Using functional diversity patterns to explore metacommunity dynamics: a framework for understanding local and regional influences on community structure

The metacommunity concept, describing how local and regional scale processes interact to structure communities, has been successfully applied to patterns of taxonomic diversity. Functional diversity has proved useful for understanding local scale processes, but has less often been applied to understanding regional scale processes. Here, we explore functional diversity patterns within a metacommunity context to help elucidate how local and regional scale processes influence community assembly. We detail how each of the four metacommunity perspectives (species sorting, mass effects, patch dynamics, neutral) predict different patterns of functional beta‐ and alpha‐diversity and spatial structure along two key gradients: dispersal limitation and environmental conditions. We then apply this conceptual model to a case study from alpine tundra plant communities. We sampled species composition in 17 ‘sky islands’ of alpine tundra in the Colorado Rocky Mountains, USA that differed in geographic isolation and area (key factors related to dispersal limitation) and temperature and elevation (key environmental factors). We quantified functional diversity in each site based on specific leaf area, leaf area, stomatal conductance, plant height and chlorophyll content. We found that colder high elevation sites were functionally more similar to each other (decreased functional beta‐diversity) and had lower functional alpha‐diversity. Geographic isolation and area did not influence functional beta‐ or alpha‐diversity. These results suggest a strong role for environmental conditions structuring alpine plant communities, patterns consistent with the species sorting metacommunity perspective. Incorporating functional diversity into metacommunity theory can help elucidate how local and regional factors structure communities and provide a framework for observationally examining the role of metacommunity dynamics in systems where experimental approaches are less tractable.     

A simulation‐based approach to understand how metacommunity characteristics influence emergent biodiversity patterns

To understand controls over biodiversity, it is necessary to take a multi‐scale approach to understand how local and regional factors affect the community assembly processes that drive emergent patterns. This need is reflected in the growing use of the metacommunity concept to interpret multi‐scale measures of biodiversity, including metrics derived from diversity partitioning (e.g. α, β and γ diversity) and variation partitioning (e.g. spatial and environmental components of compositional turnover) techniques. However, studies have shown limited success using these metrics to characterize underlying community assembly dynamics. Here we demonstrate how a metacommunity simulation package (MCSim) can be used to evaluate when and how biodiversity metrics can be used to make inferences about metacommunity characteristics. We examined a wide range of parameter settings representing ecologically relevant scenarios. We used artificial neural networks (ANNs) to assess the sensitivity of diversity and variation partitioning metrics (calculated from simulation outcomes) to metacommunity parameter settings. In the scenarios examined in this study, the niche‐neutral gradient strongly influenced most biodiversity metrics, metacommunity size exhibited a marginal influence over some metrics, and dispersal dynamics only affected a subset of variation partitioning outcomes. Variation partitioning response curves along the niche‐neutral gradient were not monotonic; however, simulation outcomes suggest other biodiversity metrics (e.g. dissimilarity saturation) can be used in combination with variation partitioning metrics to make inferences about metacommunity properties. With the growing availability of archived ecological data, we expect future work will apply simulation‐based techniques to better understand links between biodiversity and the metacommunity characteristics that are presumed to control the underlying community assembly processes.     

Variation in habitat connectivity generates positive correlations between species and genetic diversity in a metacommunity

An increasing number of studies are simultaneously investigating species diversity (SD) and genetic diversity (GD) in the same systems, looking for ‘species– genetic diversity correlations’ (SGDCs). From negative to positive SGDCs have been reported, but studies have generally not quantified the processes underlying these correlations. They were also mostly conducted at large biogeographical scales or in recently degraded habitats. Such correlations have not been looked for in natural networks of connected habitat fragments (metacommunities), and the underlying processes remain elusive in most systems. We investigated these issues by studying freshwater snails in a pond network in Guadeloupe (Lesser Antilles). We recorded SD and habitat characteristics in 232 ponds and assessed GD in 75 populations of two species. Strongly significant and positive SGDCs were detected in both species. Based on a decomposition of SGDC as a function of variance–covariance of habitat characteristics, we showed that connectivity (opportunity of water flow between a site and the nearest watershed during the rainy season) has the strongest contribution on SGDCs. More connective sites received both more alleles and more species through immigration resulting in both higher GD and higher SD. Other habitat characteristics did not contribute, or contributed negatively, to SGDCs. This is true of the desiccation frequency of ponds during the dry season, presumably because species markedly differ in their ability to tolerate desiccation. Our study shows that variation in environmental characteristics of habitat patches can promote SGDCs at metacommunity scale when the studied species respond homogeneously to these environmental characteristics.     

