The influence of niche and neutral processes on a neotropical anuran metacommunity

One of the most important questions in ecology is the relative importance of local conditions (niche processes) and dispersal ability (neutral processes) in driving metacommunity structure. Although many studies have been conducted in recent years, there is still much debate. We evaluated the processes (niche and neutral) responsible for variation in anuran composition in 28 lentic water bodies in southeastern Brazil. Because anurans depend heavily on environmental conditions, we hypothesized that environmental variables (niche processes) are the most important drivers of community composition. Additionally, as anurans have limited dispersal abilities, and the study region presents harsh conditions (high forest fragmentation, low rainfall and long dry season), we expected a lower, but significant, spatial signature in metacommunity structure, due to neutral dynamics. We used a partial redundancy analysis with variation partitioning to evaluate the relative influence of environmental and spatial variables as drivers of metacommunity structure. Additionally, we used a recently developed spatial autocorrelation analysis to test if neutral dynamics can be attributed to the pure spatial component. This analysis is based on predictions that species abundances are independent but similarly spatially structured, with correlograms similar in shape. Therefore, under neutral dynamics there is no expectation of a correlation between the pairwise distance of spatial correlograms and the pairwise correlation of species abundances predicted by the pure spatial component. We found that the environmental component explained 21.5%, the spatial component 10.2%, and the shared component 6.4% of the metacommunity structure. We found no correlation between correlograms and correlation of abundances predicted by the pure spatial component (Mantel test = ?0.109, P = 0.961). In our study, niche‐based processes are the dominant process that explained community composition. However, neutral processes are important because spatial variation can be attributed to pure neutral dynamics rather than to missing spatially structured environmental factors.     

Hydrological connectivity structures concordant plant and animal assemblages according to niche rather than dispersal processes

1. Understanding the processes that structure community assembly across landscapes is fundamental to ecology and for predicting and managing the consequences of anthropogenically induced changes to ecosystems. 2. We assessed the community similarity of fish, macroinvertebrate and vegetation communities against geographic distances ranging from 4 to 480 km (i.e. distance–decay relationships) to determine the balance between local environmental factors and regional dispersal processes, and thus whether species‐sorting (niche processes) or dispersal limitation (neutral processes) was more important in structuring these assemblages in Australia’s wet‐dry tropics. We investigated whether the balance between niche and dispersal processes depended on the degree of hydrological connectivity, predicting that dispersal processes would be more important at connected sites, and also whether there was spatial concordance among these three assemblage types. 3. There was significant but weak spatial concordance among the study communities, suggesting limited potential for surrogacy among them. Distance–decay in community similarity was not observed for any study assemblage at perennial sites, suggesting dispersal was not limiting and assemblages were structured more strongly by local niche processes at these connected sites. At intermittent sites, weak distance–decay relationships for each assemblage type were confounded by significant relationships with environmental dissimilarity, suggesting that dispersal limitation contributed, albeit weakly, to niche processes in structuring our three study assemblages at disconnected sites. 4. Two environmental factors, flow regime and channel width, explained significant proportions of variation in all three assemblages, potentially contributing to the observed spatial concordance between them and representing local environmental gradients along which these communities re‐assemble after the wet season, according to niche rather than dispersal processes.     

Beyond the continuum: a multi-dimensional phase space for neutral–niche community assembly

Neutral and niche processes are generally considered to interact in natural communities along a continuum, exhibiting community patterns bounded by pure neutral and pure niche processes. The continuum concept uses niche separation, an attribute of the community, to test the hypothesis that communities are bounded by pure niche or pure neutral conditions. It does not accommodate interactions via feedback between processes and the environment. By contrast, we introduce the Community Assembly Phase Space (CAPS), a multi-dimensional space that uses community processes (such as dispersal and niche selection) to define the limiting neutral and niche conditions and to test the continuum hypothesis. We compare the outputs of modelled communities in a heterogeneous landscape, assembled by pure neutral, pure niche and composite processes. Differences in patterns under different combinations of processes in CAPS reveal hidden complexity in neutral–niche community dynamics. The neutral–niche continuum only holds for strong dispersal limitation and niche separation. For weaker dispersal limitation and niche separation, neutral and niche processes amplify each other via feedback with the environment. This generates patterns that lie well beyond those predicted by a continuum. Inferences drawn from patterns about community assembly processes can therefore be misguided when based on the continuum perspective. CAPS also demonstrates the complementary information value of different patterns for inferring community processes and captures the complexity of community assembly. It provides a general tool for studying the processes structuring communities and can be applied to address a range of questions in community and metacommunity ecology.     

