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Diversity and heterogeneity often are conflated but are fundamentally different. An aphorism proposed by Shavit and Ellison (2021; J. Phil. 118: 525–548) for distinguishing them is that ‘a zoo is diverse whereas an ecosystem is heterogeneous.' That is, a zookeeper measuring diversity simply enumerates the different types of animals; interactions are not expected to occur between animals separated by fences or other barriers. In contrast, measures of heterogeneity ought to include both interspecific interactions and relationships between species and their heterogeneous habitats. Here, we use cross‐scale, dual scaling‐law analyses of heterogeneity and diversity of animal gut microbiomes (AGMs) to address three objectives: 1) estimate the spatial heterogeneity and diversity of animal‐gut microbiomes; 2) analyze influences of phylogeny and diets on scaling of diversity and heterogeneity; 3) explore mechanistic differences between diversity and heterogeneity in AGMs. From 4903 AGM samples collected from 318 animal species covering all six classes of vertebrates and four major classes of invertebrates, we estimated that ≈640 000 operational taxonomic units (OTUs or ‘species') make up the pool of microbial species that could inhabit animal guts, among which ≈8000 are relatively common and ≈800 are dominant. The gut of any single animal, however, includes only 0.01–0.5% of the total species pool. We applied Ma's diversity‐area relationship for scaling diversity, and extend Taylor's power law and Luna et al.'s (2020; Diversity 12: 86) interaction diversity for scaling heterogeneity. At the community scale, phylogeny significantly influenced heterogeneity, but diets did not. Phylogeny and diets had limited influence on diversity at both community and landscape scales. We demonstrated that diversity and heterogeneity measure two different properties. Further, differences in scaling of diversity and heterogeneity are a result of heterogeneity being primarily the result of evolved dispersal behavior and interspecific interactions, whereas diversity results from abundances controlled on ecological time scales. [ABSTRACT FROM AUTHOR]

Explaining the mechanisms underlying spatial and temporal variation in community composition is a major challenge. Nevertheless, the processes controlling temporal variation at a site (i.e. temporal β‐diversity, including its turnover and nestedness components) are less understood than those affecting variation among sites (i.e. spatial β‐diversity). Short‐term temporal turnover (e.g. throughout an annual cycle) is expected to correlate positively with seasonal environmental variability and landscape connectivity, but also species pool size (γ‐diversity). We use the megadiverse Amazonian freshwater ichthyofauna as a model to ask whether seasonality and landscape connectivity drive variation in temporal species turnover among geomorphological habitat types, while accounting for between‐habitat variation in γ‐diversity. We collected 11 397 fish representing 260 species during a year‐long sampling program from an area containing the lowland Amazon's four major geomorphological habitat types: rivers, floodplains, terra firme streams, and shield streams. River‐floodplain systems exhibit strong but predictable seasonality (via a high‐amplitude annual flood pulse), high connectivity, and high species richness with many rare species. Terra firme and shield streams exhibit low seasonality, low connectivity, and low species richness with proportionally fewer rare species. Based on these parameters we predicted that river‐floodplain systems should have higher temporal turnover than stream systems. Using a null model approach combined with β‐deviation calculations, we confirmed that rivers and floodplains do exhibit higher turnover (but not nestedness) than terra firme and shield streams, even when controlling for the potentially confounding effect of higher species richness in river‐floodplain systems. All habitats exhibit low temporal nestedness, indicating that short‐term changes in community composition result primarily from temporal species turnover. Our results provide a timely reminder that efforts to conserve the Amazon's threatened aquatic biodiversity should account for the distinct temporal dynamics of habitat types and variation in hydrological seasonality. [ABSTRACT FROM AUTHOR]

Relationships between area, heterogeneity and species richness are fundamental concepts in ecology yet questions remain about how area and heterogeneity tradeoff (AHTO) to constrain biodiversity. Although there is growing evidence for unimodal heterogeneity diversity relationships (HDR's) and an AHTO, tests of the concept and consequences for species richness across a landscape‐scale gradient of human‐modified ecosystems are rare. Using data from a national (Wales) field survey we analysed relationships between environmental heterogeneity and plant species richness (α and γ). We used ordination to produce a composite metric of heterogeneity and compared this to commonly used metrics. We used niche hypervolumes to categorise the breadth of plant species' ecological preferences and analysed relationships between species richness, niche width and heterogeneity. The HDR was unimodal with α diversity at the smallest scale and positive with α and γ diversity (non‐linear) at the 1 km scale although in low intensity landscapes the HDR with γ diversity was unimodal. There was a unimodal relationship between habitat diversity and γ diversity. Land use intensity was unimodally related to diversity. There were significant interactions between niche width and heterogeneity. Richness of broad niche species increased with heterogeneity with flattening of the curve at higher levels. Narrow niche species were rare and mostly unresponsive. The expected decline in narrow niche species with increasing heterogeneity was not found although they did decline with land‐use intensity. Using a unique dataset, an analysis of a large‐scale mosaic of ecosystems found that the shape of the HDR varies with land use intensity, the heterogeneity metric, spatial scale, diversity type and niche width. Although heterogeneity can increase species richness, there may be tradeoffs at higher heterogeneity. A fundamental constraint on realising the benefit of heterogeneity is the low availability of narrower niche species in local species pools in modified landscapes. [ABSTRACT FROM AUTHOR]

