e.g. proportion legume) and trait differences between invaders and the most similar resident species. Annuals and invaders with high‐specific leaf area were only successful early in stand development, whereas invaders with conservative carbon capture strategies persisted long‐term. Our results indicate that invasion is context‐dependent and long‐term experiments are required to comprehensively understand invasions.
... A reduction in genetic diversity as a result of a small founding population is not always observed (Roman & Darling, 2007), and adaptive evolution does not necessarily take a long timeframe. There is evidence that rapid adaptive evolution takes place in multiple non-indigenous species (Reznick & Ghalambor, 2001;Moles et al., 2012). However, the time lag may simply reflect the time needed for the population to reach densities that are detectable (Siegert el al. 2014). ...
The oak processionary moth (Thaumetopoea processionea; OPM) is an invasive pest species that was introduced to West London in 2006. Its gregarious larvae pose a risk to forestry and public health by defoliating oak trees and shedding toxic setae. It is not known how OPM populations will spread or what impact they will have in the UK, therefore the aim of this thesis was to explore the population ecology of oak processionary moth in West London.
The thesis focuses on three key topics. Firstly how habitat influences the temporal and spatial distribution of OPM populations. Secondly, the development of molecular methods to identify the parasitoids of OPM. Thirdly, the characterisation of interactions between OPM and its main parasitoid, Carcelia iliaca. These topics were addressed by a combination of a two-year field study at three sites in West London and lab based molecular techniques.
Oak processionary moth was found to have a strong spatially and temporally stable habitat preference for open woodland containing a high proportion of oak trees. Fieldwork and molecular techniques revealed a new tachinid fly in the UK, C. iliaca, a major parasitoid of OPM. Carcelia iliaca was responsible for the mortality of around 37% of moth pupae on average, suggesting that currently parasitism is not having a stabilising effect on OPM populations.
OPM exhibited similar habitat preferences to continental populations as well as other processionary moths, likely driven by tree apparency. It is not clear how parasitism of OPM would respond in an outbreak and current parasitism rates are lower than those in continental populations. This may be a result of OPM nest removal, which was common management practice at the time of the study. The findings of this study have been used to recommend that nests remain in situ for longer, to allow C. iliaca numbers to increase.
Other OPM management options include tree felling, pesticide application and biocontrol. This study found no evidence that OPM control currently warrants tree felling. Biocontrol could be augmented with the use of specific parasitoids such as the newly discovered C. iliaca or entomopathogenic agents identified in Chapter 3, and may be the favoured management option for stakeholders and managers, but is difficult to manage in the long term.
... In particular, large propagule sizes of the invader can increase the standard deviation of interaction strength and network connectance, thereby increasing invasibility [67]. More generalist invaders and distinct invaders will increase interaction strength, thereby making the system more susceptible to invasion [51,68]. High levels of species saturation in a community (meaning that all or most niches are occupied and most resources are used) can increase intraspecific Box ...
Invasion science is in a state of paradox, having low predictability despite strong, identifiable covariates of invasion performance. We propose shifting the foundation metaphor of biological invasions from a linear filtering scheme to one that invokes complex adaptive networks. We link invasion performance and invasibility directly to the loss of network stability and indirectly to network topology through constraints from the emergence of the stability criterion in complex systems. We propose the wind vane of an invaded network – the major axis of its adjacency matrix – which reveals how species respond dynamically to invasions. We suggest that invasion ecology should steer away from comparative macroecological studies, to rather explore the ecological network centred on the focal species.
Highlights
Invasion performance can be tentatively explained by the traits of alien species relative to those of natives, recipient site characteristics, and introduction pathways.
The rush to identify invasive traits from comparative studies has not yet led to predictability at a satisfactory level.
Synergies among invasion science, network ecology, and community ecology warrant increasing attention.
Winners and losers in recipient ecosystems are the results of the multiplayer game between natives and aliens, as well as human factors.
Statistical tools that can handle multispecies interactions are on the horizon.
... Mining may also change the spatial autocorrelation of disturbance, which can affect community composition (Liao et al. 2016). Altered disturbance regimes are also generally believed to be a key factor in successful species invasions, which can further alter community structure (Moles et al., 2012;Moles, Gruber, & Bonser, 2008). Our present findings, and our other research on different aspects of disturbance (Garrison, Miller, Roxburgh, & Shea, 2012;Miller et al., 2011;Miller, Reilly, Bauman, & Shea, 2012;, will be valuable for guiding researchers and resource managers in determining the potential effects of such operations, and understanding the impacts of early mining operations. ...
We present an investigation of the effects of disturbance and fecundity–tolerance strategies on community composition. We develop a theoretical model and apply it to macrofaunal communities at deep‐sea hydrothermal vents. We characterize community outcomes and find that dominance, coexistence, and alternative stable equilibria can all result from the interplay of disturbance regimes and fecundity–tolerance interactions. We show that fecundity–tolerance trade‐offs can permit coexistence under disturbance, but a strict fecundity–tolerance trade‐off is not required for coexistence to arise. We further describe how coexistence depends on habitat availability and disturbance regimes. Generally, our model elaborates on fecundity–tolerance strategies as a new axis of trait variation in coexistence theory. Natural disturbance regimes vary considerably across regions, and anthropogenic disturbance to vent communities will escalate with the advent of deep‐sea mineral extraction. We demonstrate how anthropogenic changes to disturbance regimes may impact species diversity, pushing communities over thresholds leading to local species extinction.
Recommendations for Resource Managers
• Conventional wisdom suggests that disturbance‐adapted communities will not be strongly affected by novel perturbations associated with resource extraction or other human activities.
• We show that differing fecundity–tolerance strategies can mediate coexistence in disturbance‐prone deep‐sea environments, but that small changes to the disturbance regime can alter community composition and result in species extirpation.
• Natural disturbance should not be used as a rationale for expected low impacts of anthropogenic disturbance, for example, mining activities in management of deep‐sea mineral extraction. Appropriate impact studies should be performed for all communities where disturbances, such as mining, are planned.
... The alternative is that nonnative plants possess more acquisitive (or fast) traits, for example, high specific leaf area, lower leaf C:N ratio, and lower wood density (Reich 2014), that allow them to take advantage of canopy gaps and forest edges created by disturbances, such as timber harvesting ("try harder/different"). This is a common assumption in much of the invasion biology literature, the idea being that invasive plants are successful because they can take advantage of changes in resource abundance and grow and spread faster than native plants ( Richardson & Pysek 2006;Catford et al. 2009;Lamarque et al. 2011;Moles et al. 2012). If this is the case, nonnative plants would occupy a different region of trait space compared to native species, indicating that nonnative plants possess growth and survival strategies that allow them to fill an unoccupied niche. ...
Question
Predicting which newly arrived species will establish and become invasive is a problem that has long vexed researchers. In a study of cold temperate oak forest stands, we examined two contrasting hypotheses regarding plant functional traits to explain the success of certain nonnative species. Under the “join the locals” hypothesis, successful invaders are expected to share traits with resident species because they employ successful growth strategies under light‐limited understory conditions. Alternatively, under the “try harder” hypothesis, successful invaders are expected to have traits different from native species in order to take advantage of unused niche space.
Location
Minnesota, USA.
Methods
We examined these two theories using 109 native and 11 nonnative plants in 68 oak forest stands. We focused on traits related to plant establishment and growth, including specific leaf area (SLA), leaf carbon to nitrogen ratio (C:N), wood density, plant maximum height, mycorrhizal type, seed mass, and growth form. We compared traits of native and nonnative species using ordinations in multidimensional trait space and compared community weighted mean (CWM) trait values across sites.
Results
We found few differences between trait spaces occupied by native and nonnative species. Nonnative species occupied smaller areas of trait space than natives, yet were within that of the native species, indicating similar growth strategies. We observed a higher proportion of nonnative species in sites with higher native woody species CWM SLA and lower CWM C:N. Higher woody CWM SLA was observed in sites with higher soil pH while lower CWM C:N was found in sites with higher light levels.
Conclusions
Nonnative plants in this system have functional traits similar to natives and are therefore “joining the locals”. However, nonnative plants may possess traits toward the acquisitive end of the native plant trait range, as evidenced by higher nonnative plant abundance in high‐resource environments.
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... The biotic resistance hypothesis [11] postulates that many introduced species fail to establish or become problematic pests due to negative interactions with native species, but it does not explain why some exotics might bypass such resistance to prosper even in highly resistant communities. More generally, the interplay between species invasiveness and community invasibility requires better integration [16][17][18][19]. Notably, some exotics become serious pests in one community but fail to do so in others [20], indicating an important role of community context in invasions [16,[21][22][23]. ...
Biological invasions present a global problem underlain by an ecological paradox that thwarts explanation: how do some exotic species, evolutionarily naïve to their new environments, outperform locally adapted natives? We propose that community assembly theory provides a framework for addressing this question. Local community assembly rules can be defined by evaluating how native species' traits interact with community filters to affect species abundance. Evaluation of exotic species against this benchmark indicates that exotics that follow assembly rules behave like natives, while those exhibiting novel interactions with community filters can greatly underperform or outperform natives. Additionally, advantages gained by exotics over natives following disturbance can be explained by accounting for extrinsic assembly processes that bias exotic traits toward ruderal strategies.
... A systematic comparison is needed. CUMULATIVE IMPACTS.-Instances of disturbance are known to facilitate invasions (Lozon & MacIsaac 1997, Lockwood et al. 2007, Moles et al. 2012. Canopy disturbance is a particularly important prerequisite for many tropical plant invasions (Denslow & DeWalt 2008). ...
Selective logging in tropical rain forests may promote population growth of invasive plants. The ability of invaders to respond, specifically in reproductive traits, to increase in resource abundance may allow them to increase their presence in the seed rain of recipient communities. The invasive pioneer tree Bellucia pentamera (Melastomataceae) is currently spreading within Gunung Palung National Park in West Kalimantan, Indonesia. The park has also experienced periods of illegal, small‐scale, selective logging that seem to have facilitated population growth and spread of this invader. We first used fruiting frequency as a proxy for fruit output to make comparisons between B. pentamera and the native tree community of over 200 genera. We then constructed two Generalized Linear Autoregressive Moving Average (GLARMA) models using 13 months of phenology data to predict both fruiting frequency and crop size of B. pentamera under selective logging versus natural treefall disturbance regimes. Bellucia pentamera fruited at considerably higher frequency than all 200 native genera considered. This invader also responded positively to selective logging with both odds of being in fruit and crop sizes at least doubling in logged plots. Prolific seed output of B. pentamera is especially problematic within the lottery competition of tropical rain forest gap tree communities, in which prominence in the seed rain is particularly important. Furthermore, the reproductive response of B. pentamera to selective logging suggests that this harvesting practice may have a considerable role in facilitating this invasion in Southeast Asia.