Community assembly is a race between immigration and adaptation: eco‐evolutionary interactions across spatial scales

Both ecological and evolutionary mechanisms have been proposed to describe how natural communities become assembled at both regional and biogeographical scales. Yet, these theories have largely been developed in isolation. Here, we unite these separate views and develop an integrated eco‐evolutionary framework of community assembly. We use a simulation approach to explore the factors determining the interplay between ecological and evolutionary mechanisms systematically across spatial scales. Our results suggest that the same set of ecological and evolutionary processes can determine community assembly at both regional and biogeographical scales. We find that the importance of evolution and community monopolization effects, defined as the eco‐evolutionary dynamics that occur when local adaptation of early established immigrants is fast enough to prevent the later immigration of better pre‐adapted species, are not restricted to adaptive radiations on remote islands. They occur at dispersal rates of up to ten individuals per generation, typical for many species at the scale of regional metacommunities. Dispersal capacity largely determines whether ecological species sorting or evolutionary monopolization structure metacommunity diversity and distribution patterns. However, other factors related to the spatial scale at which community assembly processes are acting, such as metacommunity size and the proportion of empty patches, also affect the relative importance of ecology versus evolution. We show that evolution often determines community assembly, and this conclusion is robust to a wide range of assumptions about spatial scale, mode of reproduction, and environmental structure. Moreover, we found that community monopolization effects occur even though species fully pre‐adapted to each habitat are abundant in the metacommunity, a scenario expected a priori to prevent any meaningful effect of evolution. Our results strongly support the idea that the same eco‐evolutionary processes underlie community assembly at regional and biogeographical scales.     

Metacommunity versus Biogeography: A Case Study of Two Groups of Neotropical Vegetation-Dwelling Arthropods

Biogeography and metacommunity ecology provide two different perspectives on species diversity. Both are spatial in nature but their spatial scales do not necessarily match. With recent boom of metacommunity studies, we see an increasing need for clear discrimination of spatial scales relevant for both perspectives. This discrimination is a necessary prerequisite for improved understanding of ecological phenomena across scales. Here we provide a case study to illustrate some spatial scale-dependent concepts in recent metacommunity studies and identify potential pitfalls. We presented here the diversity patterns of Neotropical lepidopterans and spiders viewed both from metacommunity and biogeographical perspectives. Specifically, we investigated how the relative importance of niche- and dispersal-based processes for community assembly change at two spatial scales: metacommunity scale, i.e. within a locality, and biogeographical scale, i.e. among localities widely scattered along a macroclimatic gradient. As expected, niche-based processes dominated the community assembly at metacommunity scale, while dispersal-based processes played a major role at biogeographical scale for both taxonomical groups. However, we also observed small but significant spatial effects at metacommunity scale and environmental effects at biogeographical scale. We also observed differences in diversity patterns between the two taxonomical groups corresponding to differences in their dispersal modes. Our results thus support the idea of continuity of processes interactively shaping diversity patterns across scales and emphasize the necessity of integration of metacommunity and biogeographical perspectives.     

Network isolation and local diversity in neutral metacommunities

Biologists seek an understanding of the biological and environmental factors determining local community diversity. Recent advances in metacommunity ecology, and neutral theory in particular, highlight the importance of dispersal processes interacting with the spatial structure of a landscape for generating spatial patterns and maintaining biodiversity. The relative spatial isolation of a community is traditionally thought to have a large influence on local diversity. However, isolation remains an elusive concept to quantify, particularly in metacommunities with complex spatial structure. We represent the metacommunity as a network of local communities, and use network centrality measures to quantify the isolation of a local community. Using spatially explicit neutral theory, we examine how node position predicts variation in alpha diversity across a metacommunity. We find that diversity increases with node centrality in the network, but only when centrality is measured on a given scale in the network that widens with increasing dispersal rates and narrows with increasing evolutionary rates. More generally, complex biodiversity patterns form only when the underlying geography has structure on this critical scale. This provides a framework for understanding the influence of spatial geographic structure on global biodiversity patterns.     