Assembly mechanisms determining high species turnover in aquatic communities over regional and continental scales

Niche and neutral processes drive community assembly and metacommunity dynamics, but their relative importance might vary with the spatial scale. The contribution of niche processes is generally expected to increase with increasing spatial extent at a higher rate than that of neutral processes. However, the extent to what community composition is limited by dispersal (usually considered a neutral process) over increasing spatial scales might depend on the dispersal capacity of composing species. To investigate the mechanisms underlying the distribution and diversity of species known to have great powers of dispersal (hundreds of kilometres), we analysed the relative importance of niche processes and dispersal limitation in determining beta‐diversity patterns of aquatic plants and cladocerans over regional (up to 300 km) and continental (up to 3300 km) scales. Both taxonomic groups were surveyed in five different European regions and presented extremely high levels of beta‐diversity, both within and among regions. High beta‐diversity was primarily explained by species replacement (turnover) rather than differences in species richness (i.e. nestedness). Abiotic and biotic variables were the main drivers of community composition. Within some regions, small‐scale connectivity and the spatial configuration of sampled communities explained a significant, though smaller, fraction of compositional variation, particularly for aquatic plants. At continental scale (among regions), a significant fraction of compositional variation was explained by a combination of spatial effects (exclusive contribution of regions) and regionally‐structured environmental variables. Our results suggest that, although dispersal limitation might affect species composition in some regions, aquatic plant and cladoceran communities are not generally limited by dispersal at the regional scale (up to 300 km). Species sorting mediated by environmental variation might explain the high species turnover of aquatic plants and cladocerans at regional scale, while biogeographic processes enhanced by dispersal limitation among regions might determine the composition of regional biotas.     

Soil Properties and Spatial Processes Influence Bacterial Metacommunities within a Grassland Restoration Experiment

Metacommunity theory proposes that a collection of local communities are linked by dispersal and the resulting compositions are a product of both niche‐based (species sorting) and spatial processes. Determining which of these factors is most important in different habitats can provide insight into the regulation of community assembly. To date, the metacommunity organization of heterotrophic soil bacteria is largely unknown. Spatial variation of soil bacterial communities could arise from (1) the resource heterogeneity produced by plant communities through root exudation and/or litter inputs; (2) the heterogeneity of soil environmental properties; and (3) pure spatial processes, including dispersal limitation and stochastic assembly. Understanding the relative importance of these factors for soil bacterial community structure and function could increase our ability to restore soil communities. We utilized an ongoing tallgrass prairie restoration experiment in northeastern Kansas to assess if restoring native plant communities produced changes in bacterial communities 6 years after restoration. We further examined the relative importance of the spatial heterogeneity of plant communities, soil properties, and pure spatial effects for bacterial community structure in the old‐field restoration site. We found that soil bacterial communities were not influenced by plant restoration, but rather, by the local heterogeneity of soil environmental properties (16.9% of bacterial community variation) and pure spatial effects (11.1%). This work also stresses the idea that restoring bacterial communities can take many years to accomplish due to the inherent changes that occur to the soil after cultivation and the time it takes for the re‐establishment of soil quality.     