Exotic plant invasions threaten biodiversity and are costly to farmers. Land use is a major pathway promoting the spread of exotic plant species; however, little is known about the processes underlying the success of exotic plants in tropical agricultural landscapes. Focussing on the heterogeneous smallholder landscapes of north‐eastern Madagascar, we studied exotic plants of understorey communities across a land‐use intensity gradient from unburned lands (old‐growth forests, forest fragments and forest‐derived vanilla agroforests) to burned ones (fallow‐derived vanilla agroforests, woody fallows and herbaceous fallows).
We quantified the absolute species richness, abundance and cover of exotic plants across land‐use types and their proportional contribution to community richness, abundance and cover as indicators of exotic plant invasion. We tested for the effects of land‐use parameters, namely land‐use history, canopy closure and landscape‐level forest cover, on exotic plants. Additionally, we tested whether the phylogenetic relatedness between exotic and native species in the same plot affected invasion success, testing Darwin's naturalisation and pre‐adaptation hypotheses.
All indicators of exotic plant invasion were lowest in old‐growth forests and forest fragments and highest in fallow‐derived vanilla agroforests, woody fallows and herbaceous fallows. Absolute and proportional exotic richness were negatively affected by canopy closure, and landscapes with high forest cover had lower proportions of exotic plant richness. High phylogenetic relatedness between exotics and natives was associated with lower proportional richness but higher proportions of exotics in abundance and cover. However, individual exotic species showed contrasting responses to land‐use parameters and relatedness to natives.
Synthesis and applications: Our results indicate that maintaining unburned lands, land‐use types with dense canopies and landscapes with high forest cover prevents the spread of exotic plants within the agricultural landscapes of north‐eastern Madagascar. Supporting Darwin's pre‐adaptation hypothesis, exotic plants that are phylogenetically closely related to native plants are more likely to become successful invaders in terms of abundance and cover. Nevertheless, individual species show different responses to land‐use changes and phylogenetic relatedness. Therefore, land‐use decisions and management choices can be tailored to limit the spread of exotic species and to preserve native plants in this global biodiversity hotspot.
Rates of species colonisation and extirpation are increasing in plant communities world‐wide. Colonisation could potentially help compensate for, or compound, resident diversity loss that results from global environmental change.
We use a multifactorial seed addition grassland experiment to examine relationships between plant colonisation, resident species diversity and key community assembly factors over 3 years. By manipulating colonist seed rate, imposing disturbance and examining abundance and diversity impacts of 14 formerly absent sown colonists in communities that varied in successional stage and time since agricultural abandonment, we were able to disentangle effects of global change factors (species introduction, novel disturbance and land use change) that are usually confounded.
Evidence suggested that cover abundance of sown colonists was most strongly influenced by successional stage of recipient communities, though number of growing seasons was also important for the group of seven colonists with resource conservative ‘slow’ life history traits. Colonist type, seed rate and disturbance had weaker relationships with colonist cover.
Factors affecting sown colonist cover were highly conditional. A negative relationship between plot‐level disturbance and colonist cover in early successional communities meant that, despite a positive relationship in late succession, colonisation was negatively related to disturbance overall, defying theoretical expectations.
Non‐sown resident diversity was negatively related to colonist cover and positively related to successional stage. Resource acquisitive colonists with ‘fast’ life history traits appeared to limit cover of ‘slow colonists’ when the two groups were sown together, likely reflecting niche pre‐emption.
Communities at earlier stages of succession had lower resident diversity and experienced higher levels of colonisation than communities at later stages of succession. Elevated colonisation and lower resident diversity both appeared to be symptoms of human‐induced land use change. However, results suggested that resource competition from plant colonists may also limit resident diversity in grasslands abandoned from agriculture more recently.
Synthesis. Our findings point to the importance of resource availability and competition on plant colonisation and colonist impacts on residents. Although colonisation is potentially a source of biodiversity in the short term, our results suggest that plant colonists that reach high abundance may be a further threat to resident plant diversity in secondary grasslands recovering from a recent history of agriculture.
Determining the drivers of non-native plant invasions is critical for managing native ecosystems and limiting the spread of invasive species1,2. Tree invasions in particular have been relatively overlooked, even though they have the potential to transform ecosystems and economies3,4. Here, leveraging global tree databases5-7, we explore how the phylogenetic and functional diversity of native tree communities, human pressure and the environment influence the establishment of non-native tree species and the subsequent invasion severity. We find that anthropogenic factors are key to predicting whether a location is invaded, but that invasion severity is underpinned by native diversity, with higher diversity predicting lower invasion severity. Temperature and precipitation emerge as strong predictors of invasion strategy, with non-native species invading successfully when they are similar to the native community in cold or dry extremes. Yet, despite the influence of these ecological forces in determining invasion strategy, we find evidence that these patterns can be obscured by human activity, with lower ecological signal in areas with higher proximity to shipping ports. Our global perspective of non-native tree invasion highlights that human drivers influence non-native tree presence, and that native phylogenetic and functional diversity have a critical role in the establishment and spread of subsequent invasions.
Peberholm is a constructed Danish island in the Øresund strait. It was primarily constructed by calcareous clay from the sea floor, and is traversed by a highway and a railway. Being constructed from material without a seed bank, Peberholm constituted a good opportunity to study primary succession in an anthropogenic context. In this study, data from a survey of the vascular plant community of Peberholm was studied. The data span over a 22 year‐period, between 1999 and 2020. The development of the flora was analysed with regards to indicators for environmental factors and vegetation types, as well as occurrence of alien species or species of conservation concern. Peberholm experienced a rapid succession during its first five years. The effects of the initial ground disturbance quickly wore off, resulting in a relative decline in plant communities associated with ruderal land. These highly anthropogenic habitats were replaced with grasslands. The shrubification also began early on. The rapid initial changes were then replaced with a much slower but also more continuous change, resulting in the development of both more natural grasslands and an increased shrubification. Although several rare or threatened species colonized Peberholm from the beginning, the conservation value of the flora on a whole increased during the succession process of forming more natural vegetation types. The succession process demonstrated at Peberholm has more in common with the succession at urban soils than with naturally occurring primary succession.
Non-native species invasion, habitat fragmentation and climate change individually impose negative impacts on natural systems, but their synergistic effects may do more harm than the sum of their parts. We examined the combined effects of these global change drivers by studying the impacts of experimental warming on seed-dispersing forest ant nesting and foraging in the Southeastern U.S. to determine if warming and forest fragmentation facilitated non-native ant invasion effects on native ants. Spring ant phenology and activities were monitored for two years (2019–2020) at weekly bait stations and in artificial nest occupancy. We found that, when combined, forest edge habitat and experimental warming favored invasive non-native ant frequency, but the experimental warming alone did not appear to facilitate non-native ant incursion into forested habitat. We did find, however, that experimental warming exacerbated the negative effects of non-native ants on native ant foraging. Moreover, fragmented edge habitat strongly limited native forest ant foraging and experimental warming increased the negative effects of non-native ant invaders on native ants. Ultimately, the non-native ants displaced native seed-dispersing ants from artificial nests, and the displacement progressively increasing with greater experimental warming. Our results suggest that global change drivers such as warming, habitat fragmentation and species invasion imposed negative impacts individually, but their combined effects were worse than the sum of their parts. Moreover, our results indicate that predicting species reactions to global change poses great challenges given that the strongest impacts were the non-additive effects.
Although closed‐canopy forests are characterized by low‐light availability and slow population dynamics, many are under threat from non‐native, invasive woody species that combine high colonization ability and fast growth potential with high low‐light survival. This ‘superinvader’ phenotype contravenes expected trade‐offs predicted by successional niche theory, posing a challenge to both invasion and forest succession theory.
We propose a parsimonious conceptual model based on the whole‐plant light compensation point (WPLCP) that, across a variety of plant strategies and growth forms, can explain greater competitive abilities of forest invaders in the context of both high‐light growth rate and shade tolerance. The model requires only that non‐native species experience relatively fewer carbon costs than native species, enabling resource‐acquisitive species to establish in low‐light conditions.
We review evidence for lower carbon costs in invasive species resulting from (1) enemy release, (2) recent environmental changes that favour less stress‐tolerant phenotypes and (3) phylogenetically constrained native floras. We also discuss implications of invader shade tolerance in the context of other life‐history strategies that, combined with canopy disturbances, facilitate their rapid numerical dominance.
Synthesis. An invasion framework driven by carbon dynamics suggests renewed focus on whole‐plant carbon costs, including below‐ground respiration and tissue turnover, which are rarely measured in functional studies of forest invaders.
Sicyos angulatus is a serious threat to riverine ecosystem functions and services worldwide. Here, we studied the effect of species identity and diversity on biotic resistance to S. angulatus under two different soil nutrient levels (unfertilized vs. fertilized). Soil nutrient levels showed no significant effect on invasion by S. angulatus in the control treatment, where intervention by native plants was absent. Species identity of native plants and its interaction with soil nutrient levels had a significant effect on biotic resistance to S. angulatus. For instance, Pennisetum alopecuroides and Lespedeza cuneata best resisted invasion in fertilized soil, whereas Lespedeza bicolor and Lactuca indica best resisted invasion in unfertilized soil. In addition, a mixture of four plant species resisted invasion equally as well as the monoculture of a species in unfertilized soil, whereas the mixed treatment resisted invasion much better in fertilized soil compared with unfertilized soil. Structural equation modeling revealed that species identity and diversity as well as fertilizer application significantly influenced biotic resistance to S. angulatus invasion, while soil nutrients did not influence invasion success directly. Based on these results, we strongly suggest sowing seed mixtures of various species after eradicating S. angulatus plants to prevent re-invasion. Overall, these results demonstrate how native plants rely on resource availability to resist colonization by an invasive plant, such as S. angulatus. This information can be used for the development of improved guidelines for plant restoration and invasive species control.
The evolution of increased competitive ability (EICA) hypothesis encapsulates the importance of evolution and ecology for biological invasions. According to this proposition, leaving specialist herbivores at home frees introduced plant species from investing limited resources in defense to instead use those resources for growth, selecting for individuals with reduced defense, enhanced growth, and, consequently, increased competitive ability. We took a multispecies approach, including ancestral and non‐native populations of seven weeds, as well as seven coexisting local weeds, to explore all three predictions (i.e., lower defense, greater growth, and better ability to compete in non‐native than ancestral populations), the generality as an invasion mechanism for a given system, and community‐level consequences of EICA. We assessed plant defenses by conducting herbivory trials with a generalist herbivore. Therefore, finding that non‐native populations are better defended than ancestral populations would lend support to the shifting defense (SD) hypothesis, an extension of EICA that incorporates the observation that introduced species escape specialists, but encounter generalists. We also manipulated water additions to evaluate how resource availability influences competition in the context of EICA and plant plasticity in our semiarid system. We found that non‐native populations of one study species, Centaurea solstitialis, were better defended, grew faster, and exerted stronger suppression on locals than ancestral populations, offering support to EICA through the SD hypothesis. The other species also displayed variation in trait attributes between ancestral and non‐native populations, but they did not fully comply with the three predictions of EICA. Notably, differences between those populations generally favored the non‐natives. Moreover, non‐native populations were, overall, superior at suppressing locals relative to ancestral populations under low water conditions. There were no differences in plasticity among all three groups. These results suggest that evolutionary change between ancestral and non‐native populations is widespread and could have facilitated invasion in our system. Additionally, although trading growth for shifted defense does not seem to be the main operational path for evolutionary change, it may explain the dominance of some introduced species in ruderal communities. Because introduced species dominate communities in disturbed environments around the world, our results are likely generalizable to other systems.
