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Invasive alien plants in South Africa: How well do we understand the ecological impacts?
Abstract
This paper examines the evidence for the effects of invasive alien plants in natural and semi-natural ecosystems in South Africa. Invasive alien plants are concentrated in the Western Cape, along the eastern seaboard, and into the eastern interior, but there is a shortage of accurate data on abundance within this range. Most information on site-specific impacts comes from the fynbos biome, and is generally poor for other biomes. The consequences of invasions for the delivery of ecosystem goods and services to people are, with the notable exception of their influence on water resources, inadequately studied. The understanding of many of the broader aspects of invasion ecology needs to be enhanced, and identify important challenges for research to address critical gaps in knowledge. Priorities for future research include the development of a predictive understanding of the rates of spread of invasive alien plants, and the development of achievable goals for ecosystem repair after clearing, including measurable criteria for assessing the success of restoration. Climate change could significantly exacerbate problems with invasive species and work is needed to accommodate plausible trajectories in planning and management frameworks. Perhaps the greatest challenge for South African ecologists is to address the twin issues of skills development and social transformation, to ensure that adequate and relevant ecological expertise is maintained to meet future research and management needs. Formal collaboration between organizations to address capacity building and educational transformation in the field of invasion ecology would represent a significant step forward.
... Both of these species are native to tropical central and southern America and are categorised as fully invasive in South Africa (Henderson, 2007;Gaertner et al. 2016). In the region these invasive species are well-known to have a substantial negative impact on native plant species through competition and replacement (Richardson and van Wilgen, 2004;Vardien et al., 2012;Urquía et al., 2019). Here, I explore the leaf-litter breakdown and nutrient release associated with these four species using an ex-situ approach (Chapter 3). ...
... Here, I aim to assess such nutrient release dynamics associated with leaf litter from key native and invasive plant species, using an ex situ mesocosms approach. and Psidium guajava are native to tropical central and southern America and are known to have a substantial negative impact on native plant species through competition and replacement (Richardson and van Wilgen, 2004;Vardien et al., 2012;Urquía et al., 2019). Both species are recognised as invasive in South Africa (Henderson, 2007;Gaertner et al. 2016). ...
... However, understanding differential colonisation responses of mosquitoes among plant species has remained elusive. (Richardson and van Wilgen, 2004;Vardien et al., 2012). Psidium guajava is also native to central and southern America and has substantial negative impacts on native species through competition and replacement (Urquía et al., 2019). ...
Leaf litter contributes to the functioning of aquatic ecosystems through allochthonous inputs of carbon, nitrogen and other elements. In many freshwater ecosystems, leaf litter inputs are among the most important cross-ecosystem nutrient contributions. However, native plant communities are under threat from invasive plant species, with largely unexplored consequences for recipient aquatic ecosystems. Broadly, ecological impacts of invasive alien species can be unpredictable and simultaneously span multiple habitat types and taxonomic groups. Invasive alien plants can have particularly severe ecological impacts, and plant inputs into aquatic environments can alter abiotic and biotic aquatic dynamics. Lakes and reservoir ecosystems are regarded as heterotrophic detritus-based habitats which are dependent upon allochthonous organic matter for the majority of energy inputs. Allochthonous detritus is extremely important for the trophic dynamics of the microbial organisms, macroinvertebrates and benthic plants in lakes and reservoirs. In the present study, leaf litter nutrient inputs, decomposition and colonisation associated with four plant species was examined using a combination of mesocosm and field experimental approaches. Native sycamore fig Ficus sycomorus L., and silver cluster–leaf Terminalia sericea Burch. ex DC. decomposition dynamics were compared to invasive tickberry Lantana camara L and guava Psidium guajava L., whereby phosphate, nitrate, nitrite, silicate and ammonium releases were quantified over time. Leaf inputs significantly reduced pH, with reductions most marked by invasive L. camara. Conductivity was heightened by all leaf input treatments, excepting native T. sericea. Leaf inputs significantly affected all nutrient levels monitored in the water over time, except for silicate. In particular, leaf litter from invasive L. camara drove significantly increased nutrient concentrations compared to other native plant species, whilst effects of invasive P. guajava