Community patterns in source-sink metacommunities

We present a model of a source-sink competitive metacommunity, defined as a regional set of communities in which local diversity is maintained by dispersal. Although the conditions of local and regional coexistence have been well defined in such systems, no study has attempted to provide clear predictions of classical community-wide patterns. Here we provide predictions for species richness, species relative abundances, and community-level functional properties (productivity and space occupation) at the local and regional scales as functions of the proportion of dispersal between communities. Local (alpha) diversity is maximal at an intermediate level of dispersal, whereas between-community (beta) and regional (gamma) diversity decline as dispersal increases because of increased homogenization of the metacommunity. The relationships between local and regional species richness and the species rank abundance distributions are strongly affected by the level of dispersal. Local productivity and space occupation tend to decline as dispersal increases, resulting in either a hump-shaped or a positive relationship between species richness and productivity, depending on the scale considered (local or regional). These effects of dispersal are buffered by decreasing species dispersal success. Our results provide a niche-based alternative to the recent neutral-metacommunity model and have important implications for conservation biology and landscape management.     

Diversity and community biomass depend on dispersal and disturbance in microalgal communities

The evidence for species diversity effects on ecosystem functions is mainly based on studies not explicitly addressing local or regional processes regulating coexistence or the importance of community structure in terms of species evenness. In experimental communities of marine benthic microalgae, we altered the successional stages and thus the strength of local species interactions by manipulating rates of dispersal and disturbance. The treatments altered realized species richness, evenness and community biomass. For species richness, dispersal mattered only at high disturbance rates; when opening new space, dispersal led to maximized richness at intermediate dispersal rates. Evenness, in contrast, decreased with dispersal at low or no disturbance, i.e. at late successional stages. Community biomass showed a non-linear hump-shaped response to increasing dispersal at all disturbance levels. We found a positive correlation between richness and biomass at early succession, and a strong negative correlation between evenness and biomass at late succession. In early succession both community biomass and richness depend directly on dispersal from the regional pool, whereas the late successional pattern shows that if interactions allow the most productive species to become dominant, diverting resources from this species (i.e. higher evenness) reduces production. Our study emphasizes the difference in biodiversity–function relationships over time, as different mechanisms contribute to the regulation of richness and evenness in early and late successional stages.     

Dispersal rates affect species composition in metacommunities of Sarracenia purpurea inquilines

Dispersal among local communities can have a variety of effects on species composition and diversity at local and regional scales. Local conditions (e.g., resource and predator densities) can have independent effects, as well as interact with dispersal, to alter these patterns. Based on metacommunity models, we predicted that local diversity would show a unimodal relationship with dispersal frequency. We manipulated dispersal frequencies, resource levels, and the presence of predators (mosquito larvae) among communities found in the water-filled leaves of the pitcher plant Sarracenia purpurea. Diversity and abundance of species of the middle trophic level, protozoa and rotifers, were measured. Increased dispersal frequencies significantly increased regional species richness and protozoan abundance while decreasing the variance among local communities. Dispersal frequency interacted with predation at the local community scale to produce patterns of diversity consistent with the model. When predators were absent, we found a unimodal relationship between dispersal frequency and diversity, and when predators were present, there was a flat relationship. Intermediate dispersal frequencies maintained some species in the inquiline communities by offsetting extinction rates. Local community composition and the degree of connectivity between communities are both important for understanding species diversity patterns at local and regional scales.     

