Quantifying the importance of regional and local filters for community trait structure in tropical and temperate zones

The influence of regional and local processes on community structure is a major focus of ecology. Classically, ecologists have used local-regional richness regressions to evaluate the role of local and regional processes in determining community structure, an approach that has numerous flaws. Here, we implemented a novel trait-based approach that treats local and regional influences as a continuum, rather than a dichotomy. Using hylid frogs (Hylidae), we compared trait dispersion among members of local species assemblages to the trait dispersion in the regional assemblage from which they were drawn. Similarly, we compared trait dispersion in the regional assemblages to dispersion in the continental species pool. We estimated the contributions of local and regional filters, and we compared their strength in temperate and tropical zones. We found that regional and local filters explained 80% of the total variation among local assemblages in community body size dispersion. Overall, regional filters reduced trait dispersion, and local filters increased it, a pattern driven by particularly strong antagonistic effects in temperate zones that reduced the realized total variation by more than 40%. In contrast, local and regional filters acted in concert in tropical regions. Patterns within the tropics did not differ from the random expectation based on a null model, but within the temperate zone, local community filtering was stronger than expected by chance. Furthermore, in temperate regions, antagonistic regional and local filtering masked from 76% to 90% of the total variation in trait dispersion. Together, these results suggest that there are fundamental differences in the scale and identity of the processes determining community structure in temperate and tropical regions.     

Latitudinal variation in local interactions and regional enrichment shape patterns of marine community diversity

While communities are shaped by both local interactions and enrichment from the regional species pool, we propose a hypothesis that the balance of these forces shifts with latitude, with regional enrichment dominating at high latitudes and local interactions dominating at low latitudes. To test this hypothesis, we conducted a latitudinal-scale experiment with marine epifaunal communities. In four regions of the North Atlantic Ocean and Caribbean Sea, we used mimics of ecosystem engineers to manipulate biogenic structural complexity. We iteratively evaluated diversity patterns of experimental communities up to one year after deployment. Additional data were also collected from one of our tropical sites 2.5 years after initial deployment. As hypothesized, we found a reciprocal latitudinal gradient in the effects of the structurally complex mimics and regional enrichment. In the tropics, local diversity was always higher in association with the mimics than in exposed areas that were more open to predation. This effect was consistent across two spatial scales and beyond the one-year timescale of the experiment. In temperate communities, no consistent effects of the mimics on diversity were observed. However, the proportion of species from the regional species pool that were present at the local scale increased from the tropics to the temperate zone, consistent with the hypothesis that higher-latitude communities may experience greater influence from the regional species pool than communities at low latitudes. This study represents the first large-scale experimental demonstration that suggests that the relative impact of local interactions and regional enrichment on community diversity may depend on latitude.     

Scale-dependent variation in coral community similarity across sites, islands, and island groups

Community similarity is the proportion of species richness in a region that is shared on average among communities within that region. The slope of local richness (alpha diversity) regressed on regional richness (gamma diversity) can serve as an index of community similarity across regions with different regional richness. We examined community similarity in corals at three spatial scales (among transects at a site, sites on an island, and islands within an island group) across a 10 000-km longitudinal diversity gradient in the west-central Pacific Ocean. When alpha diversity was regressed on gamma diversity, the slopes, and thus community similarity, increased with scale (0.085, 0.261, and 0.407, respectively) because a greater proportion of gamma diversity was subsumed within alpha diversity as scale increased. Using standard randomization methods, we also examined how community similarity differed between observed and randomized assemblages and how this difference was affected by spatial separation of species within habitat types and specialization of species to three habitat types (reef flats, crests, and slopes). If spatial separation within habitat types and/or habitat specialization (i.e., underdispersion) occurs, fewer species are shared among assemblages than the random expectation. When the locations of individual coral colonies were randomized within and among habitat types, community similarity was 46-47% higher than that for observed assemblages at all three scales. We predicted that spatial separation of coral species within habitat types should increase with scale due to dispersal/extinction dynamics in this insular system, but that specialization of species to different habitat types should not change because habitat differences do not change with scale. However, neither habitat specialization nor spatial separation within habitat types differed among scales. At the two larger scales, each accounted for 22-24% of the difference in community similarity between observed and randomized assemblages. At the smallest scale (transect-site), neither spatial separation within habitat types nor habitat specialization had significant effects on community similarity, probably due to the small size of transect samples. The results suggest that coral species can disperse among islands in an island group as easily as they can among sites on an island over time scales that are relevant to their establishment and persistence on reefs.     