were less statistically clear. The end weights of the leaf litter demonstrated decomposition differences among the species types, following a decreasing order of P. guajava > T. sericea > F. sycomorus > L. camara, further suggesting high organic inputs from invasive L. camara. Furthermore, ex-situ larval mosquito colonisation of with the above-mentioned native and invasive species leaves were assessed. Larval mosquito abundances differed significantly accordingly to leaf treatment, whilst no mosquitoes colonised leaf-free controls. Leaf litter from the invasive L. camara, invasive P. guajava and native F. sycomorus drove significant increases in mosquito abundances relative to native T. sericea. In situ macroinvertebrate colonisation, and quantify decomposition rates, of four species of native and invasive terrestrial plants was also assessed. Leaf treatments had a significant, group-specific effect on abundances and composition among focal macroinvertebrates. Invasive leaves reduced Physidae and Oligochaeta abundances, yet Ostracoda were significantly more abundant in the presence of invasive P. guajava. Chironomidae relative abundances increased under invasive L. camara treatments, whilst differences among leaf treatment effects on Coenogrionidae abundances were not statistically clear. In turn, macroinvertebrate diversity did not differ significantly among plant treatment groups, but the contributing taxa varied. The decomposition rate of the leaf litter demonstrated differences among the species types, following a decreasing order of L. camara > F. sycomorus > T. sericea > P. guajava. The study results highlight that differential leaf litter decomposition rates of …
... The strongest drying effects are documented for woody invasive species with higher leaf area index and larger root systems than members of the native flora. Such species include, for example, pines, acacias and eucalypts in South-Africa (Richardson and Wilgen 2004;Dzikiti et al. 2016), and salt cedars (Tamarix spp.) in riparian communities of the American Southwest (Gordon 1998), where Zavaleta (2000) estimated a water loss of 1.4-3.0 million m 3 annually. The high water consumption of these invasive species can lead to significant reductions in streamflows and water table levels, which are particularly problematic in water-limited regions, where the rearrangement of evapotranspiration patterns may threaten the survival of the native flora and fauna, and the water security of human populations. ...
Common milkweed (Asclepias syriaca L.) is a widespread invasive alien forb in dry sandy habitats of Central Europe. It adversely affects native plant and animal communities, but its ecosystem-level effects, particularly on hydrology, are little known. Since milkweed has an extensive, deep root system and large, broad leaves, we assumed a negative effect on the soil moisture content of the hosting ecosystem. Following the before-after control-impact protocol, we first compared the soil moisture content of the top 120 cm of the soil under seven milkweed stands to that of non-invaded reference sites. We then treated half of the stands by mechanically removing all aboveground milkweed biomass and repeated the comparative soil moisture measurements. We found that milkweed stands had significantly drier soils than reference grasslands during the growing season, but the soil under milkweed stands recharged to the level of the references in autumn and winter. However, the amount of moisture needed for this recharge was lost from deeper percolation to groundwater. Milkweed treatment prevented the depletion of moisture during the growing season, saving 21.6 l m⁻² of water on average. Treatment did not affect non-milkweed plant biomass, thus, moisture patterns could fully be attributed to the milkweed stands. Our results reinforce the importance of milkweed suppression in invaded grasslands, as, besides enabling the recovery of the native grassland ecosystem, it promotes groundwater recharge, which is particularly important in the dry regions of Central Europe, currently facing severe aridification due to climate change and unfavourable land use trends.
... Australian Acacia species have had a long history of invasion in South Africa (Shaughnessy, 1980;Poynton, 2009). Whilst many were introduced deliberately for their beneficial uses, large-scale invasion has caused detrimental impacts on native biodiversity, water resources and agriculture (De Wit et al., 2001;Richardson and van Wilgen, 2004;Gaertner et al., 2009Gaertner et al., , 2011Le Maitre et al., 2016). Included amongst the deliberately introduced species is Acacia cyclops A.Cunn. ...
... Invasive species tend to thrive in nutrient-rich soils, often escape natural enemies and are frequently quick to establish in unmanaged lands (Ehrenfeld 2003;Carreiro and Tripler 2005;Heneghan and others 2004;Johnson and Handel 2016). The presence of these species has implications for the ecosystem functions and services that rely on native biodiversity to support food webs (Narango and others 2018;Tallamy and others 2021), water resources (Richardson and van Wilgen 2004;van Wilgen and others 2008;Le Maitre and others 2000) and soil conservation (Scott and others 1998). ...