Dispersal ability links to cross‐scale species diversity patterns across the Eurasian Arctic tundra

Aim The role of dispersal in structuring biodiversity across spatial scales is controversial. If dispersal controls regional and local community assembly, it should also affect the degree of spatial species turnover as well as the extent to which regional communities are represented in local communities. Here we provide the first integrated assessment of relationships between dispersal ability and local‐to‐regional spatial aspects of species diversity across a large geographical area. Location Northern Eurasia. Methods Using a cross‐scale analysis covering local (0.64 m²) to continental (the Eurasian Arctic biome) scales, we compared slope parameters of the dissimilarity‐to‐distance relationship in species composition and the local‐to‐regional relationship in species richness among three plant‐like groups that differ in dispersal ability: lichens with the highest dispersal ability; mosses and moss allies with intermediate dispersal ability; and seed plants with the lowest dispersal ability. Results Diversity patterns generally differed between the three groups according to their dispersal ability, even after controlling for niche‐based processes. Increasing dispersal ability is linked to decreasing spatial species turnover and an increasing ratio of local to regional species richness. All comparisons supported our expectations, except for the slope of the local‐to‐regional relationship in species richness for mosses and moss allies which was not significantly steeper than that of seed plants. Main conclusions The negative link between dispersal ability and spatial species turnover and the corresponding positive link between dispersal ability and the ratio of local‐to‐regional species richness support the idea that dispersal affects community structure and diversity patterns across spatial scales.     

Warming induces synchrony and destabilizes experimental pond zooplankton metacommunities

The spatial insurance hypothesis predicts that intermediate rates of dispersal between patches in a metacommunity allow species to track favourable conditions, preserving diversity and stabilizing biomass at local and regional scales. However, theory is unclear as to whether dispersal will provide spatial insurance when environmental conditions are changing directionally. In particular, increased temperatures as a result of climate change are expected to cause synchronous growth or decline across species and communities, and this has the potential to erode the stabilizing compensatory dynamics facilitated by dispersal. Here we report on an experimental test of how dispersal affects the diversity and stability of metacommunities under warming using replicate two‐patch pond zooplankton metacommunities. Initial differences in local community composition and abiotic conditions were established by seeding each patch in the metacommunities with plankton and sediment from one of two natural ponds that differed in water chemistry and species composition. We exposed metacommunities to a 2°C increase in average ambient temperature, crossed with three rates of dispersal (none, intermediate, high). In ambient conditions, intermediate dispersal rates preserved diversity and stabilized metacommunities by promoting spatially asynchronous fluctuations in biomass, especially between local populations of the dominant genus, Ceriodaphnia. However, warming synchronized their populations so that these effects of dispersal were lost. Furthermore, because the stabilizing effect of dispersal was primarily due to asynchronous fluctuations between populations of a single genus, metacommunity biomass was stabilized, but dispersal did not stabilize local community biomass. Our results show that dispersal can preserve diversity and provide stability to metacommunities, but also show that this benefit can be eroded when warming is directional and synchronous across patches of a metacommunity, as is expected with climate warming.     

Consequences of varying regional heterogeneity in source–sink metacommunities

Although the influence of dispersal on coexistence mechanisms in metacommunities has received great emphasis, few studies have addressed how such influence is affected varying regional heterogeneity. We present a mechanistic model of resource competition in a metacommunity based on classical models of plant competition for limiting resources. We defined regional heterogeneity as the differences in resource supply rates (or resource availabilities) across local communities. As suggested by previous work, the highest diversify occurred at intermediate levels of dispersal among local communities. However our model shows how the effects of dispersal depend on the amount of heterogeneity among local communities and vice versa. Both regional and local species richness were the highest when heterogeneity was intermediate. We suggest that empirical studies that found no evidence for source–sink or mass effects at the community level may have examined communities with limited ranges of dispersal and regional heterogeneity. This model of species coexistence contributes to a broader understanding of patterns in real communities.     

