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Pathogen life-cycle leaves footprint on the spatial distribution of recruitment of their host plants
Abstract
Interactions between established and recruiting plants play an important role in species coexistence in natural plant communities. However, our knowledge on the particular ecological drivers of these interactions is still limited. We use spatial point pattern analysis to study the spatial patterns of recruitment and infection in two plant-pathogen systems, each involving a fungus with a different life cycle: the pair Quercus faginea-Trabutia quercina and the triad Crataegus monogyna-Gymnosporangium sp.-Juniperus oxycedrus. Our results show that T. quercina, an autoecious fungus, may act as a stabilizing mechanism in the population dynamics of Q. faginea. In turn, the effect of the heteroecious Gymnosporangium sp. on C. monogyna recruitment was more related to distance from the alternate host J. oxycedrus than to distance from conspecifics. These results demonstrate that the complexity of pathogen life cycle may impact recruitment and the development of interspecific plant-plant interactions in real plant communities.
... Spatial neighbourhood patterns, therefore, carry information about the operation of processes in the past, and they form the template for the processes operating in the future Page 3 of 14 (Law et al. 2009). In particular, ecological processes that occur in early life stages can leave long-lasting footprints in the spatial structure of adults (Perea et al. 2020(Perea et al. , 2021b. Then, comparisons of their spatial patterns, and the phylogenetic and phenotypic structure of their surrounding neighbourhoods, may improve our understanding of the processes and legacy effects involved in shaping plant communities (Moeur 1997, Plotkin et al. 2002, Getzin et al. 2008, Wang et al. 2016, Wiegand et al. 2017, Chun and Lee 2019. ...
... The observed phylogenetic or phenotypic repulsion of less abundant species at the sapling stage means that saplings tend to have more dissimilar sapling or adult neighbours. This pattern may be the result of strong competition (i.e. they outcompeted similar neighbour saplings; Valiente-Banuet and Verdú 2007), it could indicate heterospecific facilitation by more dissimilar adults, or be explained by Janzen-Connell effects (Perea et al. 2020). Several lines of evidence point to the role of (direct or indirect) heterospecific adult-sapling facilitation in the studied communities. ...
... generates spatial segregation between Juniperus spp. and Crataegus monogyna (Perea et al. 2020). In fact, increasing mortality of Juniperus spp. ...
The analysis of the spatial phylogenetic and phenotypic structure of plant communities can provide insight into the underlying processes and interactions governing their assembly, and how these may change during plant ontogeny. We used point pattern analysis to find out if saplings and adult plants are surrounded by phylogenetically and phenotypically more similar or more dissimilar neighbours than expected by chance, and whether these associations change from the sapling to the adult stage. To this end, we combined information on the phylogenetic structure and eight phenotypic traits of 15 woody plant species in two Mediterranean mixed forests of southeastern Spain. At the community level, we found that the sapling bank at both sites did not show phylogenetic or phenotypic spatial patterns, but adults showed phylogenetic clustering (i.e. heterospecific neighbours were more similar than expected). At the species level, we found frequently repulsive patterns in the sapling bank of less abundant species (i.e. heterospecific sapling or adult neighbours were more dissimilar than expected) in both, phylogenetic and phenotypic analyses. For the adult stage, we found phylogenetic attraction (i.e. more similar neighbours) in just one species and phenotypic clustering in four species. The processes driving the assembly of the communities of saplings and adults leave detectable signals in the spatial phylogenetic and phenotypic structure of our two forest communities. Our findings reinforce the existence of ontogenetic shifts in the mechanisms involved in plant community assembly. Facilitation between phylogenetically distant and phenotypically divergent species favours the recruitment of less abundant species. However, processes acting later in the ontogeny ameliorate the competition between close relatives and determine the spatial structure of adult plants. Nevertheless, the role of phenotype in shaping the interactions between adult plants was context‐ and trait‐dependent. The use of spatial point pattern analysis allowed a nuanced interpretation of the phylogenetic and phenotypic structures of plant communities.
... Intraspecific processes, such as resource competition or densitydependent enemies, are weaker when densities are lower and when plants are farther away from conspecific adults (Bagchi et al., 2010;Connell, 1971;Janzen, 1970;Le Roux et al., 2013;Perea et al., 2020). ...
... foraging activity and scape hypothesis; Howe, 1989;Russo et al., 2006;Verdú & García-Fayos, 1996). Conversely, seeds of dryfruited species are usually highly aggregated due to wind, gravity or scatter hoarding, which enhance the strength of density-dependent mechanisms and the spatial self-thinning, especially if they are deposited close to the mother plant (Beckman et al., 2012;Bell et al., 2006;Nathan & Muller-Landau, 2000;Perea et al., 2020). Indeed, we found that the neighbourhood density of saplings of fleshy-fruited species was on average 0.843 saplings/m 2 , less than half that of dry-fruited species (1.958 saplings/m 2 ), while thinning was on average five times stronger in dry-fruited than in fleshy-fruited species (Table 2). ...
Seed dispersal by frugivores plays a key role in structuring and maintaining tree diversity in forests. However, little is known about how the spatial legacy of seed dispersal and early recruitment shapes spatial patterns and the spatial interaction network of plant species in mature forest communities.
We analysed two fully mapped mixed Pine–Oak forest communities using spatial point pattern analysis to determine (a) the detailed structure of the intraspecific spatial patterns of saplings and adults, (b) the intra‐ and interspecific spatial interaction of saplings, adults and saplings relative to adults, (c) the spatial patterns of species richness at the community level and (d) whether seed dispersal mechanisms affect the plant–plant interaction networks and the ratio of adult to sapling neighbourhood densities used as surrogate for spatial self‐thinning.
The intraspecific spatial patterns of saplings and adults showed in general complex nested cluster structures that were similar for sapling and adult stages, despite substantial self‐thinning in some dry‐fruited species. The spatial network of saplings was characterized by positive spatial interactions. Adults of several tree species facilitated saplings in their proximity; however, adults of dry‐fruited species, but not those of fleshy‐fruited ones, lost almost all positive interactions that occurred at the sapling stage. Besides, interaction strength between adults was positive and often significantly stronger if both species were fleshy‐fruited. At the community level, the forests were structured into multispecies clumps across all life stages.
Synthesis. Our analyses highlight the importance of the spatial legacy of seed dispersal and early recruitment in the assembly of plant communities. Particularly, animal seed dispersal can lead to multispecies clusters and positive spatial associations across life stages in Mediterranean forests, with surprisingly little signatures of negative interactions. Our analysis suggests that changes of the spatial structure across plant life stages are driven by seed dispersal mechanisms and subsequent spatial self‐thinning, generating a spatial footprint at the sapling stage that conditions the long‐term interactions between adult plants. Combining spatial point pattern analysis with network analysis and species traits is a promising way to disentangle the processes underlying observed patterns of local diversity.
... Along this line, the community of fungal pathogens in our study system shows low levels of specialization (Pajares-Murgó et al., 2023). In fact, it is well known that many fungal pathogens are able, or obligated, to infect multiple hosts with differential impact depending on the plant identity (Gilbert & Webb, 2007;Hersh et al., 2012;Perea et al., 2020;Spear & Mordecai, 2018). For example, Spear and Broders (2021) show that generalist pathogens are the main drivers of seedling death and disease, with differences in pathogenic susceptibility among woody species of tropical forests. ...