Relatively unmanaged interstitial areas at the residential–wildland interface can support the development of novel woody plant communities. Community assembly processes in urban areas involve interactions between spontaneous and cultivated species pools that include native, introduced (exotic/non-native) and invasive species. The potential of these communities to spread under changing climate conditions has implications for the future trajectories of forests within and beyond urban areas. We quantified woody vegetation (including trees and shrubs) in relatively unmanaged “interstitial” areas at the residential–wildland interface and in exurban reference natural areas in six metropolitan regions across the continental USA. In addition, we analyzed soil N and C cycling processes to ensure that there were no major anthropogenic differences between reference and interstitial sites such as compaction, profile disturbance or fertilization, and to explore effects of novel plant communities on soil processes. We observed marked differences in woody plant community composition between interstitial and reference sites in most metropolitan regions. These differences appeared to be driven by the expanded species pool in urban areas. There were no obvious anthropogenic effects on soils, enabling us to determine that compositional differences between interstitial and reference areas were associated with variation in soil N availability. Our observations of the formation of novel communities in interstitial spaces in six cities across a very broad range of climates, suggest that our results have relevance for how forests within and beyond urban areas are assessed and managed to provide ecosystem services and resilience that rely on native biodiversity.
... The impact of climate change on invasive alien plants is highly complex [10][11][12][13]. Various elements of global change, including global warming, elevated atmospheric CO 2 , N-deposition and habitat fragmentation, showed an interacting influence to exacerbate or retard alien plant invasion [14]. Indeed, all ecosystems are not equally vulnerable to global changes such as climate change. ...
Invasive alien plants are one of the main causes for the decline of native biodiversity worldwide. Hence, it is crucial to understand the dynamics of invasive plants in the context of a changing climate. The main aim of this study was to evaluate the potential distribution of two major invasive alien plants, Prosopis spp and Acacia mearnsii, under current and future climate change scenarios across South Africa. The maximum entropy (MaxEnt) model was used with species occurrence data and bioclimatic variables. The Species occurrence data was obtained from the Global Biodiversity Information Facility (GBIF), while the bioclimatic variables were downloaded from the WorldClim database. The model evaluation metrics for training and test samples were the area under curve (AUC) of 0.76 and 0.77 for Prosopis spp, and 0.91 and 0.89 for A. mearnsii, respectively. It showed that MaxEnt performed well in mapping the distribution of both species. Model results indicated that the near-current potential distribution of Prosopis spp and A. mearnsii in South Africa is significant (93.8% and 9.7% of the total land area, respectively). With the projected climate, Prosopis spp showed an inconsistent result across the General Circulation Models (GCMs), projection times and climate change scenarios. However, with respect to the current potential distribution, the geographical ranges of A. mearnsii will significantly contract (by about 75%) due to climate change. Therefore, it is imperative that policy makers, environmental managers and other stakeholders implement integrated management and control strategies to restrict the distribution of Prosopis spp.
... South African biocontrol researchers have prioritised post-release evaluation studies, with a significant proportion of funding dedicated to this purpose. In this study we review postrelease evaluations of weed biocontrol programmes in South Africa that have been published in the last ten years, discuss the different ways these evaluations have been conducted, and identify gaps for considerable resources being used for their control (Richardson and van Wilgen 2004;Blackburn et al. 2014). Biological control (biocontrol), the use of host specific natural enemies from the indigenous distribution of the IAP, is regarded as a safe and effective management option for many IAP species (McFadyen 1998). ...
Biocontrol is widely considered an effective management solution for mitigating the negative impacts of invasive alien plants (weeds). Globally, post-release evaluations to assess individual biocontrol programmes are generally lacking and there have been persistent calls within the biocontrol of weeds literature to increase the quantity and quality of post-release evaluation studies. South African biocontrol researchers have prioritised post-release evaluation studies, with a significant proportion of funding dedicated to this purpose. In this study we review post-release evaluations of weed biocontrol programmes in South Africa that have been published in the last ten years, discuss the different ways these evaluations have been conducted, and identify gaps for future research. Post-release evaluations have been conducted at different scales, including physiological changes within individual plants, plant growth parameters, plant population dynamics and landscape level changes. In most cases, the results of these studies indicated that biocontrol has reduced invasions according to these metrics. While the reduction in the invasion is assumed to alleviate negative ecological and socio-economic impacts, this is usually not directly measured. Evaluations of the socio-economic and ecosystem level benefits of biocontrol were limited to just a few examples on aquatic weeds. More studies that investigate the landscape, socio-economic and ecosystem level changes due to biocontrol are required, especially for terrestrial weeds, if the true scale of the benefits provided by biocontrol are to be understood.