Inferring ecological processes from taxonomic, phylogenetic and functional trait β-diversity

Understanding the influences of dispersal limitation and environmental filtering on the structure of ecological communities is a major challenge in ecology. Insight may be gained by combining phylogenetic, functional and taxonomic data to characterize spatial turnover in community structure (β-diversity). We develop a framework that allows rigorous inference of the strengths of dispersal limitation and environmental filtering by combining these three types of β-diversity. Our framework provides model-generated expectations for patterns of taxonomic, phylogenetic and functional β-diversity across biologically relevant combinations of dispersal limitation and environmental filtering. After developing the framework we compared the model-generated expectations to the commonly used "intuitive" expectation that the variance explained by the environment or by space will, respectively, increase monotonically with the strength of environmental filtering or dispersal limitation. The model-generated expectations strongly departed from these intuitive expectations: the variance explained by the environment or by space was often a unimodal function of the strength of environmental filtering or dispersal limitation, respectively. Therefore, although it is commonly done in the literature, one cannot assume that the strength of an underlying process is a monotonic function of explained variance. To infer the strength of underlying processes, one must instead compare explained variances to model-generated expectations. Our framework provides these expectations. We show that by combining the three types of β-diversity with model-generated expectations our framework is able to provide rigorous inferences of the relative and absolute strengths of dispersal limitation and environmental filtering. Phylogenetic, functional and taxonomic β-diversity can therefore be used simultaneously to infer processes by comparing their empirical patterns to the expectations generated by frameworks similar to the one developed here.     

Dispersal limitation is stronger in communities of microorganisms than macroorganisms across Central European cities

Aim It is generally believed that communities of small organisms, or those with small propagules, are structured mainly by local niche‐based processes, and less by dispersal limitation. Conversely, weaker environmental and stronger spatial structure, indicating dispersal limitation, are expected to occur more frequently in communities of large organisms. However, this hypothesis has rarely been tested by comparing spatial and environmental effects across groups of organisms of different size (or with different size of propagules) sampled at the same set of sites. Here, we test it in urban environments. Location Thirty‐two cities in 10 countries of Central Europe and Benelux. Methods We compared effects of spatial location and climate on species composition of different groups of organisms sampled in corresponding types of urban habitats. The studied groups were: (1) subaerial cyanobacteria and algae, (2) vascular plants, (3) land snails; and subgroups of vascular plants with different life form and dispersal mode, namely: (4) herbs, (5) animal‐dispersed trees and shrubs, and (6) wind‐dispersed trees and shrubs. Data were analysed by variation partitioning based on redundancy analysis (RDA) with principal coordinates of neighbour matrices (PCNM). Eighteen PCNM eigenvectors (expressing spatial effects) and mean annual temperature, July–January temperature difference and annual precipitation sum (expressing environmental effects) were used as explanatory variables. Results Pure effects of climate on species composition, indicating niche‐based processes, were not significant for any group or subgroup of the studied organisms. In contrast, pure effects of space, indicating dispersal limitation, were significant for all groups and subgroups except herbs. Surprisingly, the community of cyanobacteria/algae possessed much stronger spatial structure independent of climate than communities of larger organisms, although cyanobacteria/algae had the lowest beta diversity among the studied cities. Main conclusions We hypothesize that the community of subaerial cyanobacteria/algae is structured by natural processes which involve dispersal limitation, whereas communities of urban plants and snails are influenced by human‐assisted dispersal of their propagules between cities, which results in weaker dispersal limitation. Our study indicates that dispersal vectors can be more important for community structure than size of organisms or of their propagules.     

Immigration,Local Dispersal Limitation,and the Repeatability of Community Composition under Neutral and Niche Dynamics

Repeatability of community composition has been a critical aspect for community structure, which is closely associated with community stability, predictability, conservation biology and ecological restoration. It has been shown that both immigration and local dispersal limitation can affect the community composition in both neutral and niche model. Hence, we use a spatially explicit individual-based model to investigate the potential influence of immigration rate and strength of local dispersal limitation on repeatability in both neutral and niche models. Similarity measures are used to quantify repeatability. We examine the repeatability of community composition among replicate communities (which means the same community repeats many times), and between niche and neutral replicate communities. We find the correlation between repeatability and immigration rate is positive in the neutral model and an inverted unimodal in the niche model. The correlation between repeatability and local dispersal distance is positive in the niche model and negative in the neutral model. High repeatability between niche communities and neutral communities is observed with high immigration rates or when high local dispersal distance appears in the niche model or low local dispersal distance in the neutral model. Our results show that repeatability of community composition is not only dependent on the types of community models (niche vs. neutrality) but also strongly determined by immigration rates and local dispersal limitation.     