A process‐based metacommunity framework linking local and regional scale community ecology

The metacommunity concept has the potential to integrate local and regional dynamics within a general community ecology framework. To this end, the concept must move beyond the discrete archetypes that have largely defined it (e.g. neutral vs. species sorting) and better incorporate local scale species interactions and coexistence mechanisms. Here, we present a fundamental reconception of the framework that explicitly links local coexistence theory to the spatial processes inherent to metacommunity theory, allowing for a continuous range of competitive community dynamics. These dynamics emerge from the three underlying processes that shape ecological communities: (1) density‐independent responses to abiotic conditions, (2) density‐dependent biotic interactions and (3) dispersal. Stochasticity is incorporated in the demographic realisation of each of these processes. We formalise this framework using a simulation model that explores a wide range of competitive metacommunity dynamics by varying the strength of the underlying processes. Using this model and framework, we show how existing theories, including the traditional metacommunity archetypes, are linked by this common set of processes. We then use the model to generate new hypotheses about how the three processes combine to interactively shape diversity, functioning and stability within metacommunities.     

Metacommunity dynamics of amphibians in years with differing rainfall

Many studies investigated the habitat preference and behaviour ecology of individual amphibian species while we know less about how their community assembly reflects changes in environmental factors, including the role of climatic extremes. Community-level studies also allow us to apply trait-based analyses that are crucial for a better understanding of the functioning of amphibian communities and metacommunities. In two years with contrasting rainfall (2012 and 2013), we found amphibian species in 85 different waterbodies of a heterogeneous landscape in Central Europe (Hungary). Within the metacommunity framework, the contributions of local, landscape and spatial variables to community assembly were assessed. We also measured the local extinction–colonisation rates in the ponds for all species between the two years. To investigate the role of dispersal traits in explaining the spatial distribution of species, we studied the relationship between body size and the pure spatial fraction of variation. According to our results, the main drivers were the same in both the dry and wet year, but their relative contribution changed. Local variables played a predominant role in the assembly of the amphibian metacommunity. Spatial signals were more evident in the dry year. This implies not only the adverse effect of decreased connectivity due to the drying out of several habitats but also a loss of breeding sites for the studied amphibians. Local colonisation rates were higher in primarily terrestrial species (Hyla arborea, Pelobates fuscus, Bufo bufo) which only visit ponds during breeding. We found a negative relationship between the pure spatial effect and body size, suggesting an increased level of dispersal limitation in small-bodied species. Our results showed that while the strength and relative role of local and spatial processes changed between years, the role of dispersal traits in explaining the spatial distribution of species was similar. Specialisation to different habitats seems to be a major process in determining vertebrate metacommunities in landscapes. Dispersal traits of different species should be taken more into consideration in the practical conservation of amphibian habitats.     

Species versus genotypic diversity of a nitrogen-fixing plant functional group in a metacommunity

Exploring species and genetic diversity interactions provides new opportunities for furthering our understanding of the ecology and evolution of population and community dynamics, and for predicting responses of ecosystems to environmental change. Theory predicts that species diversity within communities and genetic diversity within populations will covary positively, because either species and genetic diversity interact synergistically or they respond in parallel fashion to common habitat conditions. We tested the hypothesis of positive covariation between species and genotypic diversity in a metacommunity of the species-rich southwest Australian flora. We hypothesised that the connection between genotypic diversity and species diversity is strong within functional groups, but weak or non-existent if the species considered extend beyond the functional group. We show that allelic richness of Daviesia triflora, an ant-dispersed pea, covaries positively with the species richness of six co-occurring nitrogen-fixing legume species. No pattern can be detected between allelic richness of D. triflora and species richness of ant-dispersed species when four non-legumes are added. We also show that genetic diversity of D. triflora is not governed by the same environmental factors that determine the presence of a group of large-shrub/small-tree species in the same metacommunity. This study shows that species and genetic diversity covariation are more likely to be confined to within, rather than between, plant functional groups.     