... For example, invasive tree species such as pines (Pinus species), grown in plantations, and Australian wattles (Acacia species), planted mainly along the coast for dune stabilization, have spread widely into natural vegetation. Aquatic invasive species such as water hyacinth (Eichhornia crassipes) block waterways and affect water quality (Richardson & van Wilgen, 2004). The trade in ornamental plants and pets, and other enterprises that rely on non-native taxa, continue to introduce new species into the city; many of these remain undetected and/or unregulated. ...
Urbanization processes are associated with a range of human activities that enhance the spread and establishment of non-native species. As a consequence, non-native plant species play a significant role in urban regions today, and their importance will continue to increase as urbanization accelerates worldwide. The invasion of non-native plants in urban areas is associated with risks and opportunities, both of which must be considered. Some non-native plant species can affect native species and trigger ecosystem disservices. On the other hand, introduced species are often well adapted to novel urban conditions and support a variety of ecosystem services in cities. The contrasting example of Cape Town and Berlin shows that invasion patterns and associated impacts cannot be generalized. It is thus a crucial task of future research to explore urban invasions and their detrimental, beneficial or neutral consequences in different biogeographical contexts. This would inform management approaches and priority setting in urban environmental policies.
... Any net negative effects of IAPS are, however, generally more severe for poor households with high reliance on locally harvested ecosystem goods (Reynolds et al. 2020), as is the norm on the Wild Coast. Berliner (2011) argues that IAPS are significantly impacting the biodiversity and ecosystem services of the Wild Coast, although little is known on the extent of their invasion and ecological, economic and cultural impacts (Richardson and van Wilgen 2004). Complimenting the national scale mapping in Henderson (2020), based on quarter-degree squares (that includes the Wild Coast), are a few spatially localised studies that report on one or more IAPS at specific sites, such as the extensive invasion by Chromolaena odorata and Pereskia aculeata around Port St Johns (UNDP 2004;Berliner 2011), and Lantana camara at Mazeppa Bay (Jevon and Shackleton 2015), or the importance of Araujia sericifera as a medicinal plant near Nqabara (Kierungi and Fabricius 2005). ...
Invasive alien plant species (IAPS) frequently offer both ecosystem services and disservices to rural communities with high livelihood dependency on local landscapes. However, biocultural relationships with IAPS may go deeper than just provisioning uses, as they may be assimilated into local belief systems manifest in them becoming embedded in cultural constructions such as naming, stories, songs and ceremonies. It is likely that IAPS that are culturally assimilated will represent a greater conflict of interest in the face of proposed controls of IAPS in biodiversity hotspots where conservation priorities are frequently deemed paramount by external agencies. Using a mixed-methods approach we undertook roadside surveys of 17 selected IAPS along the 250 km Wild Coast section of the Maputaland-Pondoland-Albany biodiversity hotspot, accompanied by questionnaire interviews with 48 local people. The mean number of IAPS per site was four, ranging from zero (only one site) to ten, and local inhabitants deemed the abundance of all but one of the species to be increasing. All species had been in the region for decades, had a vernacular name, and all but one had direct consumptive uses. Species with multiple uses were more widely recognised. However, there was only marginal incorporation of the IAPS into stories, songs and ceremonies, although medicinal uses of some IAPS were for cultural/spiritual needs rather than physical ailments per se. These results show that despite widespread use, there was as yet relatively limited cultural assimilation of the IAPS in the Wild Coast region.
... For this matter, the models must incorporate both the complexity of the community and the complex relationships between invasive and native species (PARKER et al., 1999). The best known mathematical model is proposed by Parker et al (1999), which include three main dimensions that will determine the impact of an invasive species in a geographic scale: range, i.e., the total occupied area (R), abundance (A) and an impact measure per individual, represented by the per capita effect or biomass of the invasive species (E), resulting in the following linear equation: I = R x A x E (RICHARDSON e WILGEN, 2004;REASER et al.,2007;THIELE et al., 2010). One of the arguments of PARKER et al (1999) for the selection of these components is that any biomass, space or power taken by the invasive species is no longer an available resource for native competitors, representing potential impacts. ...