The scale-dependent importance of habitat factors and dispersal limitation in structuring Great Lakes shoreline plant communities

Niche-based and neutral models of community structure posit distinct mechanisms underlying patterns in community structure; correlation between species’ distributions and habitat factors points to niche assembly while spatial pattern independent of habitat suggests neutral assembly via dispersal limitation. The challenge is to disentangle the relative contributions when both processes are operating, and to determine the scales at which each is important. We sampled shoreline plant communities on an island in Lake Michigan, varying the extent and the grain of sampling, and used both distance-based correlation methods and variance partitioning to quantify the proportion of the variation in plant species composition that was attributable to habitat factors and to spatial configuration independent of habitat. Our results were highly scale dependent. We found no distance decay of plant community similarity at the island scale (1−33 km). All of the explained variation (32%) in species composition among samples at this scale was attributed to habitat factors. However, at a site intensively sampled at a smaller scale (5−1,200 m), similarity of species composition did decay with distance. Using a coarse sampling grain (transects), habitat factors explained 40% of the variation, but the purely spatial component explained a comparable 22%. Analyzing plots within transects revealed variation in species composition that was still jointly determined by habitat and spatial factors (18 and 11% of the variance, respectively). For both grain sizes, most of the habitat component was spatially structured, reflecting an abrupt alongshore transition from sandy dunes to cobble beach. Space per se explained more variation in species composition at a second site where the habitat transition was more gradual; here, habitat acted as a less selective filter, allowing the signal of dispersal limitation to be detected more readily. We conclude that both adaptation to specific habitat factors and habitat-independent spatial position indicative of dispersal limitation determine plant species composition in this system. Our results support the prediction that dispersal limitation—a potentially, but not necessarily, neutral driver—is relatively more important at smaller scales.     

Incorporating intraspecific variation in tests of trait-based community assembly

Environmental filtering and niche differentiation are processes proposed to drive community assembly, generating nonrandom patterns in community trait distributions. Despite the substantial intraspecific trait variation present in plant communities, most previous studies of trait-based community assembly have used species mean trait values and therefore not accounted for intraspecific variation. Using a null model approach, I tested for environmental filtering and niche differentiation acting on three key functional traits??vegetative height, specific leaf area (SLA), and leaf dry matter content (LDMC)??in old-field plant communities. I also examined how accounting for intraspecific variation at the among-plot and individual levels affected the detection of nonrandom assembly patterns. Tests using fixed species mean trait values provided evidence of environmental filtering acting on height and SLA and niche differentiation acting on SLA. Including plot-level intraspecific variation increased the strength of these patterns, indicating an important role of intraspecific variation in community assembly. Tests using individual trait data indicated strong environmental filtering acting on all traits, but provided no evidence of niche differentiation, although these signals may have been obscured by the effects of dispersal limitation and spatial aggregation of conspecific individuals. There was also strong evidence of nonrandom assembly of individuals within single species, with the strength of environmental filtering varying among species. This study demonstrates that, while analyses using fixed species mean trait values can provide insights into community assembly processes, accounting for intraspecific variation provides a more complete view of communities and the processes driving their assembly.     