Emergent neutrality (EN) suggests that species must be sufficiently similar or sufficiently different in their niches to avoid interspecific competition. Such a scenario results in a transient pattern with clumps and gaps of species abundance along the niche axis (e.g. represented by body size). From this perspective, clumps are groups of coexisting species with negligible fitness differences and stochastic abundance fluctuations. Plankton is an excellent model system for developing and testing ecological theories, especially those related to size structure and species coexistence. We tested EN predictions using the phytoplankton community along the course of a tropical river considering 1) body size structure, 2) functional clustering of species in terms of morphology‐based functional groups (MBFG) and 3) the functional similarity among species concerning their functional traits. Two main clumps in the body size axis (clump I and II) were conspicuous through time and were detected in different stretches of the river. Clump I comprised medium‐sized species from the MBFGs IV, V and VI while clump II included large‐bodied species from the MBFGs V and VI. Pairwise differences in species biovolume correlated with species functional similarity when the whole species pool was considered, but not among species within the same clump. Although clumps comprised multiple MBFGs, the dominant species within the clump belonged always to the same MBFG. Also, within‐clump species biovolume increased with functional distinctiveness considering both seasons and stretches, except the lower course. These results suggest that species within clumps behave in a quasi‐neutral state, but even minor shifts in trait composition may affect species biovolume. Our findings point that EN belongs to the plausible mechanisms explaining community assembly in river ecosystems. [ABSTRACT FROM AUTHOR]

A challenge for nature conservation is to know why many species are absent from suitable habitats and whether they might be able to disperse and to establish. Here, we used 272 dry grassland patches within a fragmented landscape to investigate the role of local abiotic conditions and dispersal filtering in determining the likelihood of vascular plants to belong to the dark diversity (i.e. absent portion of the species pool). First, we quantified the species (SD), functional (FD) and phylogenetic (PD) diversity of both observed and dark communities. Second, we determined the roles of abiotic, present‐day and historical landscape configuration variables in shaping their patterns. Third, we evaluated the importance of each variable in determining their species. Environmental filtering was assessed as effects of local abiotic conditions and dispersal filtering as the effects of present‐day and historical landscape configuration. Dispersal filtering was also estimated by comparing dispersal traits of observed and dark diversity. Finally, we assessed community completeness to determine how much of the species pool was realized within a local community. We found higher SD in the observed compared to the dark communities, but PD did not differ. Contrary to expectations, dark communities resembled higher FD compared to the observed communities. Species with low dispersal capacity, low competitive abilities and high stress‐tolerance were more often absent. Observed and dark diversities were mostly affected by local abiotic variables. In the observed communities, present‐day landscape configuration variables affected SD while historical landscape configuration variables explained FD and PD. In the dark communities, we found the opposite pattern. Completeness was affected by present‐day and historical patch size. Our results explain why dry grassland species may belong to the dark diversity and highlight the importance of local abiotic and dispersal traits of the species to conserve dry grasslands in changing landscapes. [ABSTRACT FROM AUTHOR]

Understanding variation in root and shoot growth phenology among species is crucial to understanding underlying mechanisms of temporal niche differentiation. However, little is known about the relationship between root and shoot phenology, or how this relationship varies among functional traits. We examined fine root and shoot phenology of 42 seedlings representing a variety of woody species that inhabit the cool temperate forests of northern Japan. Some aspects of phenology were common to the pool of species examined: we found positive relationships between root and shoot phenology for the end of growth, and for the duration of growth, but not for the start of growth. Further, seedlings that started root growth relatively early also ended root growth relatively late. Other aspects of phenology varied predictably with functional traits, i.e. leaf habit and successional status: first, root growth in evergreen species started significantly earlier and ended later than in deciduous species; second, early successional species had the longest duration of shoot growth among all successional types. Our results suggest that niche differentiation may be promoted by differences in phenology between root and shoots, likely contributing to the co‐existence of woody seedlings in temperate forests. [ABSTRACT FROM AUTHOR]