Species dispersal rates alter diversity and ecosystem stability in pond metacommunities

Metacommunity theory suggests that relationships between diversity and ecosystem stability can be determined by the rate of species dispersal among local communities. The predicted relationships, however, may depend upon the relative strength of local environmental processes and disturbance. Here we evaluate the role of dispersal frequency and local predation perturbations in affecting patterns of diversity and stability in pond plankton metacommunities. Pond metacommunities were composed of three mesocosm communities: one of the three communities maintained constant "press" predation from a selective predator, bluegill sunfish (Lepomis macrochirus); the second community maintained "press" conditions without predation; and the third community experienced recurrent "pulsed" predation from bluegill sunfish. The triads of pond communities were connected at either no, low (0.7%/d), or high (20%/d) planktonic dispersal. Richness and composition of zooplankton and stability of plankton biomass and ecosystem productivity were measured at local and regional spatial scales. Dispersal significantly affected diversity such that local and regional biotas at the low dispersal rate maintained the greatest number of species. The unimodal local dispersal-diversity relationship was predator-dependent, however, as selective press predation excluded species regardless of dispersal. Further, there was no effect of dispersal on beta diversity because predation generated local conditions that selected for distinct community assemblages. Spatial and temporal ecosystem stability responded to dispersal frequency but not predation. Low dispersal destabilized the spatial stability of producer biomass but stabilized temporal ecosystem productivity. The results indicate that selective predation can prevent species augmentation from mass effects but has no apparent influence on stability. Dispersal rates, in contrast, can have significant effects on both species diversity and ecosystem stability at multiple spatial scales in metacommunities.     

Paleobiology, community ecology, and scales of ecological pattern

The fossil record provides a wealth of data on the role of regional processes and historical events in shaping biological communities over a variety of time scales. The Quaternary record with its evidence of repeated climatic change shows that both terrestrial and marine species shifted independently rather than as cohesive assemblages over scales of thousands of years. Larger scale patterns also show a strong individualistic component to taxon dynamics; assemblage stability, when it occurs, is difficult to separate from shared responses to low rates of environmental change. Nevertheless, the fossil record does suggest that some biotic interactions influence large-scale ecological and evolutionary patterns, albeit in more diffuse and protracted fashions than those generally studied by community ecologists. These include: (1) the resistance by incumbents to the establishment of new or invading taxa, with episodes of explosive diversification often appearing contingent on the removal of incumbents at extinction events; (2) steady states of within-habitat and global diversity at longer time scales (10(7)-l0(8) yr), despite enormous turnover of taxa; and (3) morphological and biogeographic responses to increased intensities of predation and substratum disturbance over similarly long time scales. The behavior of species and communities over the array of temporal and spatial scales in the fossil record takes on additional significance for framing conservation strategies, and for understanding recovery of species, lineages, and communities from environmental changes.     

Metacommunity influences on community richness at multiple spatial scales: a microcosm experiment

Large-scale processes are known to be important for patterns of species richness, yet the ways in which local and larger scale processes interact is not clear. I used metacommunities consisting of five interconnected microbial aquatic communities to examine the manner in which processes at different scales affect local and metacommunity richness. Specifically, I manipulated the potential dispersal rate, whether dispersal was localized or global, and variation in initial community composition. A repeated-measures ANOVA showed that a low dispersal rate and intermediate distance dispersal enhanced local richness. Initial assembly variation had no effect on local richness, while a lack of dispersal or global dispersal reduced local richness. At the metacommunity scale, richness was enhanced throughout the time course of the experiment by initial compositional variation and was reduced by high or global dispersal. The effects of dispersal were contingent on the presence of initial compositional variation. The treatments also affected individual species occupancy patterns, with some benefiting from large-scale processes and others being adversely impacted. These results indicate that the effects of dispersal on species richness have a complex relationship with scale and are not solely divisible into "regional" vs. "local" scales. Finally, predictions of the manner in which dispersal rate structures communities appear dependent upon species compositional variation among communities.     