Phyllosphere fungal communities participate in multiple ecological functions (litter decomposition, disease‐causing, plant defence). However, there is a lack of knowledge on whether and how these functions contribute to plant community dynamics under natural conditions.
One of the aspects of plant dynamics in which these fungi can most clearly affect is recruitment, since the success of newly germinated plants can be seriously compromised by pathogenic activity or the absence of mutualistic interactions.
To determine the relationship between phyllosphere fungal communities and plant recruitment, we combined published information on the frequency of plant–plant recruitment interactions and phyllosphere fungal communities in 38 woody species from two mixed forests in southern Spain.
Our results indicate that phyllosphere pathogens and saprotrophs have a negative effect on canopy–recruit interactions, while epiphytic fungi have a positive effect.
Additionally, the presence of canopy species hosting high richness of epiphytes or counting with a high diversity of saprotrophic fungi favours the formation of an abundant sapling bank.
Synthesis . Our results suggest that phyllosphere fungi play a relevant role in the assembly of the sapling bank in forest communities, thus, potentially influencing plant community dynamics. Beyond the well‐known negative effect of pathogenic fungi on recruitment, our results show the mutualistic effect of fungal epiphytes and a dual role of saprotrophs as antagonistic, decreasing recruitment of certain species, or mutualistic, enhancing recruitment in the sapling bank.
... For example, pathogens with long-distance spore dispersal may be more likely to colonize isolated host patches, and thus their distribution is less limited by spatial connectivity of host patches than pathogens with more limited dispersal abilities (Thrall & Burdon, 1999). For pathogens utilizing multiple host species, distribution across host patches is expected to depend on the spatial distribution of all hosts (Jokela & Lively, 1995;Perea et al., 2020). Besides dispersal across patches, the transmission mode of the pathogen also may affect spread of disease within a patch. ...
Plant pathogen traits, such as transmission mode and overwintering strategy, may have important effects on dispersal and persistence, and drive disease dynamics. Still, we lack insights into how life‐history traits influence spatiotemporal disease dynamics.
We adopted a multifaceted approach, combining experimental assays, theory and field surveys, to investigate whether information about two pathogen life‐history traits – infectivity and overwintering strategy – can predict pathogen metapopulation dynamics in natural systems. For this, we focused on four fungal pathogens (two rust fungi, one chytrid fungus and one smut fungus) on the forest herb Anemone nemorosa.
Pathogens infecting new plants mostly via spores (the chytrid and smut fungi) had higher patch occupancies and colonization rates than pathogens causing mainly systemic infections and overwintering in the rhizomes (the two rust fungi). Although the rust fungi more often occupied well‐connected plant patches, the chytrid and smut fungi were equally or more common in isolated patches. Host patch size was positively related to patch occupancy and colonization rates for all pathogens.
Predicting disease dynamics is crucial for understanding the ecological and evolutionary dynamics of host–pathogen interactions, and to prevent disease outbreaks. Our study shows that combining experiments, theory and field observations is a useful way to predict disease dynamics.
The phyllosphere is a wide and complex ecosystem that provides a key support for microbial diversity. Fungal communities inhabiting the leaf are functionally variable and play important roles on plant performance. Factors conditioning the arrival and colonization of fungal communities will determine the phyllosphere fungal composition. Plant identity, leaf functional traits and host plant phylogeny have been shown to be regulators of the microbial colonization of the leaves, and can be considered as biotic filters determining the assembly of phyllosphere fungal communities. By high‐throughput sequencing we analysed the phyllosphere fungal communities from 38 Mediterranean woody plant species in two forests of south‐eastern Iberian Peninsula. We analysed the effect of plant species and site on fungal community composition. We also tested the effect of leaf functional traits and plant phylogeny on plant species differences in their fungal communities, and on the structure of the plant–fungus interaction network. Plant species account for a larger proportion than site in the variability of the composition of phyllosphere fungal communities. Leaf traits and host phylogeny influence the arrival and colonization of phyllosphere fungal communities across plant species. Plants with pubescent leaves and phylogenetically closer harbour more similar communities of decomposers, pathogens and epiphytes. Leaf habit (i.e. evergreen versus deciduous) also influences the community composition of decomposer and epiphytic fungi. Leaf carbon, leaf water content and leaf mass per area affect differentially each functional guild. Plant–fungus interaction networks present a modular structure in which plants belonging to the same module share more fungal species and are phylogenetically closer. We provide evidence that even though phyllosphere fungal communities are complex ecosystems, fungi with contrasting relationships with the plant (decomposers, epiphytes and pathogens) respond similarly to a common subset of leaf traits that impose physical limitations to the assembly of phyllosphere fungal communities.
Questions
What are the primary types of intraspecific distribution patterns and interspecific associations of tree species in the temperate–subtropical transition region? Can potential ecological mechanisms such as habitat heterogeneity, dispersal limitation, density dependence, spatial segregation, and competition between species regulate these patterns?
Location
The Qinling Mountains, north‐central China.
Methods
Ripley's K ‐function, D ‐function and pair correlation function were applied to assess the spatial distribution of 15 dominant tree species in a fully mapped 25‐ha plot. Complete spatial randomness ( CSR ), heterogeneous Poisson ( HP ) and random‐labelling ( RL ) null models were used to reveal the potential process of community construction.
Results
(a) Thirteen of 15 adult tree species displayed an aggregation distribution at various scales. (b) Eleven adult species (73.3%) showed significant aggregation, especially at small distances, after removing the effect of habitat. (c) The percentage of species showing additional aggregation relative to adults decreased from saplings to juveniles. The maximum strength of density dependence was negative with species abundance. (d) Only 29 (13.8%) of the species pairs showed significant small‐scale interactions. Most of the significant small‐scale associations did not occur in abundant species pairs. (e) Negative interactions of interspecific associations were more prevalent than positive ones, yet there was no consistency of the characteristics among species pairs.
Conclusions
Habitat heterogeneity and dispersal limitation likely contribute to the spatial pattern of tree species at different life stages. Density dependence remains the important factor in maintaining species diversity in this forest stand. Moreover, spatial segregation generates the interspecific spatial patterns of segregation and partial overlap. Unexpectedly, the species herd protection hypothesis and the low‐frequency hypothesis cannot well explain the mechanism of interspecific interaction at small distances. Resource competition might be responsible for negative associations being more frequent among species pairs.
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.
Seedling emergence in plant communities depends on the composition in the soil seed bank, in combination with species-specific responses to the environment. It is generally assumed that this juvenile transition, known as the recruitment niche, is a crucial filter that determines species’ distributions and plant community assemblies. The relative importance of this filter, however, has been widely debated. Empirical descriptions of the recruitment niche are scarce, as most field studies focus on environmental effects at later life stages. In this study, we examine the importance of the recruitment niche for predicting plant communities across a hydrological gradient in a disturbed and undisturbed area in Lake Schmiechen, Baden-Württemberg, Germany. We combine a seed bank experiment, measuring germination in water basins under standardized conditions and different water levels, with field observations of plant communities along a hydrological gradient in plowed and undisturbed transects in a former agricultural wetland. We find that hydrology consistently predicted plant community composition in both the germination experiment and in the field. The hydrological recruitment niches measured in the seed bank experiment correlated with the hydrological niche in both the plowed and undisturbed area, with slightly stronger correlation in the plowed area. We explain the latter by the fact that the seed bank experiment most closely resembles the plowed area, whereas succession and competitive interactions become more important in the undisturbed area. Our results support the view that the recruitment niche is an important driver of species composition, in both the plowed and undisturbed area. Recognizing the recruitment niche and the response of seeds within a seed bank to environmental gradients and anthropogenic disturbance is necessary to understand and predict future plant community composition.