Successful control and prevention of biological invasions depend on identifying traits of non‐native species that promote fitness advantages in competition with native species. Here, we show that, among 76 native and non‐native woody plants of deciduous forests of North America, invaders express a unique functional syndrome that combines high metabolic rate with robust leaves of longer lifespan and a greater duration of annual carbon gain, behaviours enabled by seasonally plastic xylem structure and rapid production of thin roots. This trait combination was absent in all native species examined and suggests the success of forest invaders is driven by a novel resource‐use strategy. Furthermore, two traits alone—annual leaf duration and nuclear DNA content—separated native and invasive species with 93% accuracy, supporting the use of functional traits in invader risk assessments. A trait syndrome reflecting both fast growth capacity and understorey persistence may be a key driver of forest invasions. We show that non‐native, invasive woody plants of North American deciduous forests express a unique functional syndrome combining traits associated with both high growth potential and high shade tolerance. This syndrome was absent in all native species examined and involves an integrated growth strategy reflecting leaf, stem and root traits.
Biological invasions are a leading cause of rapid ecological change and often present a significant financial burden. As a vibrant discipline, invasion biology has made important strides in identifying, mapping, and beginning to manage invasions, but questions remain surrounding the mechanisms by which invasive species spread and the impacts they bring about. Frequent, multiscalar ecological monitoring such as that provided through the National Ecological Observatory Network (NEON) can be an important tool for addressing some of these questions. We articulate a set of major outstanding questions in invasion biology, consider how NEON data science is positioned to contribute to addressing these questions, and provide suggestions to help equip a growing contingent of NEON data users in solving invasion biology problems. We demonstrate these ideas through four case studies examining the mechanisms of plant invasions in the U.S. Intermountain West. In Case Study I, we evaluate the relationships between native species richness, non‐native species richness, and probability of invasion across scales. In Case Studies II and III, we explore the relationship between environmental factors and non‐native species presence to understand invasion mechanisms. Case Study IV outlines a method for improving the ability to distinguish invasive plants from native vegetation in remotely sensed data by leveraging temporal patterns of phenology. There are many novel elements in the NEON sampling design that make it uniquely poised to shed light on the mechanisms that can help us understand invasibility, prediction, and progression, as well as on the variability, longevity, and interactions of multiple invasive species’ impacts. Thus, knowledge gained through analysis of NEON data is expected to inform sound decision‐making in unique ways for managers of systems experiencing biological invasions.
The success of invasive plants is influenced by many interacting factors, but evaluating multiple possible mechanisms of invasion success and elucidating the relative importance of abiotic and biotic drivers is challenging, and therefore rarely achieved.
We used live, sterile or inoculated soil from different soil origins (native range and introduced range plantation; and invaded plots spanning three different countries) in a fully factorial design to simultaneously examine the influence of soil origin and soil abiotic and biotic factors on the growth of invasive Pinus contorta.
Our results displayed significant context dependency in that certain soil abiotic conditions in the introduced ranges (soil nitrogen, phosphorus or carbon content) influenced responses to inoculation treatments.
Our findings do not support the enemy release hypothesis or the enhanced mutualism hypothesis, as biota from native and plantation ranges promoted growth similarly. Instead, our results support the missed mutualism hypothesis, as biota from invasive ranges were the least beneficial for seedling growth. Our study provides a novel perspective on how variation in soil abiotic factors can influence plant–soil feedbacks for an invasive tree across broad biogeographical contexts.
Aims
Plant invasion is one of the most serious threats to ecosystems worldwide. When invasive plants with the ability of clonal growth invading or colonizing in new habitat, their interconnected ramets may suffer from heterogeneous light. Effects of clonal integration on allelopathy of invasive plants are poorly understood under heterogeneous light conditions.
Methods
To investigate the effects of clonal integration on allelopathy of invasive plant Wedelia trilobata under heterogeneous light conditions, a pot experiment was conducted by using its clonal fragments with two successive ramets. The older ramets were exposed to full light, whereas the younger ones were subjected to 20% full light. The younger ramets of each clonal fragment were adjacently grown with a target plant (one tomato seedling) in a pot. Stolon between two successive ramets was either severed or retained intact. In addition, two tomato seedlings (one as target plant) were adjacently grown in a pot as contrast.
Important findings
Compared with severing stolon, biomass accumulation, foliar chlorophyll and nitrogen contents, chlorophyll fluorescence parameters and net photosynthetic rates of the target plants as well as their root length and activity, were significantly decreased when stolon between interconnected ramets of Wedelia trilobata retained intact. Under heterogeneous light conditions, transportation or sharing of carbohydrate between two successive ramets enhanced allelopathy of the young ramets subjected to 20% full light treatment. It is suggested that clonal integration may be important for invasion or colonization of invasive plants with ability of clonal growth under heterogeneous light conditions.
This chapter provides a broad overview of the interactions between invasive alien plants and climate change. Invasive alien plants are harmful nonnative plant species that have been introduced by humans outside of their “natural” geographical range. Both climate change and alien plant invasions result from human activities and can cause major environmental and socioeconomic damage. Although they can operate as independent environmental and economic threats, they can also interact. Climate change can facilitate alien plant invasions (1) by altering background environmental conditions; (2) by increasing disturbance through extreme climatic events; and (3) through human responses to climate change. While all plants, regardless of whether they are native or alien, will likely be affected by environmental change, it is widely expected that climate change will favor invasive alien plants at the expense of native plants. The way in which invasive alien plants impact the environment (e.g., modifying hydrology and soil properties, altering fire regimes) can to some extent either contribute to or exacerbate the effect of climate change. Here, we consider why and how climate change is predicted to exacerbate alien plant invasions and provide examples of how invasive alien plants can contribute to climate change. We first briefly describe what makes a plant invasive, how and why a plant becomes invasive, what stages must it go through, and what the drivers of invasion are.
Disturbance is a key factor shaping ecological communities, but little is understood about how the effects of disturbance processes accumulate over time. When disturbance regimes change, historical processes may influence future community structure, for example, by altering invasibility compared to communities with stable regimes. Here, we use an annual plant model to investigate how the history of disturbance alters invasion success. In particular, we show how two communities can have different outcomes from species introduction, solely due to past differences in disturbance regimes that generated different biotic legacies. We demonstrate that historical differences can enhance or suppress the persistence of introduced species, and that biotic legacies generated by stable disturbance history decay over time, though legacies can persist for unexpectedly long durations. This establishes a formal theoretical foundation for disturbance legacies having profound effects on communities, and highlights the value of further research on the biotic legacies of disturbance. Disturbance has many effects on communities, and much is still not known about how these effects accumulate over time. We use an annual plant model to demonstrate how legacies of disturbance can affect future community composition, long after disturbance regimes have changed. In particular, we show that differences in disturbance history can alter invasion success even when present disturbance regimes are identical.
Many invasion theories invoke resource competition as the primary mechanism of invader advantage. These include Darwin's naturalization hypothesis (DNH), which treats phylogenetic similarity as a proxy for niche overlap and competitive intensity, and the evolutionary imbalance hypothesis (EIH), which suggests the phylogenetic diversity (PD) of an introduced species’ native range is an indicator of its competitive ability.
Few tests of invasion theory, however, consider habitat characteristics associated with the role of competition in community assembly. In particular, plant invasions of habitats characterized by high environmental stress and disturbance levels should rarely be driven by competition. This suggests tests of EIH and DNH are habitat dependent, and their relative importance in invasiveness models should be predictable based on habitat qualities related to competitive intensity.
Using a dataset of plant invasions in New Zealand (NZ) natural areas that distinguishes naturalized species according to both habitat type and community impact, we evaluated the predictive ability of factors related to EIH, DNH and covariates including year of introduction, introduction mode and life history attributes, in driving species invasiveness. We hypothesized that EIH and DNH would be more important predictors of invasiveness in forested habitats and decline in importance as communities shifted towards those more dominated by herbaceous species and/or more sparsely vegetated.
We found mixed support for the role of competition linked to DNH and EIH as a driver of invasions in relation to habitat type. Native range PD was among the best predictors of invasiveness in forests, and declined in importance in more disturbed habitats, supporting EIH. In contrast, phylogenetic nearest neighbour distance (PNND) of invaders to native communities was more important in disturbed environments, suggesting competition does not drive DNH. Further, for most habitats and across all of NZ, neither PD nor PNND was as important as year of introduction or life history and growth form attributes in predicting invasiveness.
Synthesis. Although both native range PD and PNND predict the invasiveness of naturalized plants in NZ, the results of our habitat‐specific models indicate that only PD is consistent with an invasion mechanism based on competitive ability. Effects of PNND were greatest in grasslands that have been extensively modified by fire and grazing, suggesting they are more likely driven by invader pre‐adaptation to modified habitat conditions. Due to the importance of matching species' traits to environmental context, invasiveness risk assessments perform better when applied to invaders of particular habitats.
Disturbances can play a major role in biological invasions: by destroying biomass, they alter habitat and resource abundances. Previous field studies suggest that disturbance-mediated invader success is a consequence of resource influxes, but the importance of other potential covarying causes, notably the opening up of habitats, have yet to be directly tested. Using experimental populations of the bacterium Pseudomonas fluorescens, we determined the relative importance of disturbance-mediated habitat opening and resource influxes, plus any interaction between them, for invader success of two ecologically distinct morphotypes. Resource addition increased invasibility, while habitat opening had little impact and did not interact with resource addition. Both invaders behaved similarly, despite occupying different ecological niches in the microcosms. Treatment also affected the composition of the resident population, which further affected invader success. Our results provide experimental support for the observation that resource input is a key mechanism through which disturbance increases invasibility.
Invasive plant populations respond positively to light increase from windstorm‐caused canopy damage, but are typically out‐competed over time as the tree canopy recovers. Some invasive plants have the ability to slow or even completely prevent canopy regrowth. It is important to understand invasive plant dynamics in windstorm blowdowns to inform effective forest management. We hypothesized that blowdown areas are more invaded than unaffected forest and that invadedness of a blowdown will decrease with increasing time since disturbance and increase with disturbance magnitude.