Regional variation in the hierarchical partitioning of diversity in coral-dwelling fishes

The size of the regional species pool may influence local patterns of diversity. However, it is unclear whether certain spatial scales are less sensitive to regional influences than others. Additive partitioning was used to separate coral-dwelling fish diversity to its alpha and beta components, at multiple scales, in several regions across the Indo-Pacific. We then examined how the relative contribution of these components changes with increased regional diversity. By employing specific random-placement null models, we overcome methodological problems with local-regional regressions. We show that, although alpha and beta diversities within each region are consistently different from random-placement null models, the increase in beta diversities among regions was similar to that predicted once heterogeneity in coral habitat was accounted for. In contrast, alpha diversity within single coral heads was limited and increased less than predicted by the null models. This was correlated with increased intraspecific aggregation in more diverse regions and is consistent with ecological limitations on the number of coexisting species at the local scale. These results suggest that, apart from very small spatial scales, variation in the partitioning of fish diversity along regional species richness gradients is driven overwhelmingly by the corresponding gradients in coral assemblage structure.     

Life history traits predict relative abundance in an assemblage of forest caterpillars

Species in a given trophic level occur in vastly unequal abundance, a pattern commonly documented but poorly explained for most taxa. Theoretical predictions of species density such as those arising from the metabolic theory of ecology hold well at large spatial and temporal scales but are not supported in many communities sampled at a relatively small scale. At these scales ecological factors may be more important than the inherent limits to energy use set by allometric scaling of mass. These factors include the amount of resources available, and the ability of individuals to convert these resources successfully into population growth. While previous studies have demonstrated the limits of macroecological theory in explaining local abundance, few studies have tested alternative generalized mechanisms determining abundance at the community scale. Using an assemblage of forest moth species found co-occurring as caterpillars on a single host plant species, we tested whether species abundance on that plant could be explained by mass allometry, intrinsic population growth, diet breadth, or some combination of these traits. We parameterized life history traits of the caterpillars in association with the host plant in both field and laboratory settings, so that the population growth estimate was specific to the plant on which abundance was measured. Using a generalized least-squares regression method incorporating phylogenetic relatedness, we found no relationship between abundance and mass but found that abundance was best explained by both intrinsic population growth rate and diet breadth. Species population growth potential was most affected by survivorship and larval development time on the host plant. Metabolic constraints may determine upper limits to local abundance levels for species, but local community abundance is strongly predicted by the potential for population increase and the resources available to that species in the environment.     

Spatial Scale and Determination of Species Status of the Green Frog

Although the importance of spatial scale in determining species status (abundance and distribution) is widely recognized, most ecological investigations have been conducted at local scales. Our goal was to investigate the importance of spatial scale in assessing the status of the green frog ( Rana clamitans melanota) in the center of its range in eastern North America. Using repeated surveys at 160 ponds from 1992 to 1994, we investigated patterns of occupancy, abundance, and turnover at local, sub-regional, regional, and geographic scales to determine the status of the green frog in southwestern Ontario, Canada. Patterns of pond occupancy were stable at the geographic scale. Occupancy was stable in two regions and decreased in one. At the sub-regional scale, occupancy was stable in six sub-regions, increased in one, and decreased in two. Patterns of adult abundance were stable at the geographic scale. At the regional scale trends in adult abundance were increasing, decreasing, or stable in each of three regions. At the sub-regional scale abundance was stable in three, increasing in one, decreasing in two, and no trend occurred in three sub-regions. At the local scale abundance was stable at 20% of ponds, increased at 17.8%, decreased at 14.4%, and no trend existed at 47.8%. Colonization and extinction rates ranged from 0 to 0.20 and 0 to 0.35 ponds/pond occupied/year, respectively, and differed among regions. Local extinctions took place at 25% of ponds during the study, but no sub-regional or regional scale extinctions occurred. Small populations (<10 adults/pond) were prone to local extinction. Determination of the status of the green frog is scale dependent. Although green frog populations are dynamic, it is common and stable at the geographic scale, but its status varies among regions or sub-regions. Although processes that negatively affect a species may operate at the local scale, a large-scale perspective is necessary to determine status.     