A keystone ant species promotes seed dispersal in a “diffuse” mutualism

In order to understand the dynamics of co-evolution it is important to consider spatial variation in interaction dynamics. We examined the relative importance of ant activity, diversity and species identity in an ant seed dispersal mutualism at local, regional and continental scales. We also studied the determinants of seed dispersal rates and dispersal distances at eight sites in the Eneabba sandplain (29.63 S, 115.22 E), western Australia to understand local variation in seed dispersal rate and distance. To test the generality of the conclusions derived from the eight local sites, we established 16 sites along a 1650-km transect in western Australia, covering 11° of latitude and a six-fold increase in rainfall, at which we sampled the ant assemblage, estimated ant species richness and ant activity and observed the removal rate of myrmecochorous seeds. We also assessed the importance of ant species identity at a continental scale via a review of studies carried out throughout Australia which examined ant seed dispersal. Among the eight sandplain shrubland sites, ant species identity, in particular the presence of one genus, Rhytidoponera, was associated with the most dispersal and above average dispersal distances. At the landscape scale, Rhytidoponera presence was the most important determinant of seed removal rate, while seed removal rate was negatively correlated with ant species richness and latitude. Most ant seed removal studies carried out throughout Australia reinforce our observations that Rhytidoponera species were particularly important seed dispersers. It is suggested that superficially diffuse mutualisms may depend greatly on the identity of particular partners. Even at large biogeographic scales, temporal and spatial variation in what are considered to be diffuse mutualisms may often be linked to variation in the abundance of particular partners, and be only weakly – or negatively – associated with the diversity of partners.     

Sources of diversity in a grassland metacommunity: quantifying the contribution of dispersal to species richness

Metacommunity theory suggests a potentially important role for dispersal in diversity maintenance at local, as well as regional, scales. In addition, propagule addition experiments have shown that dispersal often limits local diversity. However, actual dispersal rates into local communities and the contribution of immigrants to observed local diversity are poorly known. We present a new approach that partitions the diversity of a target community into dispersal-maintained and dispersal-independent components. Specifically, we quantify distances through space and time to the nearest potential seed source for naturally occurring recruits in target communities by using hierarchical data on species pools (local, site, region, and seed bank). Using this "recruit tag" approach, we found that dispersal contributed 29%-57% of the seedling diversity in perennial grasslands with different successional histories. However, both dispersal and seedling mortality remained remarkably constant, in absolute terms, over succession. The considerable loss of diversity over secondary succession (66%), therefore, could be understood only by considering how these processes interact with the decreasing disturbance rate (i.e., frequency of gaps) in later-successional sites. We conclude that a metacommunity perspective is relevant and necessary to understand the diversity and community assembly of this study system.     

The role of habitat connectivity and landscape geometry in experimental zooplankton metacommunities

Theory predicts that inter-patch dispersal rates and patterns of patch heterogeneity both have the potential to alter patterns of local and regional species diversity. To test this, we manipulated both rates of habitat connectivity and the geometric arrangement of habitat heterogeneity within regions of experimental zooplankton communities. We found no effects of habitat geometry on any metric of species diversity or composition. Additionally, we found no effect of habitat connectivity rate on local species diversity. We did, however, find that increasing connectivity led to a decrease in regional diversity, as well as an increase in the percent similarity of local communities within regions. Of all of the species in these communities, the relatively large cladoceran Ceriodaphnia reticulata significantly responded to the treatments, and had a higher probability of achieving high densities when connectance was high. As such, we suggest that this species played a large role in driving the increased local community similarity and decreased regional species richness as connectivity increased. These findings are in opposition to previous experimental studies of metacommunities, but support the notion that increased connectance among local patches may decrease regional diversity when patches are heterogeneous.     

Community assembly along a species pool gradient: implications for multiple-scale patterns of species diversity

Various ecological processes influence patterns of species diversity at multiple spatial scales. One process that is potentially important but rarely considered is community assembly. I assembled model communities using species pools of differing size to examine how the history of community assembly may affect multi-scale diversity patterns. The model contained three scales at which diversity could be measured: local community, metacommunity, and species pool. Local species saturation occurred, as expected from the competition and predation built in the model. However, local communities did not become resistant to invasions except when the species pool was very small. Depending on dispersal rate and trophic level, the larger the species pool, the harder it was to predict which species invades which local community at a given time. Consequently, local-community dissimilarity maintained by assembly history increased linearly with pool size, even though local diversity was decoupled from pool size. These results have two implications for multi-scale diversity patterns. First, assembly history may provide an explanation for scale-dependent relationships between local and regional diversity: assembly causes the relationship to be curvilinear at one scale (local community), while linear at another (metacommunity). Second, assembly history influences how -diversity is partitioned into - and -diversity: assembly causes the relative contribution of to increase with pool size. Overall, this study suggests that community assembly history interacts with species pool size to regulate multi-scale patterns of species diversity.     