We used Landsat imagery to identify 31 blowdowns caused by one of three storms in southern Illinois of the United States: 2006 tornado, 2009 derecho or 2017 tornado. We statistically matched these blowdowns to areas of unaffected forest based on slope, aspect, elevation, distance to a road and distance to a trail. We surveyed blowdown and unaffected units for invasive plants in summer 2018. Disturbance magnitude was measured in two ways: per cent change in normalized difference vegetation index (NDVI) and area of blowdown.
Overall, blowdowns were more invaded than their statistically matched areas. Invadedness of blowdowns decreased with increasing time since disturbance, indicating that the blowdowns are recovering as tree canopy regrowth shades out invasive plants. Per cent change in NDVI was not a supported predictor of blowdown invadedness, but invadedness was strongly positively related to blowdown size. Larger blowdowns recover more slowly than smaller disturbances due to different recovery pathways, providing more time and opportunity for the spread and establishment of invasive plants.
Synthesis. Our findings suggest that although blowdowns contained higher cover of invasive plants than forest unaffected by windstorms, invasive plants were responding negatively to canopy regrowth. Land managers should prioritize treatment of invasive plants in larger and more recent blowdowns.
Mycobacteria are common inhabitants of soil, and while most members of this bacterial group are innocuous, some mycobacteria can cause environmentally acquired infections of humans and other animals. Human infections from nontuberculous mycobacteria (NTM) are increasingly prevalent worldwide, and some areas appear to be “hotspots” for NTM disease. While exposure to soil is frequently implicated as an important mode of NTM transmission, the diversity, distributions, and ecological preferences of soil mycobacteria remain poorly understood. We analyzed 143 soils from a range of ecosystems and found that mycobacteria and lineages within the group often exhibited predictable preferences for specific environmental conditions. Soils harbor large amounts of previously undescribed mycobacterial diversity, and lineages that include known pathogens were rarely detected in soil. Together, these findings suggest that soil is an unlikely source of many mycobacterial infections. The biogeographical patterns we documented lend insight into the ecology of this important group of soil-dwelling bacteria.
Mycobacterium is a diverse bacterial genus ubiquitous in many soil and aquatic environments. Members of this genus have been associated with human and other animal diseases, including the nontuberculous mycobacteria (NTM), which are of growing relevance to public health worldwide. Although soils are often considered an important source of environmentally-acquired NTM infections, the biodiversity and ecological preferences of soil mycobacteria remain largely unexplored across contrasting climates and ecosystem types. Using a culture-independent approach by combining 16S rRNA marker gene sequencing with mycobacterial-specific hsp65 gene sequencing, we analyzed the diversity, distributions, and environmental preferences of soil-dwelling mycobacteria in 143 soil samples collected from across the globe. The surveyed soils harbored highly diverse mycobacterial communities that span the full-extent of the known mycobacterial phylogeny, with most soil mycobacteria belonging to previously undescribed lineages. While the genus Mycobacterium tended to have higher relative abundances in cool, wet, and acidic soil environments, several individual mycobacterial clades had contrasting environmental preferences. We identified the environmental preferences of many mycobacterial clades, including the clinically-relevant M. avium complex that was more commonly detected in wet and acidic soils. However, most of the soil mycobacteria detected were not closely related to known pathogens, calling into question previous assumptions about the general importance of soil as a source of NTM infections. Together this work provides novel insights into the diversity, distributions and ecological preferences of soil mycobacteria, and lays the foundation for future efforts to link mycobacterial phenotypes to their distributions.
Importance
Mycobacteria are common inhabitants of soil, and while most members of the bacterial genus are innocuous, some mycobacteria can cause environmentally-acquired infections of humans and other animals. Human infections from nontuberculous mycobacteria (NTM) are increasingly prevalent worldwide, and some areas appear to be ‘hotspots’ for NTM disease. While exposure to soil is frequently implicated as an important mode of NTM transmission, the diversity, distributions and ecological preferences of soil mycobacteria remain poorly understood. We analyzed 143 soils from across the globe and found that the genus Mycobacterium and lineages within the genus often exhibited predictable preferences for specific environmental conditions. Soils harbor large amounts of previously-undescribed mycobacterial diversity, and lineages that include known pathogens were rarely detected in soil. Together these findings suggest that soil is an unlikely source of many mycobacterial infections. The biogeographical patterns we documented lend insight into the ecology of this important group of soil-dwelling bacteria.
The extent by which human activities create an inroad for the invasion of exotic plant species and the mechanisms that drive a disturbed ecosystem's resistance to exotic invasion is largely unknown. We investigated the mechanisms by which salt marsh resists cordgrass (Spartina alterniflora) invasion by two key factors: anthropogenic disturbance by ditching and herbivory by the native crab Helice tientsinensis. In northern China, although abiotic resistance (i.e., hyper‐stresses) inhibited the landward invasion of cordgrass to high marshes, our transplant experiments showed that anthropogenic ditching created windows of opportunity for cordgrass invasion to high marshes by enhancing propagule pressure and providing favorable (i.e., low‐salinity and high‐inundation) microhabitats for colonization. Furthermore, a native herbivore exclusion experiment showed that crab grazing strongly suppressed seedling establishment and growth of cordgrass in ditched high marshes, which gradually created native biotic resistance to cordgrass invasion. We conclude that grazing by native herbivores can enhance the resistance of high marshes to cordgrass invasion triggered by anthropogenic ditching disturbance in northern China. These findings highlight that it is critical to investigate ecosystem resistance associated with anthropogenic disturbance to better understand the multiple mechanisms of exotic plant invasion, and we call for integration of these findings into invasion control strategies.
Many aquatic invasive species (AIS) management programs are doing im- portant work on preventing non-indigenous species movement to our wild places. Attitudes and perspectives on aquatic non-indigenous species and their management by ecologists and the public are fundamentally a question of human values. Despite eloquent philosophical writings on treatment of non- indigenous species, management agency rhetoric on ‘invasive’ species usually degenerates to a good versus evil language, often with questionable results and lost conservation dollars. We assess and learn from an established AIS pro- gram. We discuss an ethic framework and operational directives to minimise the trap of a binary classification of species into bad or good, and we advocate for a principled pragmatic approach to minimise conflicts. We make a case for not labelling species and instead focusing on managing nuisance conditions and protecting ecosystem health.
Climate change is expected to increase the frequency and magnitude of extreme weather events. It is therefore of major importance to identify the community attributes that confer stability in ecosystem processes facing such events. By means of three experiment-based studies, we aimed to explore how plant community aspects affect ecological stability. In the first study, we compared data on biomass productivity before and after a major flood in synthetic plant communities with different values of species richness, functional diversity and community weighted means of functional traits on
different measures of stability. In the second experiment, we investigate how changes in rainfall (simulated by rainout-shelters) alter functional traits and diversity and ecosystem processes of a natural grassland under different defoliation frequencies. The third study consisted on a biodiversity experiment in which we manipulated species richness by removals for studying the effects of species diversity on the stability at the community (colonization and extinction) and ecosystem levels (variation in primary productivity). Functional traits and other community features (richness, composition) affected and were affected by the disturbances. Diversity-stability relationships presented different trends depending on the disturbance nature and intensity. Resource inputs following the flood favoured rich communities and the ones characterized by traits related to resource acquisition. Also, defoliation frequency did not affect the way vegetation responded to rainfall manipulation. Stability showed opposite trends when evaluated at different levels of ecological organization. Higher turnover in rich communities corresponded to increased stability in ecosystem processes.
The merger of two or more divergent genomes within an allopolyploid nucleus can facilitate speciation and adaptive evolution in flowering plants. Widespread changes to gene expression have been shown to result from interspecific hybridisation and polyploidy in a number of plant species, and attention has now shifted to determining the epigenetic processes that drive these changes. We present here an analysis of cytosine methylation patterns in triploid F1Senecio (ragwort) hybrids and their allohexaploid derivatives. We observe that, in common with similar studies in Arabidopsis, Spartina and Triticum, a small but significant proportion of loci display nonadditive methylation in the hybrids, largely resulting from interspecific hybridisation. Despite this, genome duplication results in a secondary effect on methylation, with reversion to additivity at some loci and novel methylation status at others. We also observe differences in methylation state between different allopolyploid generations, predominantly in cases of additive methylation with regard to which parental methylation state is dominant. These changes to methylation state in both F1 triploids and their allohexaploid derivatives largely mirror the overall patterns of nonadditive gene expression observed in our previous microarray analyses and may play a causative role in generating those expression changes. These similar global changes to DNA methylation resulting from hybridisation and genome duplication may serve as a source of epigenetic variation in natural populations, facilitating adaptive evolution. Our observations that methylation state can also vary between different generations of polyploid hybrids suggests that newly formed allopolyploid species may display a high degree of epigenetic diversity upon which natural selection can act.
This book gives a broad and up-to-date coverage of bootstrap methods, with numerous applied examples, developed in a coherent way with the necessary theoretical basis. Applications include stratified data; finite populations; censored and missing data; linear, nonlinear, and smooth regression models; classification; time series and spatial problems. Special features of the book include: extensive discussion of significance tests and confidence intervals; material on various diagnostic methods; and methods for efficient computation, including improved Monte Carlo simulation. Each chapter includes both practical and theoretical exercises. Included with the book is a disk of purpose-written S-Plus programs for implementing the methods described in the text. Computer algorithms are clearly described, and computer code is included on a 3-inch, 1.4M disk for use with IBM computers and compatible machines. Users must have the S-Plus computer application. Author resource page: http://statwww.epfl.ch/davison/BMA/
Specialization can become detrimental to a discipline if it fosters intellectual isolation. A bibliographic analysis of several research areas in plant ecology (invasion biology, succession ecology, gap/patch dynamics, and global change effects on plants) revealed that plant ecologists do not regularly make use of the findings and insights of very similar studies being conducted in other research subdisciplines, nor do they try to make their findings and insights easily accessible to researchers in other areas. Invasion papers were least likely to be cross-linked (6%) with other fields, whereas gap/patch dynamics papers were most likely to be cross-linked (15%). This tendency toward intellectual isolation may be impeding efforts to achieve more powerful generalizations in ecology by reducing the number of potentially productive exchanges among researchers. In this paper, we illustrate this problem using the example of several speciality areas that study vegetation change. We argue that, rather than characterizing studies of vegetation change on the basis of what distinguishes them from one another, plant ecologists would benefit from concentrating on what such studies have in common. As an example, we propose that several speciality areas of plant ecology could be reunified under the term ecology of vegetation change. Individual researchers, journals, and ecological societies all can take specific steps to increase the useful exchange of ideas and information among research areas. Promoting rapid and more effective communication among diverse researchers may reduce the proliferation of narrow theories, concepts, (M.A. Davis). and terminologies associated with particular research areas. In this way, we can expedite our understanding of the ecological mechanisms and consequences associated with plant communities.