Assessing the Threat Status of Ecological Communities

Abstract: Conservationists are increasingly interested in determining the threat status of ecological communities as a key part of their planning efforts. Such assessments are difficult because of conceptual challenges and a lack of generally accepted criteria. We reviewed 12 protocols for assessing the threat status of communities and identified conceptual and operational issues associated with developing a rigorous, transparent, and universal set of criteria for assessing communities, analogous to the International Union for Conservation of Nature (IUCN) Red List standards for species. We examined how each protocol defines a community and its extinction and how each applies 3 overarching criteria: decline in geographic distribution, restricted geographic distribution, and changes to ecological function. The protocols vary widely in threshold values used to assess declines and distribution size and the time frames used to assess declines, leading to inconsistent assessments of threat status. Few of the protocols specify a scale for measuring distribution size, although assessment outcomes are highly sensitive to scale. Protocols that apply different thresholds for species versus communities tend to require greater declines and more restricted distributions for communities than species to be listed in equivalent threat categories. Eleven of the protocols include a reduction in ecological function as a criterion, but almost all assess it qualitatively rather than quantitatively. We argue that criteria should be explicit and repeatable in their concepts, parameters, and scale, applicable to a broad range of communities, and address synergies between types of threats. Such criteria should focus on distribution size, declines in distribution, and changes to key ecological functions, with the latter based on workable proxies for assessing the severity, scope, and immediacy of degradation. Threat categories should be delimited by thresholds that are assessed at standard scales and are logically consistent with the viability of component species and important ecological functions.     

Paleoecology and the Coarse-Filter Approach to Maintaining Biological Diversity

Abstract: The difficulties of saving millions of species from extinction often cause conservationists to focus on a higher level of biological organization, the community. They do so for two reasons: (1) communities are considered important biological entities in their own right; and (2) conserving representative samples of communities is seen as an efficient way to maintain high levels of species diversity. This approach will work if the chosen communities contain almost all species. Because it potentially saves most but not all species, community conservation is a "coarse-filter" approach to the maintenance of biological diversity, and contrasts with the "fine-filter" approach of saving individual species. Paleoecological information on the distribution of plant taxa in North America, however, indicates that most modern plant communities are less than 8,000 years old and therefore are not highly organized units reflecting long-term co-evolution among species. Rather, they are only transitory assemblages or co-occurrences among plant taxa that have changed in abundance, distribution, and association in response to the large climate changes of the past 20,000 years. During periods when climate changes are large, communities are too ephemeral to be considered important biological entities in their own right. Large climatic changes are also likely to occur during the next century because of increased concentrations of CO₂, and we therefore propose that the coarse-filter approach to selecting nature reserves should be more strongly influenced by the distribution of physical environments than by the distribution of modern communities. Ideally, nature reserves should also encompass a broad enough range of environments to allow organisms to adjust their local distribution in response to long-term environmental change and should be connected by regional corridors that would allow species to change their geographic distributions.     

Dispersal limitation and environmental heterogeneity shape scale-dependent diversity patterns in plant communities

Understanding the large-scale distribution of species diversity requires distinguishing two of the primary factors that cause compositional differences: dispersal limitation and environmental variation. In a community with a naturally discontinuous spatial structure, we asked (1) at what scale(s) nonrandom variation in species composition occurs and (2) at what scale(s) such variation is associated with spatial separation, indicative of dispersal limitation, and at what scale(s) variation is associated with environmental heterogeneity? We sampled 50 seeps (small wetlands) on five serpentine outcrops. Using a randomization model, we showed that additive beta diversity (a measure of community dissimilarity) was lower than random within seeps and higher than random among both seeps and outcrops. Using Mantel tests, we showed that plant community dissimilarity, in both the full seep assemblage as well as in a subset of seep endemics, at the two larger scales was associated with different forms of environmental heterogeneity and, at the largest scale, was also associated with geographic distance. We conclude that diversity in this system is shaped by multiple scales of heterogeneity and by dispersal limitation at the largest scale.     