Compositional divergence and convergence in local communities and spatially structured landscapes

Community structure depends on both deterministic and stochastic processes. However, patterns of community dissimilarity (e.g. difference in species composition) are difficult to interpret in terms of the relative roles of these processes. Local communities can be more dissimilar (divergence) than, less dissimilar (convergence) than, or as dissimilar as a hypothetical control based on either null or neutral models. However, several mechanisms may result in the same pattern, or act concurrently to generate a pattern, and much research has recently been focusing on unravelling these mechanisms and their relative contributions. Using a simulation approach, we addressed the effect of a complex but realistic spatial structure in the distribution of the niche axis and we analysed patterns of species co-occurrence and beta diversity as measured by dissimilarity indices (e.g. Jaccard index) using either expectations under a null model or neutral dynamics (i.e., based on switching off the niche effect). The strength of niche processes, dispersal, and environmental noise strongly interacted so that niche-driven dynamics may result in local communities that either diverge or converge depending on the combination of these factors. Thus, a fundamental result is that, in real systems, interacting processes of community assembly can be disentangled only by measuring traits such as niche breadth and dispersal. The ability to detect the signal of the niche was also dependent on the spatial resolution of the sampling strategy, which must account for the multiple scale spatial patterns in the niche axis. Notably, some of the patterns we observed correspond to patterns of community dissimilarities previously observed in the field and suggest mechanistic explanations for them or the data required to solve them. Our framework offers a synthesis of the patterns of community dissimilarity produced by the interaction of deterministic and stochastic determinants of community assembly in a spatially explicit and complex context.     

Does dispersal ability affect the relative importance of environmental control and spatial structuring of littoral macroinvertebrate communities?

Both spatial processes and environmental control may structure metacommunities, but their relative importance may be contingent on the dispersal ability of organisms. I examined the roles of spatial and environmental factors for the structuring of littoral macroinvertebrate communities across a set of lakes in a boreal drainage basin. I hypothesized that dispersal ability would affect the relative importance of spatial processes and environmental control, and thus the biological data were divided into four groups of species differing in dispersal ability. In general, the group of the strongest aerial dispersers showed greatest relative pure environmental control and least pure spatial structuring of community structure and species richness, while spatial processes seemed to be more important for the other three dispersal ability groups. However, these results were contingent on the indirect measure of spatial processes, with the spatial variables and connectivity variables providing slightly different insights into the spatial processes and environmental control of metacommunity structuring. It appears, however, that dispersal ability has effects on the spatial processes and environmental control important in metacommunity organization, with strong dispersers being more under environmental control and less affected by spatial processes compared to weak dispersers.     

Relative contributions of neutral and niche-based processes to the structure of a desert grassland grasshopper community

Whether neutral or deterministic factors structure biotic communities remains an open question in community ecology. We studied the spatial structure of a desert grassland grasshopper community and tested predictions for species sorting based on niche differentiation (deterministic) and dispersal limitation (neutral). We contrasted the change in species relative abundance and community similarity along an elevation gradient (i.e., environmental gradient) against community change across a relatively homogeneous distance gradient. We found a significant decrease in pairwise community similarity along both elevation and distance gradients, indicating that dispersal limitation plays a role in structuring local grasshopper communities. However, the distance decay of similarity was significantly stronger across the elevational gradient, indicating that niche-based processes are important as well. To further investigate mechanisms underlying niche differentiation, we experimentally quantified the dietary preferences of two common species, Psoloessa texana and Psoloessa delicatula, for the grasses Bouteloua eriopoda and Bouteloua gracilis, which are the dominant plants (~75% of total cover) in our study area. Cover of the preferred host plant explained some of the variation in relative abundances of the two focal species, although much variance in local Psoloessa distribution remained unexplained. Our results, the first to examine these hypotheses in arid ecosystems, indicate that the composition of local communities can be influenced by both probabilistic processes and mechanisms based in the natural histories of organisms.     

History,rather than contemporary processes,determines variation in macroinvertebrate diversity in artesian springs: the expansion hypothesis