Individuals of alien species cause impacts on ecosystems by influencing the diversity, wealth and spread of native species. However, despite the recognized impacts and risks caused by such species, there is no consensus on common parameters to quantify their impacts. This study aims at discussing the definitions and methods used for measuring the invasive alien species impacts on ecological systems. It was observed that the formula proposed by Parke et al. (1999) is the most cited in scientific studies, nonetheless, it has certain mathematical and relevant data collection limitations, which hinders their use in research. Thus, abundance measures and phytosociological parameters can generate indices, which allow one to estimate the impact caused by alien plant species on native biodiversity. Phytosociology allows the characterization of plant communities through the analysis of quantitative attributes, as the Importance Value Index (IVI) allows the finding of the density, dispersion and size achieved by species. Thus, it is possible to assess the community structure in the presence of an invasive species and whether it promotes changes, occupying a high social contribution value and interfering with native species. Finally, this paper shows how impact assessment can contribute to the conservation of native biodiversity. Como definir e medir o impacto de espécies exóticas vegetais nos ecossistemas? Ao influenciar na diversidade, riqueza e distribuição de espécies nativas, as espécies exóticas causam impactos nos ecossistemas. Mesmo com as reconhecidas ameaças causadas por estas espécies, ainda não há consenso sobre parâmetros comuns para quantificar os seus impactos. Objetivou-se discutir as definições e os métodos usados para medições de impactos de espécies exóticas invasoras nos ecossistemas. A fórmula proposta por Parker et al. (1999) é a mais citada em estudos científicos. Porém, apresenta determinadas limitações matemáticas e na coleta de dados correspondentes, o que dificulta o seu uso nas pesquisas. Deste modo, medidas de abundância e parâmetros fitossociológicos podem originar índices que permitem estimar o impacto de uma espécie exótica vegetal. A fitossociologia permite através da análise de atributos quantitativos, a caracterização de comunidades vegetais e a partir do Índice de Valor de Importância (IVI) conhecer a densidade, dispersão e dimensão alcançada por uma espécie. Assim, é possível avaliar a estrutura da comunidade diante da presença de uma espécie invasora e se esta promove mudanças, ocupando um alto valor de contribuição social e interferindo nas espécies nativas. Por fim, apresenta-se como a avaliação de impactos pode contribuir para a conservação da biodiversidade nativa. Palavras-chaves: Invasão biológica; biodiversidade; medidas de impacto; abundância.
The Krka National Park is a sensible system exposed to different sorts of external
disorders. In the wider Skradinski buk area (SB) negative hydrological changes were noticed which lead to a reduction of tufa formation intensity. Major reasons were recognized in uncontrolled growth and the spreading of invasive vegetation, strengthened by anthropogenic impact and the lack of removal of deposited plant fragments in places of natural obstacles. With the aim of solving the problem, the administration board has initiated a project entitled Management and maintenance of macro-vegetation in Skradinski buk – making of a multicriteria model of sustainable management. The main goal of the project is the reactivation of
extinct tufa formation flows, the removal of invasive vegetation, and achieving sustainable conditions of tufa formation at the selected test surface. Achieving this goal has led to the implementation of numerous activities and research, including those given in this doctoral dissertation. The methodological framework of the research was divided into three levels. The macro level of the research encompasses a set of activities directed toward the selection of test surfaces within the wider SB area. Mezo level includes the installation of the system for tracking the condition of the tufa-forming environment and mapping the referent and final condition of the vegetation and hydrological network on the test surface after the removal of invasive vegetation. At the micro level of the research, the dynamics of tufa formation were studied by applying a new methodological approach based on the macro photogrammetry method. Implementation of the proposed multi-criteria model of sustainable management resulted in the achievement of the key goals of the project. At the selected test surface, the
succession of invasive macro vegetation was prevented, extinct tufa formation flows were reactivated and sustainable conditions of tufa formation were achieved within them. Results were confirmed by measured tufa growth rates, speed of water flow, physical-chemical parameters, the emergence of tufa-forming organisms, lack of invasive vegetation renewal and the dynamics of microfauna colonization. Measurements of the tufa formation dynamics resulted in an average growth rate of 5,771 mm a-1. Large variability in tufa growth rates was determined and the dynamics of tufa formation match the characteristics of most of the active tufa-forming systems. The new methodological approach (CMD) enables interval tracking of tufa-forming dynamics and generation of the model of submillimeter resolution and high-quality texture. CMD can be applied in all studies which require submillimeter data quality, comparison of interval 3D models and implementation of different morphometric parameters of a smaller object. Potential users of implemented multi-criteria model of sustainable management can be national, regional, and local authorities and boards of directors of protected and endangered tufa-forming areas.