Forest structure and diversity can regulate tree vulnerability to damage by insects and pathogens. Past work suggests that trees with diverse neighbours should experience less leaf herbivory and less damage from specialist herbivores and diseases, and that the effect of neighbourhood diversity should be strongest at small spatial scales.
In an early stage temperate tree diversity experiment, we monitored damage from leaf removing herbivores, specialist (gallers and leaf miners) herbivores, and two specialist fungal diseases (maple leaf anthracnose and cedar apple gall rust) over 3 years. The experimental design included treatments that varied independently in phylogenetic and functional diversity and we made our analyses across four spatial scales (1–16 m²).
Neighborhood diversity simultaneously increased leaf removal for some species, decreased it for others, and had no effect on yet others. Height apparency—the difference between a focal plant’s height and its neighbours’—was the best single predictor of leaf removal across species and spatial scales, but the strength and direction of its effect were also species‐specific.
Specialist pathogens and fungal foliar diseases showed signs of associational resistance and susceptibility. Oaks (Quercus spp.) were more resistant to leaf miners and maples were more resistant to anthracnose when surrounded by diverse neighbours (associational resistance). In contrast, birches (Betula papyrifera) were more susceptible to leaf miners and eastern red cedars (Juniperus virginiana) were more susceptible to cedar apple gall rust (Gymnosporangium juniperi‐virginianae) infection in diverse environments (associational susceptibility).
Herbivore and pathogen damage was better predicted by community structure and diversity at small spatial scales (1 and 4 m²) than large scales (9 and 16 m²), suggesting a characteristic spatial scale for these biodiversity‐ecosystem functioning effects.
Synthesis. Humans control forest diversity through selective harvesting and planting in natural stands and plantations. Our experimental demonstration of the role of local community structure and diversity in suppressing some forms of pest and pathogen damage to trees suggests that forest management can be most effective when diversity is considered at small spatial scales and the underlying biology of particular pests, pathogens, and hosts is taken into account. Pictured here: the “galls” formed by cedar apple gall rust (Gymnosporangium juniperae‐virginiae) on eastern red cedar (Juniperus virginiana) in early spring release wind‐dispersed teleospores. Junipers showed associational susceptibility: greater susceptibility to gall rust with more diverse neighbours.
The fungus Gymnosporangium sabinae (Dicks.) Winter was found only at three locations during the years 1965 to 1990 in Slovakia, they were the Arboretum at Mlyňany, Topoľčianky and Gbely (JUHÁSOVÁ 1973, 1975, 1998). We noticed a more frequent occurrence of this fungus during recent years. The life cycle, range of woody host plants and degree of damage on pear leaves at selected locations inSlovakia are described.
Negative distance dependence (NDisD), or reduced recruitment near adult conspecifics, is thought to explain the astounding diversity of tropical forests. While many studies show greater mortality at near vs. far distances from adults, these studies do not seek to track changes in the peak seedling curve over time, thus limiting our ability to link NDisD to coexistence. Using census data collected over 12 years from central Panama in conjunction with spatial mark-connection functions, we show evidence for NDisD for many species, and find that the peak seedling curve shifts away from conspecific adults over time. We find wide variation in the strength of NDisD, which was correlated with seed size and canopy position, but other life-history traits showed no relationship with variation in NDisD mortality. Our results document shifts in peak seedling densities over time, thus providing evidence for the hypothesized spacing mechanism necessary for diversity maintenance in tropical forests.
A higher competitive advantage of polyploid plants compared with their parental diploids is frequently invoked to explain their establishment success, colonization of novel environments and cytotypic ecological segregation, yet there is scarce experimental evidence supporting such hypotheses. Here, we investigated whether differential competitive ability of species of the Brachypodium distachyon (Poaceae) species complex, a model system for genomic, ecological and evolutionary studies of temperate grasses, contributes to explaining their ecological segregation as well as their coexistence in diploid/allotetraploid contact zones.
We conducted two field experiments in dry and humid localities to evaluate the tolerance to competition of diploids and allotetraploids in densely occupied environments, and to parameterize models of intra‐ and intercytotype competition as a mechanism for species exclusion/coexistence.
We provide experimental evidence supporting the hypothesis that, under natural field conditions, allotetraploids have superior ecological success compared with one of their parental diploids in terms of both colonizing competitive habitats and intercytotypic competition, with the balance of intra/intercytotype competition favoring polyploid population establishment.
These findings, together with previous data on ecogeographic segregation and adaptive response to water stress, suggest that the interplay between aridity and competitive outcome determines the ability to colonize competitive environments, the exclusion of diploids, especially in arid localities, and species geographic segregation.
Gymnosporangium species (Pucciniaceae, Pucciniales) cause serious diseases and significant economic losses to apple cultivars. Most of the reported species are heteroecious and complete their life cycles on two different plant hosts belonging to two unrelated genera, i.e. Juniperus and Malus. However, the phylogenetic relationships among Gymnosporangium species and the evolutionary history of Gymnosporangium on its aecial and telial hosts were still undetermined. In this study, we recognized species based on rDNA sequence data by using coalescent method of generalized mixed Yule-coalescent (GMYC) and Poisson Tree Processes (PTP) models. The evolutionary relationships of Gymnosporangium species and their hosts were investigated by comparing the cophylogenetic analyses of Gymnosporangium species with Malus species and Juniperus species, respectively. The concordant results of GMYC and PTP analyses recognized 14 species including 12 known species and two undescribed species. In addition, host alternations of 10 Gymnosporangium species were uncovered by linking the derived sequences between their aecial and telial stages. This study revealed the evolutionary process of Gymnosporangium species, and clarified that the aecial hosts played more important roles than telial hosts in the speciation of Gymnosporangium species. Host switch, losses, duplication and failure to divergence all contributed to the speciation of Gymnosporangium species.
The diversity–productivity relationship (humped‐back model ( HBM )) and the stress‐gradient ( SGH ) hypotheses may be connected when productivity is limited primarily by aridity. We analytically connect both hypotheses and assess the contribution of facilitation to woody plant richness along the aridity gradient of the Western Mediterranean floristic region.
We monitored regeneration niches of woody plants, obtaining rarefied species richness and plant relative interaction indices in 54 forests and scrublands in a 1750‐km geographical range across Spain, Morocco and the Canary Islands.
We verified the monotonic increase in facilitation with aridity postulated by SGH and the humped‐shape pattern of species richness expected from HBM , which became manifest after expanding the aridity gradient or crossing vegetation types. Along the gradient, interaction balance turned into facilitation earlier in forest than in scrublands. The effects of aridity and interaction balance on species diversity were additive rather than interdependent.