1. The relative importance of contemporary and historical processes is a fundamental question in understanding patterns of biodiversity. We tested the hypothesis that species‐sorting into different habitats (limnocrenes, rheocrenes, helocrenes), rather than history, would account for the greatest variation in macroinvertebrate diversity in desert artesian springs of The Bonneville Basin, U.S.A. These springs were isolated at the valley scale c. 9000 years ago by aridity and high salinity. Thus, the valley scale will account for the greatest variation in community composition if history and dispersal limitation are important, whereas niche‐based sorting processes will be most important if habitat accounts for the greatest variation in community composition. 2. We identified 302 taxa from 280 sites and used a partial redundancy analysis, additive partitioning and classification strength (CS) to partition the variability in diversity among the springs. The valley scale accounted for more variation in community composition in limnocrene habitats (32.5%) than all other spatial and environmental variables combined. Valleys also accounted for 58% (additive partitioning) and 83% (CS) of the regional variation in diversity in analyses that included all three habitat types. That is, the average community similarity was 25% across the region, but increased to 41% within valleys. By contrast, habitat filtering did not account for significant variation in community composition in any of the analyses. Our study is one of the few suggesting the over‐riding importance of neutral processes in determining patterns of diversity (history and dispersal limitation). 3. The ‘expansion’ hypothesis suggests that the youthful age of a region, combined with slow dispersal by a fauna dominated by generalists, will maximise the imprint of history. These communities appear to exist in a pre‐equilibrial state, where the maximum carrying capacity has not been reached and niche space is plentiful. With time, we predict that local richness will increase while β‐diversity decreases as species expand their distribution across the region. Consequently, the importance of niche‐based processes may increase with time as the imprint of history fades.     

Spatial Structures of the Environment and of Dispersal Impact Species Distribution in Competitive Metacommunities

The correspondence between species distribution and the environment depends on species’ ability to track favorable environmental conditions (via dispersal) and to maintain competitive hierarchy against the constant influx of migrants (mass effect) and demographic stochasticity (ecological drift). Here we report a simulation study of the influence of landscape structure on species distribution. We consider lottery competition for space in a spatially heterogeneous environment, where the landscape is represented as a network of localities connected by dispersal. We quantified the contribution of neutrality and species sorting to their spatial distribution. We found that neutrality increases and the strength of species-sorting decreases with the centrality of a community in the landscape when the average dispersal among communities is low, whereas the opposite was found at elevated dispersal. We also found that the strength of species-sorting increases with environmental heterogeneity. Our results illustrate that spatial structure of the environment and of dispersal must be taken into account for understanding species distribution. We stress the importance of spatial geographic structure on the relative importance of niche vs. neutral processes in controlling community dynamics.     

Integrating environmental and spatial processes in ecological community dynamics

The processes controlling the abundances of species across multiple sites form the cornerstone of modern ecology. In these metacommunities, the relative importance of local environmental and regional spatial processes is currently hotly debated, especially in terms of the validity of neutral model. I collected 158 published data sets with information on community structure, environmental and spatial variables. I showed that approximately 50% of the variation in community composition is explained by both environmental and spatial variables. The majority of the data sets were structured by species-sorting dynamics (SS), followed by a combination of SS and mass-effect dynamics. While neutral processes were the only structuring process in 8% of the collected natural communities, disregarding neutral dispersal processes would result in missing important patterns in 37% of the studied communities. Moreover, metacommunity characteristics such as dispersal type, habitat type and spatial scale predicted part of the detected variation in metacommunity structure.     

Common and rare species respond to similar niche processes in macroinvertebrate metacommunities

Ecologists have long investigated why communities are composed of a few common species and many rare species. Most studies relate rarity to either niche differentiation among species or spatial processes. There is a parallel between these processes and the processes proposed to explain the structure of metacommunities. Based on a metacommunity perspective and on data on stream macroinvertebrates from different regions of Brazil, we answer two questions. 1) Are sets of common and rare species affected by similar niche and spatial processes? 2) How does the community composition of common and of rare species differ? The main hypothesis we test is that common species are mainly affected by environmental factors, whereas rare species are mostly influenced by dispersal limitation. We used variation partitioning to determine the proportion of variation explained by the environment and space in common and rare species matrices. Contrary to our expectations, evidence supported the idea that both common and rare species are affected mainly by environmental factors, even after controlling for the differing information content between common and rare species matrices. Moreover, the abundance of some common species is also a good predictor of variation in rare species matrices. Niche differences are unlikely to be the sole cause of patterns of rarity in these metacommunities. We suggest that sets of common and rare species react to similar major environmental gradients and that rare species also respond to processes that operate at a more fine‐grained spatial scale, particularly biotic interactions. We extend the view that species sorting is the dominant process structuring metacommunities and argue that future studies focusing on rarity would benefit from a metacommunity perspective.     