Scale-dependent relationships between native richness, resource stability and exotic cover in dock fouling communities of Washington, USA

Aim In terrestrial plant communities, the relationship between native species diversity and exotic success is typically scale‐dependent. It is often proposed that within local neighbourhoods, high native diversity limits resources, thereby inhibiting exotic success. However, environmental variation that manifests over space or time can create positive correlations between native diversity and exotic success at larger scales. In marine habitats, there have been few multi‐scale surveys of this pattern, so it is unclear how diversity, resource limitation and the environment influence the success of exotic species in these systems. Location Washington, USA. Methods I analysed nested spatial and temporal surveys of fouling communities, which are assemblages of sessile marine invertebrates, to test whether the relationships between native richness, resource availability and exotic cover supported the diversity‐stability and diversity‐resistance theories, to test whether these relationships changed with spatio‐temporal scale, and to explore the temperature preferences of native and exotic fouling species. Results Survey data failed to support diversity‐stability theory: space availability actually increased with native richness at the local neighbourhood scale, and neither space availability nor variability decreased with native richness across larger spatio‐temporal scales. I did find support for diversity‐resistance theory, as richness negatively correlated with exotic cover in local neighbourhoods. Unexpectedly, this negative correlation disappeared at intermediate scales, but emerged again at the regional scale. This scale‐dependent pattern could be partially explained by contrasting water temperature preferences of native and exotic species. Main conclusions Within local neighbourhoods, native diversity may inhibit exotic abundance, but the mechanism is unlikely related to resource limitation. At the largest scale, correlations suggest that native richness is higher in cooler environments, whereas exotic richness is higher in warmer environments. This large‐scale pattern contrasts with the typical plant community pattern, and has important implications for coastal management in the face of global climate change.     

Overdispersion of body size in Australian desert lizard communities at local scales only: no evidence for the Narcissus effect

Both local and regional processes may contribute to community diversity and structure at local scales. Although many studies have investigated patterns of local or regional community structure, few have addressed the extent to which local community structure influences patterns within regional species pools. Here we investigate the role of body size in community assembly at local and regional scales in Ctenotus lizards from arid Australia. Ctenotus has long been noted for its exceptional species diversity in the Australian arid-zone, and previous studies have attempted to elucidate the processes underlying species coexistence within communities of these lizards. However, no consensus has emerged on the role of interspecific competition in the assembly and maintenance of Ctenotus communities. We studied Ctenotus communities at several hundred sites in the arid interior of Australia to test the hypothesis that body sizes within local and regional Ctenotus assemblages should be overdispersed relative to null models of community assembly, and we explored the relationship between body size dispersion at local and regional scales. Results indicate a striking pattern of community-wide overdispersion of body size at local scales, as measured by the variance in size ratios among co-occurring species. However, we find no evidence for body size overdispersion within regional species pools, suggesting a lack of correspondence between processes influencing the distribution of species phenotypes at local and regional scales. We suggest that size ratio constancy in Ctenotus communities may have resulted from contemporary ecological interactions among species or ecological character displacement, and we discuss alternative explanations for the observed patterns. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Interaction strength and extinction risk in a metacommunity