The southern African range of the habeda ibis Bostrychia hagedash has expanded from 530 900 km 2 in 1910 to 1 323 300 km 2 in 1985. Major range expansions have occurred in the fynbos biome of the south-western Cape, the Karoo, the grasslands of the eastern Cape, the Orange Free State and the Transvaal highveld. Smaller expansions are documented for Lesotho, eastern Zimbabwe, central Mozambique and westwards along the Zambezi, Okavango, Limpopo and Orange rivers. The reasons for this expansion were investigated. Important factors include reduction in human persecution following the introduction of legislation in the period 1934 to 1941 and an increase of alien trees in formerly treeless areas. The increases in artificial impoundments and areas under irrigation are thought to have played a smaller role.Die Suid-Afrikaanse verspreidingsgebied van die hadeda Bostrychia hagedash het uitgebrei vanaf 530 900 km2 in 1910 tot 1 323 300 km2 in 1985. Groot gebiedsuitbreidings het in die fynbos-bioom van die suidwestelike Kaap, die Karoo, die langgrasveld van die oostelike Kaap, die Oranje-Vrystaat en die Transvaalse Hofiveld plaasgevind. Kleiner uitbreidings is in Lesotho, die oostelike gedeelte van Zimbabwe, sentraal Mosambiek en weswaarts langs die Zambezi-, Okavango-, Limpopo-en Oranjeriviere gedokumeteer. Die redes vir hierdie uitbreiding is ondersoek. Belangrike oorsake sluit in 'n vermindering in vervolging deur die mens na die instelling van wetgewing gedurende die tydperk 1934 tot 1941, die toename in uitheemse borne in gebiede wat voorheen boomloos was en, tot 'n mindere mate, die kunsmatige opdamming van water en die toename van besproeiingsgebiede.
African penguins Spheniscus demersus recolonized Robben Island near Cape Town in 1983, after an absence of c180 yr. In the 1980s, two new mainland colonies were also formed off South Africa's SW Cape Province. At the time of establishment, all three localities were characterized by the regular presence of other seabirds or marine mammals, and by availability of shaded areas suitable for nesting by penguins. Most African penguins at Robben Island select shaded areas for breeding. In the period of 13 yr preceding their recolonization, nearly 300 rehabilitated African penguins were released at Robben Island, but data suggest that Robben Island was recolonized mainly by birds from the SW Cape Province and not by rehabilitated birds. Numbers of African penguins breeding at Robben Island increased from nine pairs in 1983 to over 2000 pairs in 1992.
The National Programme for Ecosystem Research was established in 1972 to address a wide diversity of complex environmental problems which required a multiorganizational, interdisciplinary research approach. The central goal of the programme is to develop a predictive understanding of the structure, functioning and dynamics of South African terrestrial and inland water ecosystems. - from Author
Measurements of soil pH, resistance, cation exchange capacity and six macro- and five micro-elemental concentrations were made over a 12-month period in disturbed (burnt and Acacia saligna-infested) and undisturbed habitats in a sand plain lowland fynbos community. Two null hypotheses were tested, namely that there were no differences in soil chemical properties between (i) disturbed and undisturbed habitats and (ii) different seasons. A Friedman test was used to identify significant differences in soil chemical properties between different (i) habitats against seasons and (ii) seasons against habitats. With respect to acacia infestation, the first null hypothesis was rejected (P<0.01) for all measured soil chemical properties, apart from Na, available P, Fe and Cu. With respect to disturbance by fire, this null hypothesis was rejected (P<0.01) only for soil pH, Ca, available P and Mn. The second null hypothesis was rejected (P<0.05) for soil pH, K, Na, available P, Fe and Zn. The impact of acacia infestation on soil chemical status was considerably greater than that of fire or season. The approximate two-fold increase in soil elemental concentrations evident in acacia-infested fynbos may detrimentally affect the survival of indigenous species adapted to a nutrient-impoverished environment.