Temporal variation of β-diversity and assembly mechanisms in a bacterial metacommunity

The turnover of community composition across space, β-diversity, is influenced by different assembly mechanisms, which place varying weight on local habitat factors, such as environmental conditions and species interactions, and regional factors such as dispersal and history. Several assembly mechanisms may function simultaneously; however, little is known about how their importance changes over time and why. Here, we implemented a field survey where we sampled a bacterial metacommunity consisting of 17 rock pools located at the Swedish Baltic Sea coast at 11 occasions during 1 year. We determined to which extent communities were structured by different assembly mechanisms using variation partitioning and studied changes in β-diversity across environmental gradients over time. β-Diversity was highest at times of high overall productivity and environmental heterogeneity in the metacommunity, at least partly due to species sorting, that is, selection of taxa by the prevailing environmental conditions. In contrast, dispersal-driven assembly mechanisms were primarily detected at times when β-diversity was relatively low. There were no indications for strong and persistent differences in community composition or β-diversity between permanent and temporary pools, indicating that the physical disturbance regime is of relatively minor importance. In summary, our study clearly suggests that there are temporal differences in the relative importance of different assembly mechanisms related to abiotic factors and shows that the temporal variability of those factors is important for a more complete understanding of bacterial metacommunity dynamics.     

Characterization of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> regrowth patterns suggests a trade-off between undamaged fitness and damage tolerance

Metacommunity ecology has rapidly become a dominant framework through which ecologists understand the natural world. Unfortunately, persistent misunderstandings regarding metacommunity theory and the methods for evaluating hypotheses based on the theory are common in the ecological literature. Since its beginnings, four major paradigms—species sorting, mass effects, neutrality, and patch dynamics—have been associated with metacommunity ecology. The Big 4 have been misconstrued to represent the complete set of metacommunity dynamics. As a result, many investigators attempt to evaluate community assembly processes as strictly belonging to one of the Big 4 types, rather than embracing the full scope of metacommunity theory. The Big 4 were never intended to represent the entire spectrum of metacommunity dynamics and were rather examples of historical paradigms that fit within the new framework. We argue that perpetuation of the Big 4 typology hurts community ecology and we encourage researchers to embrace the full inference space of metacommunity theory. A related, but distinct issue is that the technique of variation partitioning is often used to evaluate the dynamics of metacommunities. This methodology has produced its own set of misunderstandings, some of which are directly a product of the Big 4 typology and others which are simply the product of poor study design or statistical artefacts. However, variation partitioning is a potentially powerful technique when used appropriately and we identify several strategies for successful utilization of variation partitioning.     

Dispersal and diversity in experimental metacommunities: linking theory and practice

There has been a recent rise in the number of experiments investigating the effect of dispersal on diversity, with many of the predictions for these tests derived from metacommunity theory. Despite the promise of linking observed relationships between dispersal and diversity to underlying metacommunity processes, empirical studies have faced challenges in providing robust tests of theory. We review experimental studies that have tested how dispersal affects metacommunity diversity to determine why shortcomings emerge, and to provide a framework for empirical tests of theory that capture the processes structuring diversity in natural metacommunities. We first summarize recent experimental work to outline trends in results and to highlight common methods that cause a misalignment between empirical studies and the processes described by theory. We then identify the undesired implications of three widely used experimental methods that homogenize metacommunity structure or species traits, and present alternative methods that have been used to successfully integrate experiments and theory in a biologically relevant way. Finally, we present methodological and theoretical insights from three related ecological fields (coexistence, food web and priority effects theory) that, if integrated into metacommunity experiments, could help isolate the independent and joint effects of local interactions and dispersal on diversity, and reveal the mechanisms underlying observed dispersal–diversity patterns. Together, these methods can provide stronger tests of existing theory and stimulate new theoretical explorations. Synthesis Although metacommunity experiments offer a unique opportunity to test classic and emerging theory on the relationship between dispersal and diversity, several common challenges have hindered robust tests of theory. We outline how emerging theory on the invasion criterion, food webs and priority effects could be help clarify when and how dispersal affects metacommunity diversity, and identify when experimental approaches that homogenize metacommunities fail to test existing theory. By forging better links between theoretical and empirical work, we hope to motivate novel and improved experimental approaches to understanding the joint effects of local and regional processes on diversity.     