Facilitation is an important driver of woody species richness at macroecological scales because it added up to diversity in most sites, with enhanced contribution with increased stress. The HBM was not shaped by species interactions. Results suggest that facilitation may act in Mediterranean vegetation buffering against critical transitions between states allowing woody plant communities to cope with the rise in aridity expected with global warming.
Over the last two decades spatial point pattern analysis (SPPA) has become increasingly popular in ecological research. To direct future work in this area we review studies using SPPA techniques in ecology and related disciplines. We first summarize the key elements of SPPA in ecology (i.e., data types, summary statistics and their estimation, null models, comparison of data and models, and consideration of heterogeneity); second, we review how ecologists have used these key elements; and finally, we identify practical difficulties that are still commonly encountered and point to new methods that allow current key questions in ecology to be effectively addressed.
Our review of 308 articles published over the period 1992-2012 reveals that a standard canon of SPPA techniques in ecology has been largely identified and that most of the earlier technical issues that occupied ecologists, such as edge correction, have been solved. However, the majority of studies underused the methodological potential offered by modern SPPA. More advanced techniques of SPPA offer the potential to address a variety of highly relevant ecological questions. For example, inhomogeneous summary statistics can quantify the impact of heterogeneous environments, mark correlation function can include trait and phylogenetic information in the analysis of multivariate spatial patterns, and more refined point process models can be used to realistically characterize the structure of a wide range of patterns. Additionally, recent advances in fitting spatially-explicit simulation models of community dynamics to point pattern summary statistics hold the promise for solving the longstanding problem of linking pattern to process. All these newer developments allow ecologists to keep up with the increasing availability of spatial data sets provided by newer technologies, which allow point patterns and environmental variables to be mapped over large spatial extents at increasingly higher image resolutions.
The coexistence of numerous tree species in tropical forests is commonly explained by negative dependence of recruitment on the conspecific seed and tree density due to specialist natural enemies that attack seeds and seedlings ('Janzen-Connell' effects). Less known is whether guilds of shared seed predators can induce a negative dependence of recruitment on the density of different species of the same plant functional group. We studied 54 plots in tropical forest on Barro Colorado Island, Panama, with contrasting mature tree densities of three coexisting large seeded tree species with shared seed predators. Levels of seed predation were far better explained by incorporating seed densities of all three focal species than by conspecific seed density alone. Both positive and negative density dependencies were observed for different species combinations. Thus, indirect interactions via shared seed predators can either promote or reduce the coexistence of different plant functional groups in tropical forest.
© 2015 John Wiley & Sons Ltd/CNRS.
The Janzen–Connell hypothesis proposes that specialist natural enemies, such as herbivores and pathogens, maintain diversity in plant communities by reducing survival rates of conspecific seeds and seedlings located close to reproductive adults or in areas of high conspecific density. Variation in the strength of distance- and density-dependent effects is hypothesized to explain variation in plant species richness along climatic gradients, with effects predicted to be stronger in the tropics than the temperate zone and in wetter habitats compared to drier habitats.
We conducted a comprehensive literature search to identify peer-reviewed experimental studies published in the 40+ years since the hypothesis was first proposed. Using data from these studies, we conducted a meta-analysis to assess the current weight of evidence for the distance and density predictions of the Janzen–Connell hypothesis.
Overall, we found significant support for both the distance- and density-dependent predictions. For all studies combined, survival rates were significantly reduced near conspecifics compared to far from conspecifics, and in areas with high densities of conspecifics compared to areas with low conspecific densities. There was no indication that these results were due to publication bias.
The strength of distance and density effects varied widely among studies. Contrary to expectations, this variation was unrelated to latitude, and there was no significant effect of study region. However, we did find a trend for stronger distance and density dependence in wetter sites compared to sites with lower annual precipitation. In addition, effects were significantly stronger at the seedling stage compared to the seed stage.
Synthesis. Our study provides support for the idea that distance- and density-dependent mortality occurs in plant communities world-wide. Available evidence suggests that natural enemies are frequently the cause of such patterns, consistent with the Janzen–Connell hypothesis, but additional studies are needed to rule out other mechanisms (e.g. intraspecific competition). With the widespread existence of density and distance dependence clearly established, future research should focus on assessing the degree to which these effects permit species coexistence and contribute to the maintenance of diversity in plant communities.
Numerous studies have demonstrated that tree survival is influenced by negative density dependence (NDD) and differences among species in shade tolerance could enhance coexistence via resource partitioning, but it is still unclear how NDD affects tree species with different shade-tolerance guilds at later life stages. In this study, we analyzed the spatial patterns for trees with dbh (diameter at breast height) ≥2 cm using the pair-correlation g(r) function to test for NDD in a temperate forest in South Korea after removing the effects of habitat heterogeneity. The analyses were implemented for the most abundant shade-tolerant (Chamaecyparis obtusa) and shade-intolerant (Quercus serrata) species. We found NDD existed for both species at later life stages. We also found Quercus serrata experienced greater NDD compared with Chamaecyparis obtusa. This study indicates that NDD regulates the two abundant tree species at later life stages and it is important to consider variation in species' shade tolerance in NDD study.
Recent hypotheses argue that phylogenetic relatedness should predict both the niche differences that stabilise coexistence and the average fitness differences that drive competitive dominance. These still largely untested predictions complicate Darwin's hypothesis that more closely related species less easily coexist, and challenge the use of community phylogenetic patterns to infer competition. We field parameterised models of competitor dynamics with pairs of 18 California annual plant species, and then related species' niche and fitness differences to their phylogenetic distance. Stabilising niche differences were unrelated to phylogenetic distance, while species' average fitness showed phylogenetic structure. This meant that more distant relatives had greater competitive asymmetry, which should favour the coexistence of close relatives. Nonetheless, coexistence proved unrelated to phylogeny, due in part to increasing variance in fitness differences with phylogenetic distance, a previously overlooked property of such relationships. Together, these findings question the expectation that distant relatives should more readily coexist.
Tropical forests are important reservoirs of biodiversity, but the processes that maintain this diversity remain poorly understood. The Janzen-Connell hypothesis suggests that specialized natural enemies such as insect herbivores and fungal pathogens maintain high diversity by elevating mortality when plant species occur at high density (negative density dependence; NDD). NDD has been detected widely in tropical forests, but the prediction that NDD caused by insects and pathogens has a community-wide role in maintaining tropical plant diversity remains untested. We show experimentally that changes in plant diversity and species composition are caused by fungal pathogens and insect herbivores. Effective plant species richness increased across the seed-to-seedling transition, corresponding to large changes in species composition. Treating seeds and young seedlings with fungicides significantly reduced the diversity of the seedling assemblage, consistent with the Janzen-Connell hypothesis. Although suppressing insect herbivores using insecticides did not alter species diversity, it greatly increased seedling recruitment and caused a marked shift in seedling species composition. Overall, seedling recruitment was significantly reduced at high conspecific seed densities and this NDD was greatest for the species that were most abundant as seeds. Suppressing fungi reduced the negative effects of density on recruitment, confirming that the diversity-enhancing effect of fungi is mediated by NDD. Our study provides an overall test of the Janzen-Connell hypothesis and demonstrates the crucial role that insects and pathogens have both in structuring tropical plant communities and in maintaining their remarkable diversity.