Some alien tree species used in commercial forestry, and agroforestry cause major problems as invaders of natural and seminatural ecosystems. The magnitude of the problem has increased significantly over the past few, decades, with a rapid increase in afforestation and changes in land use. Trends can be explained by analyzing natural experiments created by the widespread planting of a small number of species in different parts of the world. The species that cause the greatest problems are general those that have been planted most widely and for the longest time. The most affected areas have the longest histories of intensive planting. Pinus spp. are especially problematic, and at least 19 species are invasive over large areas in the southern hemisphere, where some species cause major problems. The most invasive Pinus species have a predictable set of life-history, attributes, including low, seed mass, short juvenile period, and short interval between large seed crops. Pine invasions have severely, impacted large areas of grassland and scrub-brushland in the southern hemisphere by causing shifts in life-form dominance, reduced structural diversity, increased biomass, disruption of prevailing vegetation dynamics, and changing nutrient cycling patterns. The (unavoidable) negative impacts of forestry with alien species are thus spilling over into areas set aside for conservation or water production. There is an urgent need to integrate the various means available for reducing the negative impacts of current invaders and to implement protocols to regulate the translocation of species that are known to be invasive.
Examined stands of the invasive Australian Acacia saligna and surrounding sand plain lowland fynbos vegetation after an autumn fire. Indigenous forms regenerated less successfully in acacia-infested than in natural habitats. Seedling:parent ratios of proteoids were 32%, restioids 42% and erocoids 65% of those in natural vegetation. Seedling mortalities of all plant forms were not different in the two habitats. The risk of local extinction after fire was three to four times greater for ericoids, five times greater for proteoids and seven times greater for restioids in acacia-infested than in natural habitats. Within a 10m2 acacia stand area, the probability of extinction of proteoids was high (99.7%) and moderate (56-57%) for ericoid and restioid forms. Soil N, Ca, Mg, K, Mn and B concentrations were higher and soil Fe concentrations lower in acacia-infested habitats, but the only significant concentration increases recorded for all plant forms were in the N and K of seedling leaves in acacia stands. Increased shoot:root ratios observed among indigenous forms in acacia-infested habitats reflected responses to shading by burnt acacia parental remnants rather than soil mineral enrichment. Acacia shoot:root ratios were unaffected by the different environmental conditions in acacia stands. The depletion of indigenous taxa, particularly obligate reseeding forms, beneath acacia stands results mainly from their poor seed regeneration success and associated increased risk of local extinction from stochastic causes after fire. -from Author
Models of plant migration based on estimates of biological parameters severely underestimate the rate of spread when compared to empirical estimates of plant migration rates. This is disturbing, since an ability to predict migration and colonization rates is needed for predicting how native species will distribute themselves in response to habitat loss and climate change and how rapidly invasive species will spread. Part of the problem is the difficulty of formally including rare long‐distance dispersal events in spread models. In this article, we explore the process of making predictions about plant migration rates. In particular, we examine the links between data, statistical models, and ecological predictions. We fit mixtures of Weibull distributions to several dispersal data sets and show that statistical and biological criteria for selecting the most appropriate statistical model conflict. Fitting a two‐component mixture model to the same data increases the spread‐rate prediction by an average factor of 4.5. Data limit our ability to fit more components. Using simulations, we show that a small proportion (0.001) of seeds moving long‐distances (1–10 km) can lead to an order of magnitude increase in predicted spread rate. The analysis also suggests that most existing data sets on dispersal will not resolve the problem; more effort needs to be devoted to collecting data on long‐distance dispersal. Although dispersal had the strongest effect on the predicted spread rate, we showed that dispersal interacts strongly with plant life history, disturbance, and habitat loss in influencing the predicted rate of spread. The importance of these interactions means that an approach that integrates local and long‐distance dispersal with plant life history, disturbance, and habitat availability is essential for predicting migration rates.