Although biological invasions are clearly one of the most important impacts humans have had on the Earth's ecosystems, we still do not have reliable tools which can help us to predict which species are potential invaders. At present, several limited generalizations are available for seed plants: (1) invasiveness of woody species in disturbed landscapes is significantly associated with small seed mass, short juvenile period, and short mean interval between large seed crops; (2) vertebrate dispersal is responsible for the success of many woody invaders in disturbed as well as ‘undisturbed’ habitats; (3) primary (native) latitudinal range of herbaceous Gramineae, Compositae, and Fabaceae seems to be the best predictor of their invasiveness, at least for species introduced from Eurasia to North America; (4) low nuclear DNA content (genome size) seems to be a result of selection for short minimum generation time and, therefore, may be associated with plant invasiveness in disturbed landscapes; (5) analysis of exotic Gramineae and Compositae introduced from Europe to California supports Darwin's suggestion that alien species belonging to exotic genera are more likely to be invasive than alien species from genera represented in the native flora. Fortunately, these seemingly disparate stories can be brought together and provide a foundation for building a general theory of seed plant invasiveness.
▪ Abstract Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other ci...
■ Abstract Contributions from the field of population biology hold promise for understanding and managing invasiveness; invasive species also offer excellent oppor- tunities to study basic processes in population biology. Life history studies and demo- graphic models may be valuable for examining the introduction of invasive species and identifying life history stages where management will be most effective. Evolution- ary processes may be key features in determining whether invasive species establish and spread. Studies of genetic diversity and evolutionary changes should be useful for
The introduction of new predators and pathogens has caused numerous well-documented extinctions of long-term resident species, particularly in spatially restricted environments such as islands and lakes. However, there are surprisingly few instances in which extinctions of resident species can be attributed to competition from new species. This suggests either that competition-driven extinctions take longer to occur than those caused by predation or that biological invasions are much more likely to threaten species through intertrophic than through intratrophic interactions. The likely threat of introduced species to resident controphics (species in the same trophic level) can be assessed with the help of existing biodiversity and extinction data sets and of two recent theories: (1) the fluctuating resource availability hypothesis, developed to account for changes in the invasibility of communities, and (2) the unified neutral theory, proposed to account for patterns of biodiversity at the community and metacommunity levels. Taken together, theory and data suggest that, compared to intertrophic interactions and habitat loss, competition from introduced species is not likely to be a common cause of extinctions of long-term resident species at global, metacommunity, and even most community levels.
Summary • Many studies have searched for traits that characterize successful invaders. Unfortunately, very few generalizations have emerged from this work. It seems that the traits of successful invaders are idiosyncratic and context-dependent. Unless we are to study each potential invader in each possible target community individually, we will need a new approach. • We introduce a framework for predicting traits that are likely to confer success in a given ecosystem. Our approach considers the prevailing environmental conditions, the traits of resident species, and the traits of potentially invading species. • Our approach can be applied to ecosystems where the environmental conditions and/or disturbance regime have recently changed, to predict the range of trait space occupied by (i) native species at risk of local extinction, (ii) native species that can persist under the present conditions, and (iii) successful invaders. Our approach can also be used to identify unoccupied viable trait space (i.e. vacant niches) that might be at risk of invasion. • Synthesis. Understanding invasions resulting from rapid changes in environmental conditions and invasions resulting from the colonization of vacant niches would be a major step forward for invasion biology. The conceptual framework described here is not limited to plant invasions: the same approach can be used for any taxa (e.g. insects, fish, mammals and marine invertebrates) and could also be used to predict species responses to environmental change.
Biotic resistance describes the ability of resident species in a community to reduce the success of exotic invasions. Although resistance is a well-accepted phenomenon, less clear are the processes that contribute most to it, and whether those processes are strong enough to completely repel invaders. Current perceptions of strong, competition-driven biotic resistance stem from classic ecological theory, Elton's formulation of ecological resistance, and the general acceptance of the enemies-release hypothesis. We conducted a meta-analysis of the plant invasions literature to quantify the contribution of resident competitors, diversity, herbivores and soil fungal communities to biotic resistance. Results indicated large negative effects of all factors except fungal communities on invader establishment and performance. Contrary to predictions derived from the natural enemies hypothesis, resident herbivores reduced invasion success as effectively as resident competitors. Although biotic resistance significantly reduced the establishment of individual invaders, we found little evidence that species interactions completely repelled invasions. We conclude that ecological interactions rarely enable communities to resist invasion, but instead constrain the abundance of invasive species once they have successfully established.
We investigated the influence of channel migration and expansion on riparian plant species diversity along the lower Colorado River near the United States–Mexico border. Using repeat aerial photography in a GIS we identified and classed areas of low, moderate, and high disturbance frequency caused by channel expansion and migration. Replicate vegetation plots (12m12m) were sampled in each of the three disturbance classes. One-way ANOVA was used to test for differences in species richness, species diversity (using the Shannon–Weiner Index) and overall percent ground cover of plants between the three disturbance classes. Regardless of disturbance class, plots were dominated by trees or shrubs, especially the non-native Tamarix ramosissima, as well as Pluchea sericea, Baccharis salicifolia and Salix goodingii. Clearly woody species constitute the great bulk of overall species richness, percent ground cover, and species diversity (H) in each disturbance group. No overall statistically significant differences were revealed among the disturbance groups for values of species richness, percent ground cover, or the Shannon–Wiener Index, though paired contrasts of means revealed that total percent ground cover on low disturbance plots was significantly higher than on moderately disturbed plots. Spatial and temporal variability in riparian diversity in the study area appears to hinge on factors other than disturbance frequency such as salt or drought stress. Alternately, our results could be interpreted as suggesting that in the presence of intensive flow regulation, disturbance plays a secondary role to ecological stresses, similar to that demonstrated by others. Intentional flood pulses are advocated as a restorative management strategy for improving plant productivity, management of exotic species (particularly T. ramosissima), and restoration of overall biodiversity.
The level of genetic diversity within populations of introduced species has received increasing attention as an important
factor influencing their survival and adaptive potential. We examined this issue with the Guatemalan potato tuber moth Tecia solanivora, an agricultural pest which has successfully invaded South America and the Canary Islands within the last 20years. To analyse
changes in T.solanivora genetic diversity, the mitochondrial marker cytochrome b was sequenced from individuals collected across its known distribution area. High haplotypic diversity was observed in Guatemala,
whereas only three haplotypes have been found in Venezuela and a single one in the remaining invaded South American countries
and the Canary Islands. Invasive haplotypes were not observed in our samples from Guatemala but are closely related to Guatemalan
haplotypes. These results are consistent with the hypotheses that (i) either a few individuals were introduced into Venezuela leading to a strong initial genetic bottleneck, or selection pressure
may have lead to the disparition of all but a reduced number of introduced haplotypes, (ii) a second bottleneck occured between Venezuela and Colombia, and (iii) the invasion of the Canary Islands originated from South America. We further reviewed the recent literature to compare this
change in genetic diversity with those reported for other invasive species. We quantified the changes in genetic diversity
between native and introduced ranges for 57 biological invasions. We found that the genetic homogenization in T.solanivora was among the strongest reported and discuss factors that can explain the success of invasive populations with low genetic
diversity.
Any organism must be equipped for life in a given environment, otherwise it will die. The fundamental question is how well does an organism need to be “equipped”,or what syndrome of traits must it possess to survive and flourish at a given locality.
In the current human-mediated biodiversity crisis, where alien species play an important role, we need to know whether some
species are inherently better equipped to become invasive when moved to new areas by humans. If so, we can identify such species
and consider management options to prevent, or at least reduce the damaging effects of biological invasions.
Despite the importance of chance and timing in the establishment and spread of alien plants (Crawley 1989), invasions are
clearly not entirely random events (Crawley et al. 1996). Much of the early work on invasions was directed at collating traits
associated with invasiveness (Booth et al. 2003). The question of whether is it possible to determine a set of traits that
predispose a species to be invasive has been a central theme since the emergence of invasion ecology as a discrete field of
study.
Tropical East Asia (TEA) has numerous islands, both continental and oceanic. This study uses information on invasive aliens
in terrestrial habitats on these islands to test the generality of the continental-oceanic contrast in invasibility, assess
the conservation impacts of invasive species, and suggest ways to mitigate these. The continental islands of Hong Kong and
Singapore are worst-case scenarios for continental invasibility and alien species often dominate in chronically disturbed
sites, but very few have successfully invaded closed forests, with the exception of birds in Hong Kong. On other, less densely
populated, continental islands, closed-canopy forests appear to resist invasions by all taxa, with few known exceptions. Forests
on oceanic islands isolated by <100km during the last glacial maximum appear no more susceptible to plant and invertebrate
invasions than those on continental islands, but invasions by mammals are widespread. Snake invasions may be under-recognized.
The remote oceanic Ogasawara (Bonin) Islands, >1000km from the nearest continent, have a native biota of largely tropical
East Asian origin and are suffering from alien forest invasions across the taxonomic spectrum. These patterns of invasibility
are consistent with the idea that alien invasion is facilitated by the absence of native species in the same functional group.
Alien invasives are not yet a major conservation problem in TEA, except on remote islands, but their dominance on disturbed
sites may slow or prevent recovery of native biodiversity. Strict quarantine is impractical in TEA, although some major introduction
routes could be blocked. Management efforts should focus on early recognition and immediate control of potential problem species.
While biological invasions have been the subject of considerable attention both historically and recently, the factors controlling the susceptibility of communities to plant invasions remain controversial. We surveyed 44 sites in southeastern New York State to examine the relationships between plant community characteristics, soil characteristics, and nonnative plant invasion. Soil nitrogen mineralization and nitrification rates were strongly related to the degree of site invasion (F= 30.2, P F= 11.8, P R 2= 0.22, P r= 0.70, P P
A long-term field experiment in limestone grassland near Buxton (North Derbyshire, United Kingdom) was designed to identify plant attributes and vegetation characteristics conducive to successful invasion. Plots containing crossed, continuous gradients of fertilizer addition and disturbance intensity were subjected to a single-seed inoculum comprising a wide range of plant functional types and 54 species not originally present at the site. Several disturbance treatments were applied; these included the creation of gaps of contrasting size and the mowing of the vegetation to different heights and at different times of the year. This paper analyzes the factors controlling the initial phase of the resulting invasions within the plots subject to gap creation. The susceptibility of the indigenous community to invasion was strongly related to the availability of bare ground created, but greatest success occurred where disturbance coincided with eutrophication. Disturbance damage to the indigenous dominants (particularly Festuca ovina) was an important determinant of seedling establishment by the sown invaders. Large seed size was identified as an important characteristic allowing certain species to establish relatively evenly across the productivity-disturbance matrix; smaller-seeded species were more dependent on disturbance for establishment. Successful and unsuccessful invaders were also distinguished to some extent by differences in germination requirements and present geographical distribution.