Recruitment limitation of tree population dynamics is poorly understood, because fecundity and dispersal are difficult to characterize in closed stands. We present an approach that estimates seed production and dispersal under closed canopies and four limitations on recruitment: tree density and location, fecundity, seed dispersal, and estab- lishment. Consistent estimates are obtained for 14 canopy species using 5 yr of census data from 100 seed traps and several thousand mapped,trees and seedlings from five southern Appalachian forest stands that span gradients in elevation and moisture. Fecundity (seed production per square centimeter of basal area) ranged over four orders of magnitude, from 10° cm2 basal area/yr (Carya, Cornus, Nyssa, Quercus) to >10 3 cmVyr (Betula). Mean dispersal distance ranged from 20 m(Acer, Betula, Liriodendron, Tsuga) and was positively correlated with fecundity. Species also differ in the degree of seed clumping at fine (1 m2) spatial scales. Dispersal patterns can be classed in two groups based on dispersal vector: wind-dispersed taxa with high fecundities, long-distance dis- persal, and low clumping vs. animal-dispersal taxa with low fecundities, short-distance dispersal, and a high degree of clumping. "Colonization" limitations caused by sizes and locations of parent trees, fecundity, and dispersal were quantified as the fraction of sites receiving seed relative to that expected under null models that assume dispersal is nonlocal (i.e., long-distance) and not clumped (i.e., Poisson). Difference among species in coloni- zation levels ranged from those capable of saturating the forest floor with seed in most stands (Acer, Betula, Liriodendron) to ones that leave much of the forest floor without seed, despite presence of adults (Carya, Cornus, Nyssa, Oxydendrum). Seedling establish- ment is one of the strongest filters on recruitment in our study area. Taken together, our results indicate (1) that fecundity and dispersal can be resolved, even under a closed canopy, and (2) that recruitment of many species is limited by the density and location of source, dispersal patterns, or both. Key -words: dispersal; establishment; fecundity; forest dynamics; negative binomial; recruitment;
This book provides information on the historical and theoretical perspectives of biodiversity and ecology in tropical forests, plant and animal behaviour towards seed dispersal and plant-animal interactions within forest communities, consequences of seed dispersal, and conservation, biodiversity and management.
Quercus canariensis Willd. is an oak species endemic to northwestern Africa and the Iberian Peninsula. This species is particularly abundant in the southwestern Andalucía Region of southern Spain. During a disease survey in this area from 1997 to 1998, we observed Q. canariensis trees affected by a tar spot disease. Tar spot lesions were clearly differentiated by a black, crustose, and shiny stromata (10 to 20 mm in diameter) on the upper surface of leaves mainly arranged along leaf veins but also scattered randomly over the leaf surface. On most leaves there was little necrosis, and on most trees the damage was not serious, although some trees located in the most humid areas were severely affected with leaf chlorosis and heavy defoliation. Other Quercus species in the area, such as Q. ilex and Q. suber, did not show signs of tar spot. One fungus species was consistently associated with tar spots. The fungus formed ascomata, 250 to 400 µm in diameter, embedded in the stromata. Asci were clavate or saccate, 45 to 55 × 17 to 22 µm, eight spored, and short stalked. The ascus apex was acute to obtuse. Ascospores were arranged irregularly, 20 to 25 (-30) × 5 to 8.5 µm, fusiform to ellipsoidal fusiform, often curved and flattened on one side, thin walled, hyaline, aseptate, and smooth without a gelatinous sheath or appendage. Based on these characteristics, the fungus was identified as Trabutia quercina (Rudolphi ex Fr.) Sacc. & Roum., a pathogen causing tar spot in several Quercus species in the Northern Hemisphere (1). Samples were deposited in the Plant Pathology Mycotheca at the University of Córdoba, Spain (MIC-888 to MIC-897). To our knowledge, this is the first report of T. quercina on Q. canariensis and the first report of this pathogen in Spain.
Reference: (1) P. F. Cannon. Mycopathologia 135:69, 1996.
A high number of tree species, low density of adults of each species, and long distances between conspecific adults are characteristic of many low-land tropical forest habitats. I propose that these three traits, in large part, are the result of the action of predators on seeds and seedlings. A model is presented that allows detailed examination of the effect of different predators, dispersal agents, seed-crop sizes, etc. on these three traits. In short, any event that increases the efficiency of the predators at eating seeds and seedlings of a given tree species may lead to a reduction in population density of the adults of that species and/or to increased distance between new adults and their parents. Either event will lead to more space in the habitat for other species of trees, and therefore higher total number of tree species, provided seed sources are available over evolutionary time. As one moves from the wet lowland tropics to the dry tropics or temperate zones, the seed and seedling predators in a habitat are hypothesized to be progressively less efficient at keeping one or a few tree species from monopolizing the habitat through competitive superiority. This lowered efficiency of the predators is brought about by the increased severity and unpredictability of the physical environment, which in turn leads to regular or erratic escape of large seed or seedling cohorts from the predators.
Trabutia possesses unitunicate asci and must be removed from the Loculoascomycetes. The characteristics of biotrophic habit, centrum of thin-walled asci at maturity among paraphyses, and pycnidial anamorph dictate its disposition in the Phyllachoraceae of the Phyllachorales.
Plant pathogens reduce the performance of their hosts and therefore may contribute to ecological mechanisms of coexistence. In Chesson’s framework, pathogens contribute to stabilizing mechanisms when they intensify negative intraspecific interactions, such as density‐dependent disease. Additionally, pathogens contribute to equalizing mechanisms when they reduce differences in performance among species. Life‐history trade‐offs predict higher susceptibility to pathogens in rapidly growing species, which could equalize performance among fast‐ and slow‐growing species in the presence of pathogens.
In a coastal prairie in California, we studied the impact of leaf diseases on the performance of 17 co‐occurring species of Trifolium and Medicago (“clovers”). We transplanted clovers in randomized arrays into the natural prairie community in 3 years of common garden experiments. We quantified infection rates by isolating fungi from leaves, and we measured disease severity as per cent leaf area damaged. In a fungicide experiment, we measured the impact of infection on biomass and survival. We assessed whether disease on transplants was positively related to natural abundance of that species in the surrounding community, which we monitored over 5 years. We assessed life‐history trade‐offs by testing whether more rapidly growing species were more susceptible to pathogens.
Rank abundance of clover species was stable over 5 years despite marked environmental fluctuations. Across hosts, fungal infection was not linearly related to density, although transplants of species that were locally absent showed lower and more variable infection. Disease severity was greater for more abundant species, but in only 1 of 3 years, and response to fungicide was not stronger in more abundant species. As predicted by life‐history trade‐offs, faster‐growing species experienced greater fungal infection. However, the impact of that infection was less negative, not more negative, on faster‐growing species.
Our results suggest that life‐history trade‐offs in plant–pathogen interactions may influence equalizing mechanisms among species in this guild, but the combined effects of greater infection with greater tolerance may limit rather than promote coexistence. We also found modest evidence that density‐dependent disease may contribute to stabilizing mechanisms. Lack of host specificity, rapid evolution of host use and temporal variation in climatic conditions may all influence the role that pathogens play in coexistence of these closely related plants.