Species interactions and connectivity are both central to explaining the stability of ecological communities and the problem of species extinction. Yet, the role of species interactions for the stability of spatially subdivided communities still eludes ecologists. Ecological models currently address the problem of stability by exploring the role of interaction strength in well mixed habitats, or of connectivity in subdivided communities. Here I propose a unification of interaction strength and connectivity as mechanisms explaining regional community stability. I introduce a metacommunity model based on succession dynamics in coastal ecosystems, incorporating limited dispersal and facilitative interactions. I report a sharp transition in regional stability and extinction probability at intermediate interaction strength, shown to correspond to a phase transition that generates scale-invariant distribution and high regional stability. In contrast with previous studies, stability results from intermediate interaction strength only in subdivided communities, and is associated with large-scale (scale-invariant) synchrony. These results can be generalized to other systems exhibiting phase transitions to show how local interaction strength can be used to resolve the link between regional community stability and pattern formation.     

Dark diversity in dry calcareous grasslands is determined by dispersal ability and stress‐tolerance

Temperate calcareous grasslands are characterized by high levels of species richness at small spatial scales. Nevertheless, many species from a habitat‐specific regional species pool may be absent from local communities and represent the ‘dark diversity’ of these sites. Here we investigate dry calcareous grasslands in northern Europe to determine what proportion of the habitat‐specific species pool is realized at small scales (i.e. how the community completeness varies) and which mechanisms may be contributing to the relative sizes of the observed and dark diversity. We test whether the absence of particular species in potentially suitable grassland sites is a consequence of dispersal limitation and/or a low ability to tolerate stress (e.g. drought and grazing). We analysed a total of 1223 vegetation plots (1 × 1 m) from dry calcareous grasslands in Sweden, Estonia and western Russia. The species co‐occurrence approach was used to estimate the dark diversity for each plot. We calculated the maximum dispersal distance for each of the 291 species in our dataset by using simple plant traits (dispersal syndrome, growth form and seed characteristics). Large seed size was used as proxy for small seed number; tall plant height and low S‐strategy type scores were used to characterise low stress‐tolerance. Levels of small‐scale community completeness were relatively low (more species were absent than present) and varied between the grasslands in different geographic areas. Species in the dark diversity were generally characterized by shorter dispersal distances and greater seed weight (fewer seeds) than species in the observed diversity. Species within the dark diversity were generally taller and had a lower tolerance of stressful conditions. We conclude that, even if temperate grasslands have high levels of small‐scale plant diversity, the majority of potentially suitable species in the regional species pool may be absent as a result of dispersal limitation and low stress‐tolerance.     

Dispersal, spatial scale, and species diversity in a hierarchically structured experimental landscape

Although there has been growing interest in the effect of dispersal on species diversity, much remains unknown about how dispersal occurring at multiple scales influences diversity. We used an experimental microbial landscape to determine whether dispersal occurring at two different scales - among local communities and among metacommunities - affects diversity differently. At the local scale, dispersal initially had a positive effect and subsequently a neutral effect on diversity, whereas at the metacommunity and landscape scales, dispersal showed a consistently negative effect. The timing in which dispersal affected beta diversity also differed sharply between local communities and metacommunities. These patterns were explained by scale- and time-dependent effects of dispersal in allowing spread of species and in removing spatial refuges from predators. Our results suggest that the relative contribution of opposing mechanisms by which dispersal affects diversity changes considerably over time and space in hierarchical landscapes in which dispersal occurs at multiple scales.     

Dispersal and seed limitation affect diversity and productivity of montane grasslands

So far, seed limitation as a local process, and dispersal limitation as a regional process have been largely neglected in biodiversity–ecosystem functioning research. However, these processes can influence both local plant species diversity and ecosystem processes, such as biomass production. We added seeds of 60 species from the regional species pool to grassland communities at 20 montane grassland sites in Germany. In these sites, plant species diversity ranged from 10 to 34 species m⁻² and, before manipulation, diversity was not related to aboveground biomass, which ranged from 108 to 687 g m⁻². One year after seed addition, local plant species richness had increased on average by six species m⁻² (29%) compared with control plots, and this increase was highest in grasslands with intermediate productivity. The increased diversity after adding seeds was associated with an average increase of aboveground biomass of 36 g m⁻² (14.8%) compared with control plots. Thus, our results demonstrate that a positive relationship between changes in species richness and productivity, as previously reported from experimental plant communities, also holds for natural grassland ecosystems. Our results show that local plant communities are dispersal limited and a hump‐shaped model appears to be the limiting outline of the natural diversity–productivity relationship. Hence, the effects of dispersal on local diversity can substantially affect the functioning of natural ecosystems.     