Determinism of bacterial metacommunity dynamics in the southern East China Sea varies depending on hydrography

Spatial variation of communities composition (metacommunities) results from multiple assembly mechanisms, including environmental filtering and dispersal; however, whether and why the relative importance of the assembly mechanisms in shaping bacterial metacommunity changes through time in marine pelagic systems remains poorly studied. Here, we applied the elements of metacommunity structure framework and the variation partitioning framework to examine whether temporal variation of hydrographic conditions influences bacterioplankton metacommunity dynamics in the southern East China Sea (ECS). The spatiotemporal variation of bacterial communities composition was revealed using 454 pyrosequencing of 16S rDNA. In addition to the whole bacterial community, we analyzed four dominant taxonomic groups (Cyanobacteria, Alphaproteobacteria, Gammaproteobacteria, and Actinobacteria) separately. Our analyses indicate that, considering the whole community level, the determinism of metacommunity structure varied among seasons. When the degree of connectivity was low (December), the metacommunity exhibited random distribution and was explained mainly by the environmental component. However, Clementsian metacommunity was found at intermediate connectivity (May), during which the environmental and spatial predictors were both significant. When connectivity was high (August), a random distribution pattern was found and no significant effect of environmental filtering or dispersal limitation was detected. Nevertheless, when considering different taxonomic groups, the differences in metacommunity dynamics among groups were found. Our results suggest that the driving forces of metacommunity dynamics varied depending on hydrography, as the degrees of environmental heterogeneity and connectivity among habitat patches were determined by circulation pattern. Moreover, mechanisms varied among different taxonomic groups, suggesting that differential dispersal capacity among taxonomic groups should be integrated into community assembly studies.     

Genetic structure of a foundation species: scaling community phenotypes from the individual to the region

Understanding the local and regional patterns of species distributions has been a major goal of ecological and evolutionary research. The notion that these patterns can be understood through simple quantitative rules is attractive, but while numerous scaling laws exist (e.g., metabolic, fractals), we are aware of no studies that have placed individual traits and community structure together within a genetics based scaling framework. We document the potential for a genetic basis to the scaling of ecological communities, largely based upon our long-term studies of poplars (Populus spp.). The genetic structure and diversity of these foundation species affects riparian ecosystems and determines a much larger community of dependent organisms. Three examples illustrate these ideas. First, there is a strong genetic basis to phytochemistry and tree architecture (both above- and belowground), which can affect diverse organisms and ecosystem processes. Second, empirical studies in the wild show that the local patterns of genetics based community structure scale up to western North America. At multiple spatial scales the arthropod community phenotype is related to the genetic distance among plants that these arthropods depend upon for survival. Third, we suggest that the familiar species-area curve, in which species richness is a function of area, is also a function of genetic diversity. We find that arthropod species richness is closely correlated with the genetic marker diversity and trait variance suggesting a genetic component to these curves. Finally, we discuss how genetic variation can interact with environmental variation to affect community attributes across geographic scales along with conservation implications.     

Recruitment facilitation can promote coexistence and buffer population growth in metacommunities

Although positive species interactions are ubiquitous in nature, theory has generally focused on the role of negative interactions to explain patterns of species diversity. Here, we incorporate recruitment facilitation, a positive interaction prevalent in marine and terrestrial systems, into a metacommunity framework to assess how the interplay between colonisation, competition and facilitation mediates coexistence. We show that when subordinate species facilitate the recruitment of dominant species, multi-species metacommunities can persist stably even if the colonisation rate of the dominant species is greater than that of the subordinate species. In addition, recruitment facilitation can buffer population growth from changes in colonisation rates, and thus explain the paradoxical mismatch between patterns of abundance and recruitment in marine systems. Overall, our results demonstrate that recruitment facilitation can have profound effects on the assembly, dissolution and regulation of metacommunities by mediating the relative influence of local and regional processes on population abundance and species diversity.     

What can observational data reveal about metacommunity processes?