Theory predicts that intraspecific competition should be stronger than interspecific competition for any pair of stably coexisting species, yet previous literature reviews found little support for this pattern. We screened over 5400 publications and identified 39 studies that quantified phenomenological intraspecific and interspecific interactions in terrestrial plant communities. Of the 67% of species pairs in which both intra‐ and interspecific effects were negative (competitive), intraspecific competition was, on average, four to five‐fold stronger than interspecific competition. Of the remaining pairs, 93% featured intraspecific competition and interspecific facilitation, a situation that stabilises coexistence. The difference between intra‐ and interspecific effects tended to be larger in observational than experimental data sets, in field than greenhouse studies, and in studies that quantified population growth over the full life cycle rather than single fitness components. Our results imply that processes promoting stable coexistence at local scales are common and consequential across terrestrial plant communities.
Interactions between established (canopy) and recruiting individuals (recruits) are pervasive in plant communities. Studies on recruitment in forests have mainly focused on negative density‐dependent conspecific interactions, while the outcomes of heterospecific canopy–recruit interactions have received much less attention and are generally assumed to be driven by stochastic processes.
Herein, we explore the relative influence of stochastic (abundance) and deterministic (species identity and phylogenetic distance) effects on the frequency of canopy–recruit interactions and characterize the interactions in terms of their spatial consistency and effect on recruitment (depressing, neutral or enhancing).
In 12 plots (50 × 50 m) of mixed pine–oak forests in southern Spain, we identified all saplings recruiting beneath 56 shrub and tree species, and in open areas not covered by woody plants. We used generalized linear mixed models to investigate the influence of stochastic and deterministic processes on the frequency of canopy–recruit interactions, on their spatial consistency and their effects on recruitment, and applied neutral null models to evaluate the spatial consistency in the occurrence of interactions across plots.
Deterministic and stochastic interactions were equally common, emphasizing the prevalence of non‐neutral effects. Among the realized interactions, 36.8% enhanced recruitment, 49.05% were neutral, and 14.1% depressed recruitment. Many potential interactions (42.08%) were not observed in any study sites, presumably due to the scarcity of the interacting species. Moreover, the probability that two species formed a canopy–recruit interaction, the frequency of their interaction and the probability that the interaction had an enhancing effect on recruitment, all increased with the phylogenetic distance between the interacting species. However, the prevalence of these effects depended on the recruitment environment (heterospecific, conspecific or open). Recruitment‐enhancing interactions between heterospecifics were more consistently realized in different sites than neutral or depressing interactions.
The establishment of canopy–recruit interactions (which species recruits beneath which others, and how often) is not simply determined by stochastic events. Indeed, due to their prevalence, we argue that deterministic canopy–recruit interactions are important drivers of plant community dynamics.
A plain language summary is available for this article.
The tremendous diversity of species in ecological communities has motivated a century of research into the mechanisms that maintain biodiversity. However, much of this work examines the coexistence of just pairs of competitors. This approach ignores those mechanisms of coexistence that emerge only in diverse competitive networks. Despite the potential for these mechanisms to create conditions under which the loss of one competitor triggers the loss of others, we lack the knowledge needed to judge their importance for coexistence in nature. Progress requires borrowing insight from the study of multitrophic interaction networks, and coupling empirical data to models of competition.
We present strong evidence that pathogens play a critical role in structuring plant communities and maintaining plant diversity. Pathogens mediate plant species coexistence through trade-offs between competitive ability and resistance to pathogens and through pathogen specialization. Experimental tests of individual plant-pathogen interactions, tests of feedback through host-specific changes in soil communities, and field patterns and field experimentation consistently identify pathogens as important to plant species coexistence. These direct tests are supported by observations of the role of pathogens in generating the productivity gains from manipulations of plant diversity and by evidence that escape from native pathogens contributes to success of introduced plant species. Further work is necessary to test the role of pathogen dynamics in large-scale patterns of plant diversity and range limits, the robustness of coexistence to coevolutionary dynamics, the contribution of different pathogens, and the role of pathogens in plant succession.
In a greenhouse experiment, we studied the winter survival of the rust Gymnosporangium cornutum on the alternative host Sorbus aucuparia and in a field experiment we studied dispersal patterns of spores of G. cornulum between the primary host Juniperus communis and the alternative host S. aucuparia. The results show that the rust G. cornutum cannot survive the winter in S. aucuparia. Instead, the rust relies on the wind to spread its spores between the host plants. Spore dispersal was effective, since up to 22 m from the source, a J. communis bush, more than 50% of S. aucuparia plants were infected and up to 7 m from the source pustules covered more than 50% of the leaf area. At 55-57 m from the source about 21% of the plants were infected and the pustules were discrete on the leaves. It is concluded that the position in the field and the distance from the source is of importance for the transmission of the rust spores between J. communis and S. aucuparia.
The chapters of this book on seed dispersal are divided into four parts: (1) frugivores and frugivory (8 chapters); (2) seed and seedling shadows (7 chapters); (3) seed fate and establishment (eight chapters); and (4) management implications and conservation (six chapters). The book presents both recent advances and reviews of current knowledge.
Niche differences and average fitness differences jointly determine coexistence. However, little empirical information about the magnitude of these two mechanisms is available. Using multispecies population models fit to long-term demographic data for common, co-occurring species in five grassland and shrubland plant communities in western North America, we estimated the strength of stabilizing niche differences and average fitness differences. In all five communities, both pairwise and full community comparisons showed evidence for strong stabilizing mechanisms and relatively small average fitness differences. For a total of 17 species pairs, a measure of niche differences based on simulations of invasion growth rates ranged from 0.59 to 0.93 with a mean of 0.81, where 0 indicates complete niche overlap and 1 indicates zero niche overlap. A corresponding measure of average fitness differences ranged from 1.02 to 2.54 with a mean of 1.53, where 1 indicates identical fitness and a value of 2 indicates a fourfold difference in sensitivity to competition. Comparisons of full communities displayed similar patterns: niche differences ranged from 0.58 to 0.69 with a mean of 0.64, and the average fitness differences ranged from 1.42 to 1.63 with a mean of 1.47. In almost every case, the stabilizing mechanisms were much stronger than minimally necessary to prevent competitive exclusion. Considering that all but one of the species we studied are perennial grasses, which are often grouped in the same functional type, the magnitude of these niche differences is surprising. In all five communities, differences between intra- and interspecific effects at the recruitment stage contributed far more to stabilization than interactions affecting growth and survival. Our results indicate that for these abundant, cooccurring species (1) dynamics are far from neutral, with strong niche differences and weak fitness differences combining to stabilize coexistence, and (2) processes operating at early life stages account for a large proportion of the stabilizing effect. Given the limitations of our inductive approach, both these findings represent hypotheses in need of experimental testing.
1. The spatial distribution of Pistacia lentiscus, a Mediterranean bird-dispersed plant, in abandoned orchards was found to be strongly linked to the presence of trees or shrubs that act as perches. 2. These perches not only attract seed-disperser birds but also produce favourable microenvironmental conditions for seed germination and seedling establishment. 3. Soil moisture content after a rainfall was always greater beneath perches than not beneath perches. Favourable water potentials for seed germination were maintained for a longer time beneath a perch than elsewhere. 4. After a rainfall, soil was compacted faster where not beneath perches. Seedling radicle penetration into soil was strongly associated with soil compactation.