1 Dramatic reductions in dispersal potential are characteristic of many diverse taxa, both plants and animals, on oceanic islands. This paper documents the same trend of reduced dispersal ability over the course of just a few generations in some weedy, short-lived and wind-dispersed plants of inshore islands in British Columbia, Canada. 2 We measured dispersal-related morphological characteristics of diaspores from island populations of known ages, and from mainland populations. In two of three species with sufficiently large sample sizes, older island populations show increasingly reduced dispersal potential relative to mainland populations or to young island populations. 3 These and other morphological differences are consistent with results expected from strong selection for reduced dispersal potential, and may be striking examples of short-term evolution in small and isolated natural populations.
In their search for generalizations, ecologists have postulated many concepts and processes. Some of these have become reified. Reification consists of accepting a designation as if it has empirical meaning when, in fact, its existence has either never been tested or it has been found empty. The distinction between a hypothesis and a reification is that hypotheses are created to be tested and replaced, whereas a reification is taken as an untestable axiom. When a research area loses its dynamism, its hypotheses become reifications. Conversely, if reifications are permitted to accumulate, they can destroy the dynamism of a research area. If a science retains an excess number of reifications, it stagnates and ultimately loses its status as a science. Reified concepts include the logistic equation (together with the theoretical constructs based on it), the idea of constant ecological efficiency, the concept of an integrated community, and certain aspects of species diversity, particularly in the context 'good', 'bad' and alien species. Also, there are reified metaphors, which, if taken seriously, can be obfuscatory. For example, natural communities have been likened to aeroplanes, and each species to parts of an aeroplane. A metaphor is then constructed in which the removal of a species from a community is likened to the removal of an aeroplane part. Just as removal of one or more parts will cause the plane to crash, the metaphor asserts that the removal of one too many species from a community will result in collapse of the community. On closer examination, this is seen as empty. There is no 'aeroplane'. Also, the designation of certain kinds of species as good or bad – specifically, alien species are bad and 'native' species are good – is empty and misleading. While invasive species, in some cases, actually do damage native species, the generalization that invaders will reduce species diversity is not well founded. Fields that are required to focus on research defined by social needs, like ecology and medi-cine, rather than on scientific capabilities, like astronomy and hydrodynamics, generate reifica-tions. Reifications are dangerous to the health of a research area and should be avoided. Only vigorous extirpation of reifications permits a field to preserve scientific integrity.
Disturbance is an important component of many ecosystems, and variations in disturbance regime can affect ecosystem and community structure and functioning. The “intermediate disturbance hypothesis” suggests that species diversity should be highest at moderate levels of disturbance. However, disturbance is also known to increase the invasibility of communities. Disturbance therefore poses an important problem for conservation management, Here, we review the effects of disturbances such as fire grazing, soil disturbance and nutrient addition on plant species diversity and invasion with particular emphasis on grassland vegetation. Individual components of the disturbance regime can have marked effects on species diversity, but it is often modifications of the existing regime that have the largest influence. Similarly, disturbance can enhance invasion of natural communities, but frequently it is the interaction between different disturbances that has the largest effect. The natural disturbance regime is now unlikely to persist within conservation areas since fragmentation and human intervention have usually modified physical and biotic conditions. Active management decisions must now be made on what disturbance regime is required and this requires decisions on what species are to be encouraged or discouraged.
This paper considers key issues in plant invasion ecology, where findings published since 1990 have significantly improved our understanding of many aspects of invasions. The review focuses on vascular plants invading natural and semi-natural ecosystems, and on fundamental ecological issues relating to species invasiveness and community invasibility. Three big questions addressed by the SCOPE programme in the 1980s (which species invade; which habitats are invaded; and how can we manage invasions?) still underpin most work in invasion ecology. Some organizing and unifying themes in the field are organism-focused and relate to species invasiveness (the tens rule; the concept of residence time; taxonomic patterns and Darwin's naturalization hypothesis; issues of phenotypic plasticity and rapid evolutionary change, including evolution of increased competitive ability hypothesis; the role of long-distance dispersal). Others are ecosystem-centred and deal with determinants of the invasibility of communities, habitats and regions (levels of invasion, invasibility and propagule pressure; the biotic resistance hypothesis and the links between diversity and invasibility; synergisms, mutualisms, and invasional meltdown). Some theories have taken an overarching approach to plant invasions by integrating the concepts of species invasiveness and community invasibility (a theory of seed plant invasiveness; fluctuating resources theory of invasibility). Concepts, hypotheses and theories reviewed here can be linked to the naturalization-invasion continuum concept, which relates invasion processes with a sequence of environmental and biotic barriers that an introduced species must negotiate to become casual, naturalized and invasive. New research tools and improved research links between invasion ecology and succession ecology, community ecology, conservation biology and weed science, respectively, have strengthened the conceptual pillars of invasion ecology.
Changes in historical disturbance regimes have been shown to facilitate non-native plant invasions, but reinstatement of disturbance can be successful only if native colonizers are able to outcompete colonizing invasives. Reintroduction of flooding in the southwestern United States is being promoted as a means of reestablishing Populus deltoides subsp. wislizenii, but flooding can also promote establishment of an introduced, invasive species, Tamarix ramosissima. We investigated competition between Populus and Tamarix at the seedling stage to aid in characterizing the process by which Tamarix may invade and to determine the potential ability of Populus to establish itself with competitive pressure from Tamarix. We planted seedlings of Tamarix and Populus in five ratios at three densities for a total of 15 treatments. The growth response of each species was measured in terms of height, above-ground biomass, and tissue concentrations of nitrogen and phosphorous. These measurements across treatments were modeled as three-dimensional response surfaces. For both species, Populus density was more important than Tamarix density for determining growth response. Both species were negatively affected by increasing numbers of Populus seedlings. Due to the larger size of the native Populus, we predict that its superior competitive ability can lead to its dominance when conditions allow native establishment. Our results suggest that even in the presence of an invader that positively responds to disturbance, reestablishment of historical flooding regimes and post-flood hydrology can restore this ecosystem by promoting its dominant plant species.
The floristic and structural changes in the vegetation resulting from the long-term exclusion of large herbivores from experimental plots in the Murchison Falls National Park, Uganda, are described.Two experimental plots were surveyed, one in Sporobolus-Setaria grassland and the other in Combretum-Terminalia woodland. The survey shows that long-term removal of grazing and browsing pressure results in (1) marked tree regeneration in both grassland and woodland plots; (2) a relatively species-poor herbaceous layer in the grassland plot; and (3) the development of a more diverse all-aged stand in the woodland plot. These results are discussed in the context of the poaching of large herbivores during the 1978 civil war and the concomitant vegetation changes, especially Acacia sieberiana regeneration, that are now occurring throughout the Park. The implications for future management of the Park are discussed.
Much as Rachel Carson's "Silent Spring" was a call to action against the pesticides that were devastating bird populations, Charles S. Elton's classic "The Ecology of Invasions by Animals and Plants" sounded an early warning about an environmental catastrophe that has become all too familiar today-the invasion of nonnative species. From kudzu to zebra mussels to Asian long-horned beetles, nonnative species are colonizing new habitats around the world at an alarming rate thanks to accidental and intentional human intervention. One of the leading causes of extinctions of native animals and plants, invasive species also wreak severe economic havoc, causing $79 billion worth of damage in the United States alone. Elton explains the devastating effects that invasive species can have on local ecosystems in clear, concise language and with numerous examples. The first book on invasion biology, and still the most cited, Elton's masterpiece provides an accessible, engaging introduction to one of the most important environmental crises of our time. Charles S. Elton was one of the founders of ecology, who also established and led Oxford University's Bureau of Animal Population. His work has influenced generations of ecologists and zoologists, and his publications remain central to the literature in modern biology. "History has caught up with Charles Elton's foresight, and "The Ecology of Invasions" can now be seen as one of the central scientific books of our century."-David Quammen, from the Foreword to "Killer Algae: The True Tale of a Biological Invasion"
Biologists are nearly unanimous in their belief that humanity is in the process of extirpating a significant portion of the earth's spe cies. The ways in which we are doing so reflect the magnitude and scale of human enterprise. Everything from highway construction to cattle ranch ing to leaky bait buckets has been implicated in the demise or endan germent of particular species. Ac cording to Wilson (1992), most of these activities fall into four major categories, which he terms "the mind less horsemen of the environmental apocalypse": overexploitation, habi tat destruction, the introduction of non-native (alien) species, and the spread of diseases carried by alien species. To these categories may be added a fifth, pollution, although it can also be considered a form of habitat destruction. Surprisingly, there have been reIa tively few analyses of the extent to which each of these factors-much less the more specific deeds encomDavid S. Wilcove is a senior ecologist at the Environmental Defense Fund, Wash ington, DC 20009. David Rothstein re ceived his J.D. in 1997 from Northeastern
Although the impacts of exotic plant invasions on community structure and ecosystem processes are well appreciated, the pathways or mechanisms that underlie these impacts are poorly understood. Better exploration of these processes is essential to understanding why exotic plants impact only certain systems, and why only some invaders have large impacts. Here, we review over 150 studies to evaluate the mechanisms underlying the impacts of exotic plant invasions on plant and animal community structure, nutrient cycling, hydrology and fire regimes. We find that, while numerous studies have examined the impacts of invasions on plant diversity and composition, less than 5% test whether these effects arise through competition , allelopathy, alteration of ecosystem variables or other processes. Nonetheless, competition was often hypothesized, and nearly all studies competing native and alien plants against each other found strong competitive effects of exotic species. In contrast to studies of the impacts on plant community structure and higher trophic levels, research examining impacts on nitrogen cycling, hydrology and fire regimes is generally highly mechanistic, often motivated by specific invader traits. We encourage future studies that link impacts on community structure to ecosystem processes, and relate the controls over invasibility to the controls over impact.
It is suggested that evolution in plants may be associated with the emergence of three primary strategies, each of which may be identified by reference to a number of characteristics including morphological features, resource allocation, phenology, and response to stress. The competitive strategy prevails in productive, relatively undisturbed vegetation, the stress-tolerant strategy is associated with continuously unproductive conditions, and the ruderal strategy is characteristic of severely disturbed but potentially productive habitats. A triangular model based upon the three strategies may be reconciled with the theory of r- and K-selection, provides an insight into the processes of vegetation succession and dominance, and appears to be capable of extension to fungi and to animals.