Separating the effect of mechanisms shaping species‐abundance distributions at multiple scales in a subtropical forest

Species abundance distributions (SADs) play an important role in the current dispute over mechanisms shaping community assembly. Niche theory assumes differential occurrence of species in different habitats while neutral theory emphasizes stochastic events and dispersal. The previous tests of niche and neutral models shaping SADs lead to the claim that SADs are not informative for inferring underlying processes. Using spatial statistical models in a fully mapped 24‐ha subtropical forest in China, we first demonstrate that one can not distinguish between the effect of habitat heterogeneity and dispersal limitation on SADs by inspecting whether the observed SADs fall within 95% confidence intervals of the simulated SADs. Subsequently, we demonstrate that SADs can be used to detect mechanisms shaping SADS by comparing alternative process‐based models using model selection techniques. We found that dispersal limitation explain SADs at smaller spatial scales, while the combination of niche and dispersal limitation explain SADs at larger scales. These processes are linked with the degree of conspecific aggregation, informing further attempts to refine and parameterize the statistical theory of sampling SADs.     

The importance of metacommunity ecology for environmental assessment research in the freshwater realm

Most bioassessment programs rest on the assumption that species have different niches, and that abiotic environmental conditions and changes therein determine community structure. This assumption is thus equivalent to the species sorting perspective (i.e. that species differ in their responses to environmental variation) in metacommunity ecology. The degree to which basing bioassessment on the species sorting perspective is reasonable is likely to be related to the spatial extent of a study and the characteristics of the organism groups (e.g. dispersal ability) with which the effects of anthropogenic changes are assessed. Recent findings in metacommunity research have stressed that community structure is determined not only by local abiotic environmental conditions but also by biotic interactions and dispersal‐related effects. For example, dispersal limitation may prevent community structure recovery from the effects of a putative stressor, as organisms may not be able to disperse to all sites in a region. Mass effects (i.e. the presence of species in environmentally suboptimal sites due to high dispersal rates from environmentally suitable sites) may, in turn, obscure the effects of a stressor, as dispersal from source sites (e.g. an unaltered site) allows persistence at sink sites (e.g. an anthropogenically altered site). Better bioassessment should thus take both niche‐ and dispersal‐related processes simultaneously into consideration, which can be accomplished by explicitly modelling spatial location as a proxy for dispersal effects. Such an integrated approach should be included in bioassessment programs using general multivariate approaches, predictive modelling, and multimetric indices.     

Dispersal Ability Determines the Role of Environmental,Spatial and Temporal Drivers of Metacommunity Structure

Recently, community ecologists are focusing on the relative importance of local environmental factors and proxies to dispersal limitation to explain spatial variation in community structure. Albeit less explored, temporal processes may also be important in explaining species composition variation in metacommunities occupying dynamic systems. We aimed to evaluate the relative role of environmental, spatial and temporal variables on the metacommunity structure of different organism groups in the Upper Paraná River floodplain (Brazil). We used data on macrophytes, fish, benthic macroinvertebrates, zooplankton, periphyton, and phytoplankton collected in up to 36 habitats during a total of eight sampling campaigns over two years. According to variation partitioning results, the importance of predictors varied among biological groups. Spatial predictors were particularly important for organisms with comparatively lower dispersal ability, such as aquatic macrophytes and fish. On the other hand, environmental predictors were particularly important for organisms with high dispersal ability, such as microalgae, indicating the importance of species sorting processes in shaping the community structure of these organisms. The importance of watercourse distances increased when spatial variables were the main predictors of metacommunity structure. The contribution of temporal predictors was low. Our results emphasize the strength of a trait-based analysis and of better defining spatial variables. More importantly, they supported the view that “all-or- nothing” interpretations on the mechanisms structuring metacommunities are rather the exception than the rule.