An eight-year study in a Mediterranean area near Montpellier (southern France) allows us to document and discuss the relationships between morphological and chemical characteristics of fleshy fruits of the native plant species and the choices made by their bird and mammal seed dispersers. This study is based on qualitative (presence-absence) data. Dispersers have been identified for 50 of the 65 existing fleshy-fruited taxa. Twenty bird and 5 mammal species act as seed dispersers. Five important bird species together disperse 98% of the bird-dispersed taxa (68% of all taxa), and 2 mammal species 91% of the mammal-dispersed taxa (32% of all taxa). There is a wide overlap in the choices among the different disperser species and also among birds and mammals. The choice of fruits organizes itself along a gradient of body weight and gape size of the dispersers. The size of the fruit is the major characteristic to explain the studied choices. Mammals seem to preferently consume lipid-poor fruits and to forsake protein-rich ones, probably because they avoid associated secondary compounds present in lipid- and protein-rich fruits. It is suggested that the evolution of the seed dispersal system is mainly determined by a few dispersers and by a few plants, the other species being only satellites of the key-species. It is emphasized that as dispersers, mammals are probably important to plants, but that their precise actions are as yet poorly known.
QuestionsWhen a tree or a shrub dies, the space it occupied can be overtaken by plants that were recruiting beneath it or colonized by new species. Such replacement processes can drive the temporal change in species abundance in a plant community. Can we predict the dynamics of a real plant community using observational data on plant–plant recruitment interactions? What would be the relative importance of recruitment interactions vs life-history traits in determining community dynamics?LocationForest communities dominated by Pinus halepensis and Quercus ilex in SE Spain.Methods
We develop a continuous time non-linear model that can be easily parameterized with empirical data for the interactions between adult and juvenile plants recruiting beneath them. These interactions form a complex replacement network (or matrix) that is the backbone of the model. We parameterize the model with life-history data from the literature and from recruitment interactions observed in a successional community 12 yr after a forest fire. We explore the behaviour of the model under different intensities of chronic disturbance, and after modifications of the structure of the replacement network and the values of the parameters.ResultsThe model predicts that the current community of the burned area will develop into a forest very similar quantitatively to the surrounding mature forests, as long as the rates of chronic disturbance remain very low. For increasingly higher levels of chronic disturbance, the community would reach stable states resembling a mixed pine–oak forest, a degraded oak forest, an oak dehesa and, finally, a steppe-like vegetation. All these types of plant assemblages can currently be found throughout the study area. These predictions are less sensitive to variation in the estimates of species' life history (i.e. growth, death and colonization rates) than to variation in the structure of the recruitment matrix.Conclusions
The model projects realistic community dynamics. The analysis of the model suggest that understanding the structure of replacement networks and how they are assembled can contribute significantly to our knowledge of the dynamics and stability of forest plant communities.
Soil-borne pathogens are posited to maintain forest diversity. However, their insitu impact and spatial variation are largely unknown. We examined spatial patterns ofpathogenic activity in a deciduous forest using a common garden experiment and also in anatural experiment around replicated trees, and we quantified Pythium (a soil-borne pathogen)density around individual Prunus serotina trees. In both experiments, P. serotina seedlingsurvival was 52-57 % greater in plots treated with a metalaxyl-based fungicide specific tooomycetes (i.e., Pythium ) than in untreated plots. Disease dynamics were not densitydependent, but pathogenic activity and Pythium density were spatially variable. In thecommon garden and natural experiments, pathogenic activity of soil inoculum varied amongtrees, while in the natural experiment disease dynamics were also distance dependent andpathogenic activity decreased away from P. serotina trees. Disease and Pythium density werenot always related but displayed considerable spatial variation. We found that Pythium density did not vary with distance away from P. serotina trees but did vary among trees.Understanding the spatial complexity of soil-borne pathogens is critical to accuratelycharacterizing their effects on populations and ultimately on forest diversity.
Pathogens, like other consumers, mediate the outcome of competitive interactions between their host species. Ongoing efforts to integrate pathogens into plant community ecology could be accelerated by greater conceptual unification. Research on plant pathogens has mainly focused on a variety of disparate mechanisms—the Janzen-Connell hypothesis, plant—soil feedbacks, competition—defense trade-offs, escape of invasive plants from their enemies, and epidemic-driven community shifts—with limited recognition of how these mechanisms fit into the broader context of plant coexistence. Here, I extend an emerging theoretical framework for understanding species coexistence to include various pathogen impacts on plant communities. Pathogens can promote coexistence by regulating relative abundance or by reducing the disparities between species in fitness that make coexistence more difficult. Conversely, pathogens may undermine coexistence by creating positive feedbacks or by increasing between-species fitness differences. I review the evidence for these pathogen-mediated mechanisms, and I reframe the major hypotheses in a community ecology context in order to understand how the mechanisms are related. This approach generates predictions about how various modes of pathogen attack affect plant coexistence, even when direct impacts on host relative abundance are difficult to measure. Surprisingly, no study gives direct empirical evidence for pathogen effects on mutual invasibility, a key criterion for coexistence. Future studies should investigate the relationship between pathogen attack and host relative abundance, in order to distinguish between mechanisms.
The Janzen-Connell (JC) hypothesis, one of the most influential hypotheses explaining forest diversity, is inconsistent with evidence that tree species share the same natural enemies. Through the discussion of seedling diseases from a pathogen-centered perspective, we expand the JC hypothesis to tie in host-pathogen-environment interactions at three levels: local adaptation, host specificity of the combined effect of multiple infections, and environmental modulation of disease. We present evidence from plant pathology, disease ecology, and host-parasite evolution relevant to (but not commonly associated with) forest species diversity maintenance. This expanded view of the JC hypothesis suggests ways to direct new experiments to integrate research on pathogen local adaptation, co-infection, and environmental effects on infection by using high-throughput molecular techniques and statistical models.
We present a model of community regulation that incorporates the effects of abiotic disturbance, predation, competition, and recruitment density. We assume that mobile organisms (i.e., consumers) are more strongly affected by environmental stress than are sessile organisms and that food-web complexity decreases with increasing stress. The model makes three predictions under conditions of high recruitment. First, in stressful environments, consumers have no effect because they are absent or inactive, and competition for space is prevented. Both mobile and sessile organisms are regulated directly by environmental stress. Second, in moderate environments, consumers are still ineffective, but sessile organisms are less affected by stress and frequently attain high densities, leading to competition for space. Finally, in benign environments, consumers prevent competition for space unless the prey can escape a predation bottleneck and reach a high abundance. A reduction in recruitment density reduces the import...
A prescribed statistical model is a parametric specification of the distribution of a random vector, whilst an implicit statistical model is one defined at a more fundamental level in terms of a generating stochastic mechanism. This paper develops methods of inference which can be used for implicit statistical models whose distribution theory is intractable. The kernel method of probability density estimation is advocated for estimating a log‐likelihood from simulations of such a model. The development and testing of an algorithm for maximizing this estimated log‐likelihood function is described. An illustrative example involving a stochastic model for quantal response assays is given. Possible applications of the maximization algorithm to ad hoc methods of parameter estimation are noted briefly, and illustrated by an example involving a model for the spatial pattern of displaced amacrine cells in the retina of a rabbit.