Spatial location dominates over host plant genotype in structuring an herbivore community

Recent work has shown a potential role for both host plant genotype and spatial context in structuring insect communities. In this study, we use three separate data sets on herbivorous insects on oak (Quercus robur) to estimate the relative effects of host plant genotype (G), location (E), and the G x E interaction on herbivore community structure: a common garden experiment replicated at the landscape scale (approximately 5 km2); two common gardens separated at the regional scale (approximately 10 000 km2); and survey data on wild trees in various spatial settings. Our experiments and survey reveal that, at the landscape scale, the insect community is strongly affected by the spatial setting, with 32% of the variation in species richness explained by spatial connectivity. In contrast, G and G x E play minor roles in structuring the insect community. Results remained similar when extending the spatial scale of the study from the more local (landscape) level to the regional level. We conclude that in our study system, spatial processes play a major role in structuring these insect communities at both the landscape and regional scales, whereas host plant genotype seems of secondary importance.     

Single and joint effects of regional- and local-scale variables on tropical seagrass fish assemblages

Seagrass beds are highly important for tropical ecosystems by supporting abundant and diverse fish assemblages that form the basis for artisanal fisheries. Although a number of local- and regional-scale variables are known to influence the abundance, diversity and assemblage structure of seagrass-associated fish assemblages, few studies have evaluated the relative and joint (interacting) influences of variables, especially those acting at different scales. Here, we examined the relative importance of local- and regional-scale factors structuring seagrass-associated fish assemblages, using a field survey in six seagrass (Thalassodendron ciliatum) areas around Unguja Island (Zanzibar, Tanzania). Fish density and assemblage structure were mostly affected by two regional-scale variables; distance to coral reefs, which positively affected fish density, and level of human development, which negatively affected fish density. On the local scale, seagrass biomass had a positive (but weaker) influence on fish density. However, the positive effect of seagrass biomass decreased with increasing level of human development. In summary, our results highlight the importance of assessing how multiple local and regional variables, alone and together, influence fish communities, in order to improve management of seagrass ecosystems and their services.     

The invasibility of marine algal assemblages: role of functional diversity and identity

The emergence of the biodiversity-ecosystem functioning debate in the last decade has renewed interest in understanding why some communities are more easily invaded than others and how the impact of invasion on recipient communities and ecosystems varies. To date most of the research on invasibility has focused on taxonomic diversity, i.e., species richness. However, functional diversity of the communities should be more relevant for the resistance of the community to invasions, as the extent of functional differences among the species in an assemblage is a major determinant of ecosystem processes. Although coastal marine habitats are among the most heavily invaded ecosystems, studies on community invasibility and vulnerability in these habitats are scarce. We carried out a manipulative field experiment in tide pools of the rocky intertidal to test the hypothesis that increasing functional richness reduces the susceptibility of macroalgal communities to invasion. We selected a priori four functional groups on the basis of previous knowledge of local species characteristics: encrusting, turf, subcanopy, and canopy species. Synthetic assemblages containing one, two, three, or four different functional groups of seaweeds were created, and invasion by native species was monitored over an eight-month period. Cover and resource availability in the assemblages with only one functional group showed different patterns in the use of space and light, confirming true functional differences among our groups. Experimental results showed that the identity of functional groups was more important than functional richness in determining the ability of macroalgal communities to resist invasion and that resistance to invasion was resource-mediated.     