■ Abstract In the search to identify factors that make some plant species trou- blesome invaders, many studies have compared various measures of native and alien invasive plant performance. These comparative studies provide insights into the more general question "Do alien invasive plants usually outperform co-occurring native species, and to what degree does the answer depend on growing conditions?" Based on 79 independent native-invasive plant comparisons, the alien invaders were not statisti- cally more likely to have higher growth rates, competitive ability, or fecundity. Rather, the relative performance of invaders and co-occurring natives often depended on grow- ing conditions. In 94% of 55 comparisons involving more than one growing condition, the native's performance was equal or superior to that of the invader, at least for some key performance measures in some growing conditions. Most commonly, these con- ditions involved reduced resources (nutrients, light, water) and/or specific disturbance regimes. Independently of growing conditions, invaders were more likely to have higher leaf area and lower tissue construction costs (advantageous under high light and nutrient conditions) and greater phenotypic plasticity (particularly advantageous in disturbed environments where conditions are in frequent flux). There appear to be few "super in- vaders" that have universal performance advantages over co-occurring natives; rather, increased resource availability and altered disturbance regimes associated with hu- man activities often differentially increase the performance of invaders over that of natives.
Traditional models of genetic drift predict a linear decrease in additive genetic variance for populations passing through a bottleneck. This perceived lack of heritable variance limits the scope of founder-effect models of speciation. We produced 55 replicate bottleneck populations maintained at two male-female pairs through four generations of inbreeding (average F = 0.39). These populations were formed from an F-2 intercross of the LG/J and SM/J inbred mouse strains. Two contemporaneous control strains maintained with more than 60 mating pairs pet generation were formed from this same source population. The average level of within-strain additive genetic variance for adult body weight was compared between the control and experimental lines. Additive genetic variance for adult body weight within experimental bottleneck strains was significantly higher than expected under an additive genetic model. This enhancement of additive genetic variance under inbreeding is likely to be due to epistasis, which retards or reverses the loss of additive genetic variance under inbreeding for adult body weight in this population. Therefore, founder-effect speciation processes may not be constrained by a loss of heritable variance due to population bottlenecks.
Exotic species affect the biogeochemical pools and fluxes of materials and energy, thereby altering the fundamental structure and function of their ecosystems. Rapidly accumulating evidence from many species of both animal and plant invaders suggests that invasive species often increase pool sizes, particularly of biomass, and promote accelerated flux rates, but many exceptions can be found. Ecosystem dynamics are altered through a variety of interacting, mutually reinforcing mechanistic pathways, including species' resource acquisition traits; population densities; ability to engineer changes to physical environmental conditions; effects on disturbance, especially fire; regimes; the ability to structure habitat for other species; and their impact on food webs. Local factors of landscape setting, history, and other sources of disturbance constrain ecosystem responses to invasions. New research directions are suggested, including the need for whole-system budgets, the quantification of abundance-impact rel...
Although most studies of factors contributing to successful establishment and spread of non-native species have focused on species traits and characteristics (both biotic and abiotic), increasing empirical and statistical evidence implicates propagule pressure—propagule sizes, propagule numbers, and temporal and spatial patterns of propagule arrival—as important in both facets of invasion. Increasing propagule size enhances establishment probability primarily by lessening effects of demographic stochasticity, whereas propagule number acts primarily by diminishing impacts of environmental stochasticity. A continuing rain of propagules, particularly from a variety of sources, may erase or vitiate the expected genetic bottleneck for invasions initiated by few individuals (as most are), thereby enhancing likelihood of survival. For a few species, recent molecular evidence suggests ongoing propagule pressure aids an invasion to spread by introducing genetic variation adaptive for new areas and habitats. This phenomenon may also explain some time lags between establishment of a non-native species and its spread to become an invasive pest.
With a simple model, I show that comparisons of invasibility between regions are impossible to make unless one can control for all of the variables besides invasibility that influence exotic richness, including the rates of immigration of species and the characteristics of the invading species themselves. Using data from the literature for 184 sites around the world, I found that nature reserves had one-half of the exotic fraction of sites outside reserves, and island sites had nearly three times the exotic fraction of mainland sites. However, the exotic fraction and the number of exotics were also dependent on site area, and this had to be taken into account to make valid comparisons between sites. The number of native species was used as a surrogate for site area and habitat diversity. Nearly 70% of the variation in the number of exotic species was accounted for by a multiple regression containing the following predictors: the number of native species, whether the site was an island or on the mainland, and whether or not it was a nature reserve. After controlling for scale, there were significant differences among biomes, but not continents, in their level of invasion. Multiple biome regions and temperate agricultural or urban sites were among the most invaded biomes, and deserts and savannas were among the least. However, there was considerable within-group variation in the mean degree of invasion. Scale-controlled analysis also showed that the New World is significantly more invaded than the Old World, but only when site native richness (probably a surrogate for habitat diversity) is factored out. Contrary to expectation, communities richer in native species had more, not fewer, exotics. For mainland sites, the degree of invasion increased with latitude, but there was no such relationship for islands. Although islands are more invaded than mainland sites, this is apparently not because of low native species richness, as the islands in this data set were no less rich in native species than were mainland sites of similar area. The number of exotic species in nature reserves increases with the number of visitors. However, it is difficult to draw conclusions about relative invasibility, invasion potential, or the roles of dispersal and disturbance from any of these results. Most of the observed patterns here and in the literature could potentially be explained by differences between regions in species properties, ecosystem properties, or propagule pressure.
The effect of fire on annual plants was examined in two vegetation types at remnant vegetation edges in the Western Australian wheatbelt. Density and cover of non-native species were consistently greatest at the reserve edges, decreasing rapidly with increasing distance from reserve edge. Numbers of native species showed little effect of distance from reserve edge. Fire had no apparent effect on abundance of non-natives in Allocasuarina shrubland but abundance of native plants increased. Density of both non-native and native plants in Acacia acuminata-Eucalyptus loxophleba woodland decreased after fire. Fewer non-native species were found in the shrubland than in the woodland in both unburnt and burnt areas, this difference being smallest between burnt areas. Levels of soil phosphorus and nitrate were higher in burnt areas of both communities and ammonium also increased in the shrubland. Levels of soil phosphorus and nitrate were higher at the reserve edge in the unburnt shrubland, but not in the woodland. There was a strong correlation between soil phosphorus levels and abundance of non-native species in the unburnt shrubland, but not after fire or in the woodland. Removal of non-native plants in the burnt shrubland had a strong positive effect on total abundance of native plants, apparently due to increases in growth of smaller, suppressed native plants in response to decreased competition. Two native species showed increased seed production in plots where non-native plants had been removed. There was a general indication that, in the short term, fire does not necessarily increase invasion of these communities by non-native species and could, therefore be a useful management tool in remnant vegetation, providing other disturbances are minimised.
1. Ecological count data typically exhibit complexities such as overdispersion and zero-inflation, and are often weakly associated with a relatively large number of correlated covariates. The use of an appropriate statistical model for inference is therefore essential. A common selection criteria for choosing between nested models is the likelihood ratio test (LRT). Widely used alternatives to the LRT are based on information-theoretic metrics such as the Akaike Information Criterion.
2. It is widely believed that the LRT can only be used to compare the performance of nested models – i.e. in situations where one model is a special case of another. There are many situations in which it is important to compare non-nested models, so, if true, this would be a substantial drawback of using LRTs for model comparison. In reality, however, it is actually possible to use the LRT for comparing both nested and non-nested models. This fact is well-established in the statistical literature, but not widely used in ecological studies.
3. The main obstacle to the use of the LRT with non-nested models has, until relatively recently, been the fact that it is difficult to explicitly write down a formula for the distribution of the LRT statistic under the null hypothesis that one of the models is true. With modern computing power it is possible to overcome this difficulty by using a simulation-based approach.
4. To demonstrate the practical application of the LRT to both nested and non-nested model comparisons, a case study involving data on questing tick (Ixodes ricinus) abundance is presented. These data contain complexities typical in ecological analyses, such as zero-inflation and overdispersion, for which comparison between models of differing structure – e.g. non-nested models – is of particular importance.
5. Choosing between competing statistical models is an essential part of any applied ecological analysis. The LRT is a standard statistical test for comparing nested models. By use of simulation the LRT can also be used in an analogous fashion to compare non-nested models, thereby providing a unified approach for model comparison within the null hypothesis testing paradigm. A simple practical guide is provided in how to apply this approach to the key models required in the analyses of count data.
To investigate factors affecting the ability of introduced species to invade natural communities in the Western Australian wheatbelt, five communities were examined within a nature reserve near Kellerberrin. Transect studies indicated that introduced annuals were more abundant in woodland than in shrub communities, despite an input of introduced seed into all communities. The response of native and introduced annuals to soil disturbance and fertilizer addition was examined. Small areas were disturbed and/or provided with fertilizer prior to addition of seed of introduced annuals. In most communities, the introduced species used (Avena fatua and Ursinia anthemoides) established well only where the soil had been disturbed, but their growth was increased greatly when fertilizer was also added. Establishment and growth of other introduced species also increased where nutrient addition and soil disturbance were combined. Growth of several native annuals increased greatly with fertilizer addition, but showed little response to disturbance. Fertilizer addition also significantly increased the number of native species present in most communities. This indicates that growth of both native and introduced species is limited by nutrient availability in these communities, but also that introduced species respond more to a combination of nutrient addition and soil disturbance.
1. Recent debates about the role of biotic resistance in controlling invasion success have focused on effects of species richness. However, functional composition could be a stronger control: species already in the community with similar functional traits to those of the invaders should have the greatest competitive effect on invaders. Still, experiments assessing effects of functional similarity have found contradictory results.
2. We used experimental communities in a serpentine grassland in California, USA, to assess the extent to which functional composition and functional diversity influenced success of two different types of invading plants: early season annuals (E) and late-season annuals (L) that have been previously shown to differ in patterns of resource acquisition.
3. We seeded known quantities of seed of six different species (three in each functional group) into experimental plots containing established communities differing in functional composition and functional diversity. The experimental communities contained different combinations of E, L, perennial bunchgrass (P) and nitrogen-fixer (N) functional groups, with functional diversity ranging from 0 to 4 groups. Each invading species was seeded into a separate quadrat within each plot to minimize competitive effects of invaders on each other. We measured both seedling and adult success of invaders for two full growing seasons to further understand mechanisms underlying biotic resistance.
4. More functionally diverse communities were less invaded overall, as measured by the average success of individual invaders. However, assessment of invaders by functional groups was more informative: Es in the extant community suppressed E invaders the most, and Ls in the extant community suppressed L invaders the most.
5. We observed a variety of interactions among extant functional groups in reducing invader success, including synergism, complementarity and ‘basement’ effects, where two or more groups negatively affected invaders, but combinations of groups were no more suppressive than single groups. The extant community influenced invaders more strongly through suppression of adult plant growth than through effects on seedling establishment.