Fruit production and patterns of seed dispersal by birds were studied at two elevations in the mediterranean scrublands of southern Spain. Fleshy-fruit-producing species represent a very prominent fraction of woody plants in terms of cover (57-76%) and species number (49-66%). Fruit production occurs year round in the lowland site but is confined to August-February upslope. Ripe fruits are most abundant (>los ripe fruitsiha) in November-December. Fruit abundance fluctuates widely between years at the highland locality but only slightly in the lowlands. In both communities, the dominant species ripen fruits in autumn-winter, display the highest within-plant fruit densities, and tend to have the most lipid-rich fruits. Fruits differ in pulp nutritive value, seediness, and relative amount of pulp among species but are remarkably uniform in size (mostly 5-10 mm transverse di- ameter). Two-thirds of the passerine species at each site eat some fruit. Of these species, 69% (highland) and 26% (lowland) are resident "fruit predators," feeding on either pulp or seeds alone, and damaging the seeds when eating pulp and seeds together. The rest are overwintering or migratory seed dispersers that ingest whole fruits without damaging seeds. Seed dispersers are most common in late autumn- winter, coincident with the peak in fruit abundance and the predominance of lipid-rich fruits. A few small (12-18 g body mass) disperser species (Erithrrcus rubecula, Sylvia rrtricapilla, Sylvirr rnelano- cephala) account for most of the frugivory at each site and disperse the majority of seeds. Fruit predators either are relatively scarce or eat fruits infrequently, or fruits represent a negligible fraction of their diets. Fruit removal was very high (89-100% of crops) among species with fruits smaller than the gape width of the abundant small-sized dispersers, and very low among species with fruits larger than gape width. Removal success was negatively correlated with fruit size among species having fruits smaller than dominant dispersers' gape width. No relation has been found between removal success and fruit quality, fruiting time, ripening rate, or within-plant fruit density. The principal dispersers at each site ate mainly the most nutritious fruits, although not to the exclusion of less nutritious fruits. Substantial pairwise plant-bird reciprocity is not common. (The avian species disperses a substantial fraction of a plant's seeds, which in turn provide the bulk of the bird's energy supply.) Current bird-plant seed dispersal interactions are the result of evolutionary, climatic, and geo- graphical factors in the Mediterranean. Mutualistic congruency largely is, in these cases, an epiphe- nomenon of these factors, not resulting necessarily from mutual adaptations (coevolution). It is sug- gested that actual coevolution involving a smaller set of bird and plant species may facilitate the persistence of noncoevolving (or very slowly coevolving) plant species, thus favoring the existence of a chronic "anachronism load" (with regard to dispersal) in the plant community.
1. Seed dispersal and natural enemies both influence spatial patterns of seedlings, which in turn influence future abiotic and biotic interactions, with consequences for plant populations, distributions and diversity. Clumped seed deposition is common, especially for vertebrate-dispersed seeds, and has the potential to significantly affect interactions with density-responsive enemies, yet has received relatively little attention.
2. We used spatially explicit simulation models to examine how different patterns of seed dispersal and natural enemy attack structure seedling spatial patterns. We simulated clumped seed dispersal by combining a two-dimensional Student’s T dispersal kernel for expected seed rain with a negative binomial distribution for seed deposition. We based our models for seed mortality on published data reflecting differing life histories of insect seed predators and soil-borne pathogens. We varied dispersal distance, degree of clumping, type of enemy, enemy dispersal distance and fecundity among simulations.
3. Under insect seed predation, seeds escaped predation by dispersing longer distances than insects, resulting in ‘Janzen–Connell’ patterns in which seedling recruitment peaks at intermediate distances. When insects dispersed longer distances than seeds, higher seed densities near the tree satiated insects, resulting in ‘McCanny’ patterns in which seed deposition, survivorship and seedling establishment all decrease with distance from the parent tree. Total seedling establishment was lowest when insects and seeds dispersed similar distances.
4. Under pathogen attack, Janzen–Connell patterns predominated except when seedling survival was virtually zero or one everywhere, or in the case where pathogen dispersal distances exceeded seed dispersal distances, producing ‘Hubbell’ patterns in which seed deposition and seedling establishment decrease with distance, though survivorship increases.
5. Clumped seed deposition increased the probability of seedling establishment under both insect seed predation and pathogen attack as it led to local satiation of insect seed predators and made it harder for pathogen distributions to track seeds.
6. Synthesis. Our modelling study suggests that the relative dispersal distances of seeds and natural enemies are crucial to determining establishment rates and spatial patterns of seedlings. Better characterization of the movement and natural histories of natural enemies is critical to improving our understanding of seedling distributions and plant–enemy interactions.
Spatial point processes are mathematical models used to describe and analyse the geometrical structure of patterns formed by objects that are irregularly or randomly distributed in one-, two- or three-dimensional space. Examples include locations of trees in a forest, blood particles on a glass plate, galaxies in the universe, and particle centres in samples of material. Numerous aspects of the nature of a specific spatial point pattern may be described using the appropriate statistical methods. Statistical Analysis and Modelling of Spatial Point Patterns provides a practical guide to the use of these specialised methods. The application-oriented approach helps demonstrate the benefits of this increasingly popular branch of statistics to a broad audience. The book: Provides an introduction to spatial point patterns for researchers across numerous areas of application Adopts an extremely accessible style, allowing the non-statistician complete understanding Describes the process of extracting knowledge from the data, emphasising the marked point process Demonstrates the analysis of complex datasets, using applied examples from areas including biology, forestry, and materials science Features a supplementary website containing example datasets. Statistical Analysis and Modelling of Spatial Point Patterns is ideally suited for researchers in the many areas of application, including environmental statistics, ecology, physics, materials science, geostatistics, and biology. It is also suitable for students of statistics, mathematics, computer science, biology and geoinformatics.
Complementary beneficial effects of different arbuscular mycorrhizal fungi ( AMF ) can result in a more efficient exploitation of the soil nutrients available, thus influencing plant communities. Here, we hypothesize that plant– AMF specificity is mediated by phylogenetic constraints defining possible interactions, and that plant– AMF interaction patterns can influence plant–plant facilitation specificity.
We reanalyzed previous data describing plant–plant and plant– AMF interaction at the community level to specifically test for a phylogenetic signal on plant and AMF interactions and for a relationship between plant–plant facilitation specificity and plant species differences in their AMF associates.
Closely related AMF operational taxonomical units ( OTU s) tend to interact with the same plant species, but there is not a significant signal in the interaction through the plant phylogeny. This indicates that the similarity in the AMF associates of two plant species is independent of their phylogenetic relatedness. Interestingly, plant– AMF interactions match plant facilitation specificity, with pairs of plant species recruiting more frequently under each other tending to have different AMF associates.
An increment of AMF diversity in the rhizosphere, as a result of plant– AMF and plant–plant selectivity, is suggested as a potential driver of plant–plant facilitation. This study highlights the role of plant– AMF interactions in shaping plant community assemblages.