A key challenge for community ecology is to understand to what extent observational data can be used to infer the underlying community assembly processes. As different processes can lead to similar or even identical patterns, statistical analyses of non‐manipulative observational data never yield undisputable causal inference on the underlying processes. Still, most empirical studies in community ecology are based on observational data, and hence understanding under which circumstances such data can shed light on assembly processes is a central concern for community ecologists. We simulated a spatial agent‐based model that generates variation in metacommunity dynamics across multiple axes, including the four classic metacommunity paradigms as special cases. We further simulated a virtual ecologist who analysed snapshot data sampled from the simulations using eighteen output metrics derived from beta‐diversity and habitat variation indices, variation partitioning and joint species distribution modelling. Our results indicated two main axes of variation in the output metrics. The first axis of variation described whether the landscape has patchy or continuous variation, and thus was essentially independent of the properties of the species community. The second axis of variation related to the level of predictability of the metacommunity. The most predictable communities were niche‐based metacommunities inhabiting static landscapes with marked environmental heterogeneity, such as metacommunities following the species sorting paradigm or the mass effects paradigm. The most unpredictable communities were neutral‐based metacommunities inhabiting dynamics landscapes with little spatial heterogeneity, such as metacommunities following the neutral or patch sorting paradigms. The output metrics from joint species distribution modelling yielded generally the highest resolution to disentangle among the simulated scenarios. Yet, the different types of statistical approaches utilized in this study carried complementary information, and thus our results suggest that the most comprehensive evaluation of metacommunity structure can be obtained by combining them.     

Species and individual replacements contribute more than nestedness to shape vertebrate scavenger metacommunities

Understanding the mechanisms that organize biodiversity is central in ecology and conservation. Beta diversity links local (alfa) and regional (gamma) diversity, giving insight into how communities organize spatially. Metacommunity ecology provides the framework to interpret regional and local processes interacting to shape communities. However, the lack of metacommunity studies for large vertebrates may limit the understanding and compromise the preservation of ecosystem functions and services. We aim to understand the mechanisms underlying differences in species composition among vertebrate scavenger communities ? which provide key ecosystem functions, e.g. carrion consumption ? within a metacommunity context. We obtained species richness and abundances at scavenger communities consuming ungulate carcasses monitored through motion‐triggered remote cameras in seven terrestrial ecosystems in Spain. We partitioned beta diversity to decompose incidence‐based (species presence/absence) and abundance‐based dissimilarities into their components (turnover/balanced variation and nestedness/abundance gradient, respectively). We identified the environmental factors explaining the observed patterns. The vertebrate scavenger metacommunity consisted of 3101 individuals from 30 species. Changes in composition among ecosystems were mostly (> 84%) due to species or individual replacement (i.e. turnover or balanced variation). Species or individual loss/gain (i.e. nestedness or abundance gradient) accounted for 13–16% of these changes. Mean carcass weight, elevation and habitat diversity were the main factors explaining species/individual replacement. Our findings suggest that local processes such as species‐sorting through habitat heterogeneity would dominate scavenger metacommunity dynamics together with stochastic forces (i.e. related to carrion unpredictability and scavenging being a widespread strategy among vertebrates). The presence of structured patterns (i.e. nestedness) in beta diversity could reflect a role of deterministic processes: mass‐effects through dispersal and defaunation. Vultures are long‐distance foragers and functionally dominant species, which would connect local assemblages within the metacommunity, supporting scavenger diversity and functions across space. These results highlight the importance of managing vertebrate scavenger assemblages within a metacommunity context.     

Genetic variation of loci potentially under selection confounds species–genetic diversity correlations in a fragmented habitat

Positive species–genetic diversity correlations (SGDCs) are often thought to result from the parallel influence of neutral processes on genetic and species diversity. Yet, confounding effects of non‐neutral mechanisms have not been explored. Here, we investigate the impact of non‐neutral genetic diversity on SGDCs in high Andean wetlands. We compare correlations between plant species diversity and genetic diversity (GD) calculated with and without loci potentially under selection (outlier loci). The study system includes 2188 specimens from five species (three common aquatic macroinvertebrate and two dominant plant species) that were genotyped for 396 amplified fragment length polymorphism loci. We also appraise the importance of neutral processes on SGDCs by investigating the influence of habitat fragmentation features. Significant positive SGDCs were detected for all five species (mean SGDC = 0.52 ± 0.05). While only a few outlier loci were detected in each species, they resulted in significant decreases in GD and in SGDCs. This supports the hypothesis that neutral processes drive species–genetic diversity relationships in high Andean wetlands. Unexpectedly, the effects on genetic diversity GD of the habitat fragmentation characteristics in this study increased with the presence of outlier loci in two species. Overall, our results reveal pitfalls in using habitat features to infer processes driving SGDCs and show that a few loci potentially under selection are enough to cause a significant downward bias in SGDC. Investigating confounding effects of outlier loci thus represents a useful approach to evidence the contribution of neutral processes on species–genetic diversity relationships.