6.Synthesis. Contrary to predictions from neutral theory, these results indicate that niche overlap was an important component of biotic resistance in these experimental plant communities and summed up to significant effects of species richness.
A widespread but controversial idea in ecology states that the number of invaders of a species assemblage depends on its species richness. Both negative and positive relationships have been reported. We examined whether a simple neutral model where assemblages are generated by drawing individuals from two pools of identical species (native and alien) can predict this relationship. We performed a meta-analysis of published data on this relationship. The neutral model showed that in communities with low and fixed numbers of indi-viduals, the relationship between the number of aliens and native species is strong and negative. This becomes weaker as the proportion of species from both pools already present in the community increases. The relationship between alien and native species richness becomes positive when the number of individuals is allowed to vary, because the richness depends on number of individuals and/or area sampled. The meta-analysis showed that scale-dependence of the relationship between alien and native species richness is universal and compatible with the neutral model. Unless more evidence is available to refute it, the relationship between native and alien species richness should be considered a result of a neutral processes due to constraints on the number of individuals in the community.
While small-scale studies show that more diverse native communities are less invasible by exotics, studies at large spatial scales often find positive correlations between native and exotic diversity. This large-scale pattern is thought to arise because landscapes with favorable conditions for native species also have favorable conditions for exotic species. From theory, we proposed an alternative hypothesis: the positive relationship at large scales is driven by spatial heterogeneity in species composition, which is driven by spatial heterogeneity in the environment. Landscapes with more spatial heterogeneity in the environment can sustain more native and more exotic species, leading to a positive correlation of native and exotic diversity at large scales. In a nested data set for grassland plants, we detected negative relationships between native and exotic diversity at small spatial scales and positive relationships at large spatial scales. Supporting our hypothesis, the positive relationships between native and exotic diversity at large scales were driven by positive relationships between native and exotic beta diversity. Further, both native and exotic diversity were positively correlated with spatial heterogeneity in abiotic conditions (variance of soil depth, soil nitrogen, and aspect) but were uncorrelated with average abiotic conditions, supporting the spatial-heterogeneity hypothesis but not the favorable-conditions hypothesis.
The current rate of invasive species introductions is unprecedented, and the dramatic impacts of exotic invasive plants on community and ecosystem properties have been well documented. Despite the pressing management implications, the mechanisms that control exotic plant invasion remain poorly understood. Several factors, such as disturbance, propagule pressure, species diversity, and herbivory, are widely believed to play a critical role in exotic plant invasions. However, few studies have examined the relative importance of these factors, and little is known about how propagule pressure interacts with various mechanisms of ecological resistance to determine invasion success.
We quantified the relative importance of canopy disturbance, propagule pressure, species diversity, and herbivory in determining exotic plant invasion in 10 eastern hemlock forests in Pennsylvania and New Jersey (USA). Use of a maximum‐likelihood estimation framework and information theoretics allowed us to quantify the strength of evidence for alternative models of the influence of these factors on changes in exotic plant abundance. In addition, we developed models to determine the importance of interactions between ecosystem properties and propagule pressure. These analyses were conducted for three abundant, aggressive exotic species that represent a range of life histories: Alliaria petiolata , Berberis thunbergii , and Microstegium vimineum .
Of the four hypothesized determinants of exotic plant invasion considered in this study, canopy disturbance and propagule pressure appear to be the most important predictors of A. petiolata , B. thunbergii , and M. vimineum invasion. Herbivory was also found to be important in contributing to the invasion of some species. In addition, we found compelling evidence of an important interaction between propagule pressure and canopy disturbance. This is the first study to demonstrate the dominant role of the interaction between canopy disturbance and propagule pressure in determining forest invasibility relative to other potential controlling factors. The importance of the disturbance–propagule supply interaction, and its nonlinear functional form, has profound implications for the management of exotic plant species populations. Improving our ability to predict exotic plant invasions will require enhanced understanding of the interaction between propagule pressure and ecological resistance mechanisms.
1. While previous studies have demonstrated rapid evolution in introduced plants and animals, most focus on single species. They are therefore unable to show whether these are special cases, or if rapid evolution is a common phenomenon in introduced species.
2. We used over 1900 herbarium specimens to determine whether morphological traits [plant height, leaf mass per area (LMA), leaf area or leaf shape] have shown significant change during the last ∼150 years in 23 plant species introduced to New South Wales, Australia.
3. Seventy per cent of our study species showed a change in at least one trait through time. The most common change was in plant height (eight out of 21 species). Six of these showed a decrease in height through time. Decreases in height mainly occurred in western New South Wales where dry, low nutrient conditions may favour shorter plants. We also found changes in leaf traits, including one decrease in LMA, five changes in leaf shape, and three changes in leaf area. The magnitude of these changes was surprisingly large, up to 125% in 100 years.
4. We used specimens of both native Australian species, and of the introduced species taken from their native range to investigate the possibility that the morphological changes were a response to temporal environmental change rather than to the species’ introduction to Australia. These control species showed significantly (P < 0.05) fewer changes than the introduced species. We included in our analyses information on the region in which each specimen was collected, to account for the possibility that changes through time might be caused by populations radiating in to new environments where a different phenotype might be expressed. Overall, rapid evolution seems the most likely explanation for the changes we observed.
5. Synthesis. The majority of our study species showed morphological change through time. While common garden experiments will be required to rule out phenotypic plasticity as an alternative explanation for these patterns, our results suggest that rapid evolution in introduced plant species could be much more common, and of a much greater magnitude than previously thought.
1. A prevalent question in the study of plant invasions has been whether or not invasions can be explained on the basis of traits. Despite many attempts, a synthetic view of multi-trait differences between alien and native species is not yet available.
2. We compiled a database of three ecologically important traits (specific leaf area, typical maximum canopy height, individual seed mass) for 4473 species sampled over 95 communities (3784 species measured in their native range, 689 species in their introduced range, 207 in both ranges).
3. Considering each trait separately, co-occurring native and alien species significantly differed in their traits. These differences, although modest, were expressed in a combined 15% higher specific leaf area, 16% lower canopy height and 26% smaller seeds.
4. Using three novel multi-trait metrics of functional diversity, aliens showed significantly smaller trait ranges, larger divergences and a consistent differentiation from the median trait combination of co-occurring natives.
5. We conclude that the simultaneous evaluation of multiple traits is an important novel direction in understanding invasion success. Our results support the phenotypic divergence hypothesis that predicts functional trait differences contribute to the success of alien species.
Aim This paper reviews the main geographical determinants of naturalization in plants.
Location Global.
Methods Comparative studies of large data sets of alien floras are the main source of information on global patterns of naturalization.
Results Temperate mainland regions are more invaded than tropical mainland regions but there seems to be no difference in invasibility of temperate and tropical islands. Islands are more invaded than the mainland. The number of naturalized species in temperate regions decreases with latitude and their geographical ranges increase with latitude. The number of naturalized species on islands increases with temperature. Naturalized species contribute to floristic homogenization, but the phenomenon is scale-dependent.
Main conclusions Some robust patterns are evident from currently available data, but further research is needed on several aspects to advance our understanding of the biogeography of naturalization of alien plants. For example, measures of propagule pressure are needed to determine the invasibility of communities/ecosystems/regions. The patterns discussed in this paper are derived largely from numbers and proportions of naturalized species, and little is known about the proportion of introduced species that become naturalized. Further insights on naturalization rates, i.e. the proportion of aliens that successfully naturalize within regions, and on geographical and other determinants of its variation would provide us with better understanding of the invasion process. Comparative studies, and resulting generalizations, are almost exclusively based on numbers of species, but alien species differ in their impact on native biodiversity and ecosystem processes.
While exotic plant species often come to dominate disturbed communities, long-term patterns of invasion are poorly known. Here we present data from 40 yr of continuous vegetation sampling, documenting the temporal distribution of exotic plant species in old field succession. The relative cover of exotic species decreased with time since abandonment, with significant declines occurring ≥20 yr post-abandonment. The number of exotic species per plot also declined with time since abandonment while field-scale richness of exotics did not change. This suggests displacement occurring at small spatial scales. Life history types changed from short-lived herbaceous species to long-lived woody species for both native and exotic plant species. However, shrubs and lianas dominated woody cover of exotic plants while trees dominated native woody cover. The species richness of exotic and native species was positively correlated at most times. In abandoned hay fields, however, the proportion of exotic plant cover per plot was inversely related to total species richness. This relationship suggests that it is not the presence, but the abundance of exotic species that may cause a reduction in community diversity. While the development of closed-canopy forest appears to limit most introduced plant species, several shade-adapted exotic species are increasing within the fields. These invasions may cause a reversal of the patterns seen in the first 40 yr of succession and may result in further impacts on community structure.
Study of interactions between pairs or larger groups of nonindigenous species has been subordinated in the literature to study of interactions between nonindigenous and native species. To the extent that interactions among introduced species are depicted at all, the emphasis has been on negative interactions, primarily resource competition and interference. However, a literature search reveals that introduced species frequently interact with one another and that facilitative interactions are at least as common as detrimental ones. The population significance of these interactions has rarely been determined, but a great variety of types of direct and indirect interactions among individuals of different nonindigenous species is observed, and many are plausibly believed to have consequences at the population level. In particular, mutualisms between plants and the animals that disperse and/or pollinate them and modification of habitat by both animals and plants seem common and often important in facilitating invasions. There is little evidence that interference among introduced species at levels currently observed significantly impedes further invasions, and synergistic interactions among invaders may well lead to accelerated impacts on native ecosystems – an invasional meltdown process.
Two hypotheses for the decline of native species are the superior exploitation of disturbance by exotic species and the competitive
displacement of native species by their exotic counterparts. Theory predicts that functional similarity will increase the
intensity of competition between native and invasive species. Ecologically important “foundation” species, Zostera marina and other seagrasses have globally declined during the past century. This study used transplant and vegetation removal experiments
to test the hypotheses that disturbance and competitive interactions with an invasive congener (Z. japonica) are contributing to the decline of native Z. marina in the northeastern Pacific. Interspecific competition reduced Z. marina and Z. japonica above-ground biomass by 44 and 96%, respectively, relative to intraspecific competition. Disturbance substantially enhanced
Z. japonica productivity and fitness, and concomitantly decreased Z. marina performance, effects that persisted two years following substratum disturbance. These results demonstrate that disturbance
and competitive interactions with Z. japonica reduce Z. marina performance, and suggest that Z. japonica’s success as an invasive species stems dually from its ability to persist in competition with Z. marina and its positive response to disturbance. These results highlight the importance of understanding the interconnected roles
of species interactions and disturbance in the decline of seagrass habitats, and provide a rationale for amending conservation
policy in Washington State. In the interest of conserving native eelgrass populations, the current policy of protecting both
native and invasive Zostera spp. should be refined to differentiate between native and invader, and to rescind the protection of invasive eelgrass.