Connectivity, scale-dependence, and the productivity–diversity relationship

We surveyed freshwater ponds (localities) nested within watersheds (regions) to evaluate the relationship between productivity and animal species richness at different spatial scales. In watersheds where the ponds were relatively distant from one another (likely reducing the level of interpond dispersal of many organisms), we found a scale‐dependent productivity–diversity relationship; at local scales (among ponds), diversity was a hump‐shaped function of productivity, whereas at regional scales (among watersheds), diversity monotonically increased with productivity. Furthermore, this relationship emerged because there was a strong relationship between productivity and pond‐to‐pond species compositional differences. Alternatively, in watersheds where ponds were relatively close together (likely leading to higher rates of dispersal of many organisms), we found no scale‐dependence; diversity was a hump‐shaped function of productivity at both local and regional scales. Here, the relationship between species compositional dissimilarity and productivity was much weaker. We conclude that whether or not scale‐dependence is observed in productivity–diversity relationships will depend, at least in part, on the degree of connectivity among localities within regions.     

Regional effects as important determinants of local diversity in both marine and terrestrial systems

Community ecologists have struggled to create unified theories across diverse ecosystems, but it has been difficult to acertain whether marine and terrestrial communities differ in the mechanisms responsible for structure and dynamics. One apparent difference between marine and terrestrial ecology is that the influence of regional processes on local populations and communities is better established in the marine literature. We examine three potential explanations: 1) influential early studies emphasized local interactions in terrestrial communities and regional dispersal in marine communities. 2) regional‐scale processes are actually more important in marine than in terrestrial communities. 3) recruitment from a regional species pool is easier to study in marine than terrestrial communities. We conclude that these are interrelated, but that the second and especially the third explanations are more important than the first. We also conclude that in both marine and terrestrial systems, there are ways to improve our understanding of regional influences on local community diversity. In particular, we advocate examining local vs regional diversity relationships at localities within environmentally similar regions that differ in their diversity either because of their sizes or their varying degrees of isolation from a species source.     

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.     

Disentangling the effects of local and regional processes on biodiversity patterns through taxon‐contingent metacommunity network analysis

Metacommunity theory, which has gained a central position in ecology, accounts for the role of migration in patterns of diversity among communities at different scales. Community isolation has a main role in this theory, but is difficult to estimate empirically, partly due to the taxon‐dependent nature of dispersal. Landscapes could be perceived as either fragmented or connected for organisms with contrasting dispersal abilities. Indeed, the dispersal ability of a taxon, and the spatial scale at which eco‐evolutionary processes shape local diversity, determine a taxon‐dependent metacommunity network. In this paper, we introduce a methodology using graph theory to define this taxon‐dependent metacommunity network and then to estimate the isolation of local communities. We analyzed the relative importance of local conditions versus community isolation as determinants of community richness for 25 taxa inhabiting 18 temporary ponds. Although local factors have been the foci of most previous empirical and theoretical considerations, we demonstrate that the metacommunity network is an equally important contributor to local diversity. We also found that the relative effect of local conditions and the metacommunity network depend on body size and taxon abundance. Local diversity of larger species was more affected by patch isolation, while taxon abundances were associated with positive or negative effects of isolation. Our results provide empirical support for the proposed role of metacommunity networks as determinants of community diversity and show the taxon‐dependent nature of these networks.