Using vertebrate environmental DNA from seawater in biomonitoring of marine habitats

Conservation and management of marine biodiversity depends on biomonitoring of marine habitats, but current approaches are resource-intensive and require different approaches for different organisms. Environmental DNA (eDNA) extracted from water samples is an efficient and versatile approach to detecting aquatic animals. In the ocean, eDNA composition reflects local fauna at fine spatial scales, but little is known about the effectiveness of eDNA-based monitoring of marine communities at larger scales. We investigated the potential of eDNA to characterize and distinguish marine communities at large spatial scales by comparing vertebrate species composition among marine habitats in Qatar, the Arabian Gulf (also known as the Persian Gulf), based on eDNA metabarcoding of seawater samples. We conducted species accumulation analyses to estimate how much of the vertebrate diversity we detected. We obtained eDNA sequences from a diverse assemblage of marine vertebrates, spanning 191 taxa in 73 families. These included rare and endangered species and covered 36% of the bony fish genera previously recorded in the Gulf. Sites of similar habitat type were also similar in eDNA composition. The species accumulation analyses showed that the number of sample replicates was insufficient for some sampling sites but suggested that a few hundred eDNA samples could potentially capture >90% of the marine vertebrate diversity in the study area. Our results confirm that seawater samples contain habitat-characteristic molecular signatures and that eDNA monitoring can efficiently cover vertebrate diversity at scales relevant to national and regional conservation and management.     

Seed arrival and ecological filters interact to assemble high-diversity plant communities

Two prominent mechanisms proposed to structure biodiversity are niche-based ecological filtering and chance arrival of propagules from the species pool. Seed arrival is hypothesized to play a particularly strong role in high-diversity plant communities with large potential species pools and many rare species, but few studies have explored how seed arrival and local ecological filters interactively assemble species-rich communities in space and time. We experimentally manipulated seed arrival and multiple ecological filters in high-diversity, herbaceous-dominated groundcover communities in longleaf pine savannas, which contain the highest small-scale species richness in North America (up to > 40 species/m2). We tested three hypotheses: (1) local communities constitute relatively open-membership assemblages, in which increased seed arrival from the species pool strongly increases species richness; (2) ecological filters imposed by local fire intensity and soil moisture influence recruitment and richness of immigrating species; and (3) ecological filters increase similarity in the composition of immigrating species. In a two-year factorial field experiment, we manipulated local fire intensity by increasing pre-fire fuel loads, soil moisture using rain shelters and irrigation, and seed arrival by adding seeds from the local species pool. Seed arrival increased species richness regardless of fire intensity and soil moisture but interacted with both ecological filters to influence community assembly. High-intensity fire decreased richness of resident species, suggesting an important abiotic filter. In contrast, high-intensity fire increased recruitment and richness of immigrating species, presumably by decreasing effects of other ecological filters (competition and resource limitation) in postfire environments. Drought decreased recruitment and richness of immigrating species, whereas wet soil conditions increased recruitment but decreased or had little effect on richness. Moreover, some ecological filters (wet soil conditions and, to a lesser extent, high-intensity fire) increased similarity in the composition of immigrating species, illustrating conditions that influence deterministic community assembly in species-rich communities. Our experiment provides insights into how dispersal-assembly mechanisms may interact with niche-assembly mechanisms in space (spatial variation in disturbance) and time (temporal variation in resource availability) to structure high-diversity communities and can help guide conservation of threatened longleaf pine ecosystems in the face of habitat fragmentation and environmental change.     

On the relationship between regional and local species richness: a test of saturation theory

What are the local community consequences of changes in regional species richness and composition? To answer this question we followed the assembly of microarthropod communities in defaunated areas of moss, embedded in a larger moss "region." Regions were created by combining moss from spatially distinct sites, resulting in regional species pools that differed in both microarthropod richness and composition, but not area. Regional effects were less important than seasonality for local richness. Initial differences in regional richness had no direct effect on local species richness at any time along a successional gradient of 0.5-16 months. The structure of the regional pool affected both local richness and local composition, but these effects were seasonally dependent. Local species richness differed substantially between dates along the successional gradient and continued to increase 16 months after assembly began. To the best of our knowledge, this is the first critical test of saturation theory that experimentally manipulates regional richness. Further, our results failed to support the most important mechanisms proposed to explain the local richness-regional richness relationship. The results demonstrate that complicated interactions between assembly time, seasonality, and regional species pools contribute to structuring local species richness and composition in this community.