(1) South African fynbos vegetation is fire-prone and susceptible to invasion by alien shrubs. Alien shrubs change the nature of the fuel bed and thus affect fire behaviour. (2) Changes in biomass, size and distribution of plant parts as fuel and plant moisture and energy contents were determined at two sites invaded by the important alien shrubs Hakea sericea Schrad. and Acacia saligna (Labill.) Wendl. (3) The data were used to define fuel models and to simulate fire behaviour using Rothermel's fire model. This simulation was used to test the hypothesis that invasion increases fire hazard through increasing fuel loads. (4) Invasion by H. sericea resulted in a 60% increase in fuel load and lowered the moisture content of live foliage from 155 to 110%. Simulated rates of fire spread and intensity were nonetheless lower than in fynbos due to a densely-packed fuel bed. (5) Invasion by A. saligna resulted in a 50% increase in fuel load. The high moisture content of foliage of this shrub (about 270%) effectively reduces the fuel load and fuel bed depth, resulting in low rates of fire spread and intensity in the simulation. (6) Shortcomings in Rothermel's model prevented the accurate simulation of high intensity fires which have occurred in invaded areas under extreme weather conditions. Such fires vigorously consume the increased biomass of shrub crowns, are difficult to content of foliage of this shrub (about 270%) effectively reduces the fuel load and fuel bed Under such conditions, the fire hazard will be increased by invasion.
1. The impact of dense stands of the alien invasive species Acacia saligna (Labill) Wendl. on the guild structure of indigenous fynbos vegetation was investigated at three sites on the Cape Peninsula, South Africa. Two Acacia stands of either recent (1-2 fire cycles) or longer (>2 fire cycles) origin were compared with neighbouring uninvaded vegetation. 2. At one site fynbos recruitment was monitored following complete removal of vegetation in the three different stands in order to assess the restoration potential of invaded vegetation. 3. Fynbos species richness, cover and frequency all declined through each invasion stage in the standing vegetation. Guild structure also changed: species with vertebrate-dispersed seeds were relatively more frequent in long-invaded stands and at two of the three sites long-invaded stands had relatively more tall shrubs and fewer shrubs with leptophyllous (ericoid) leaves compared to fynbos. Serotinous shrubs were virtually absent in long-invaded stands. 4. Richness per plot of species recruited by seedlings was highest in fynbos and declined both with stage of invasion and time after clearing. However, the total stand species richness was nearly as high in recently invaded as in uninvaded fynbos. 5. Ephemeral forbs constituted the majority of seedlings in all stands, but after two years they remained the most important growth form in terms of density and cover only in the long-invaded stand. 6. At 18 months after clearing, indigenous canopy cover was about 50% in all stands, but only in uninvaded fynbos did this continue to increase over the second summer. After two years, resprouters formed about half the canopy cover in fynbos and recently invaded stands, but were insignificant in the long-invaded stand. 7. After clearing, many species recruiting in the long-invaded stand were not present in the standing vegetation, indicating that persistent seed banks exist. 8. As representatives of all the major fynbos growth forms were recruited into the long-invaded stand after clearing, albeit at a low density, such stands could revert to vegetation resembling fynbos in structure. However, in order to speed up the restoration process and to improve the probability of a fully functioning ecosystem being established, guilds which have been eliminated, such as serotinous Proteaceae, should be reintroduced.
(1) Production, nitrogen and phosphorus return, and decomposition of leaf litter of the invasive alien, Acacia saligna, was compared with that of the indigenous sclerophyllous shrub, Leucospermum parile, in sand-plain lowland fynbos with acid soils low in P during the early stages of alien invasion. The same was done for A. cyclops and Pterocelastrus tricuspidatus in strandveld with alkaline soils high in P. (2) The Acacia spp. had twice the leaf N concentrations of the indigenous species, whereas P concentrations were highest in the strandveld species. (3) The Acacia spp. tended to produce more litter, with three times the N content of that of the indigenous species. No significant differences in P return were found between the acacias and indigenous species in either vegetation. (4) Decomposition turnover times were longer in the fynbos species than those of the strandveld. Nitrogen was immobilized in the leaf litter of the indigenous species, while the N contents of the acacias varied little. Phosphorus was immobilized in the fynbos species compared with a release of about 50% from P. tricuspidatus after 2 years. (5) Soil N concentrations and litter-layer N contents were elevated under acacia canopies. (6) The N status of the fynbos and strandveld ecosystems is elevated by the invasion of alien acacias. The results for P cycling are equivocal and P availability does not appear to limit plant growth in the strandveld.