Ecosystem biomass, soil conditions and the diversity of different taxa are often interrelated. These relationships could originate from biogeographic affinity (varying species pools) or from direct ecological effects within local communities. Disentangling regional and local causes is challenging as the former might mask the latter in natural ecosystems with varying habitat conditions. However, when the species pool contribution is considered in statistics, local ecological effects might be detected. In this study we disentangle the indirect effects of the species pool and direct ecological effects on the complex relationships among wood volume, soil conditions and diversities of different plant and fungal groups in 100 old‐growth forest sites (10 × 10 m) at the border of boreal and nemoral zones in northern Europe. We recorded all species for different vegetation groups: woody and herbaceous vascular plants, terricolous and epiphytic bryophytes and lichens. Fungal communities were detected by DNA‐based analyses from soil samples. Above‐ground wood volume was used as a proxy of biomass. We measured soil pH and nutrient content and obtained modelled climate parameters for each site. Species pool effect was considered by dividing sites into boreal and nemoral groups based on community composition. In order to disentangle direct and indirect effects, we applied variation partitioning, and raw and partial correlations. We found many significant positive relationships among studied variables. Many of these relationships were associated to boreal and nemoral species pools, thus indicating that biogeographic affinity of interacting plants and fungi largely defines forest diversity and functioning. At the same time, several relationships were significant also after considering biogeography: woody plant and ectomycorrhizal fungi diversities with wood volume, many plant and fungal groups with each other, or with soil conditions. These direct ecological interactions could be considered in forestry practices to achieve both economic gain and maintenance of biodiversity. [ABSTRACT FROM AUTHOR]

The role of deterministic and stochastic processes in community assembly is a key question in community ecology. We evaluated the effect of an abiotic filter (hydroperiod) on the partitioned diversity of three taxonomic groups (birds, vegetation, macroinvertebrates) from prairie pothole wetlands in Alberta, Canada, which naturally vary in water permanence. We observed that alpha and gamma diversity were higher in permanent than temporary wetlands (16–25% and 34–47% respectively, depending on the taxon). This suggests an influence of deterministic constraints on the number of species a wetland can support. Taxa which cannot persist in shallow, temporary wetlands are excluded by the deterministic constraints that a shortened hydroperiod imposes. In contrast, we observed that beta diversity was significantly higher (2–12%) in temporary wetlands than permanent ones, and temporary wetlands supported more unique combinations of community composition than permanent wetlands, despite having a smaller regional species pool. This observation contradicts prior mesocosm studies that found beta diversity mirrored the pattern in gamma diversity along an environmental filtering gradient. We conclude that deterministic processes are more influential in more stable permanent wetlands, whereas stochastic processes play a more important role in assembly in dynamic temporary wetlands that must disassemble and re‐establish annually. Considering three distinct taxonomic groups differing in their relative mobility, our large‐scale field study demonstrates that both stochastic and deterministic processes act together to influence the assembly of multiple communities and that the relative importance of the two processes varies consistently along a gradient of environmental filtering. [ABSTRACT FROM AUTHOR]

Understanding the imprint of environmental filtering on community assembly along environmental gradients is a key objective of trait‐gradient analyses. Depending on local constraints, this filtering generally entails that species departing from an optimum trait value have lower abundances in the community. The community‐weighted mean (CWM) and variance (CWV) of trait values are then expected to depict the optimum and intensity of filtering, respectively. However, the trait distribution within the regional species pool and its limits can also affect local CWM and CWV values apart from the effect of environmental filtering. The regional trait range limits are more likely to be reached in communities at the extremes of environmental gradients. Analogous to the mid‐domain effect in biogeography, decreasing CWV values in extreme environments can then represent the influence of regional trait range limits rather than stronger filtering in the local environment. We name this effect the 'trait‐gradient boundary effect' (TGBE). First, we use a community assembly framework to build simulated communities along a gradient from a species pool and environmental filtering with either constant or varying intensity while accounting for immigration processes. We demonstrate the significant influence of TGBE, in parallel to environmental filtering, on CWM and CWV at the extremes of the environmental gradient. We provide a statistical tool based on Approximate Bayesian Computation to decipher the respective influence of local environmental filtering and regional trait range limits. Second, as a case study, we reanalyze the functional composition of alpine plant communities distributed along a gradient of snow cover duration. We show that leaf trait convergence found in communities at the extremes of the gradient reflect an influence of trait range limits rather than stronger environmental filtering. These findings challenge correlative trait–environment relationships and call for more explicitly identifying the mechanisms responsible of trait convergence/divergence along environmental gradients. [ABSTRACT FROM AUTHOR]

Synthesis Metacommunity theory aims to elucidate the relative influence of local and regional-scale processes in generating diversity patterns across the landscape. Metacommunity research has focused largely on assemblages of competing organisms within a single trophic level. Here, we test the ability of metacommunity models to predict the network structure of the aquatic food web found in the leaves of the northern pitcher plant Sarracenia purpurea. The species-sorting and patch-dynamics models most accurately reproduced nine food web properties, suggesting that local-scale interactions play an important role in structuring Sarracenia food webs. Our approach can be applied to any well-resolved food web for which data are available from multiple locations. The metacommunity framework explores the relative influence of local and regional-scale processes in generating diversity patterns across the landscape. Metacommunity models and empirical studies have focused mostly on assemblages of competing organisms within a single trophic level. Studies of multi-trophic metacommunities are predominantly restricted to simplified trophic motifs and rarely consider entire food webs. We tested the ability of the patch-dynamics, species-sorting, mass-effects, and neutral metacommunity models, as well as three hybrid models, to reproduce empirical patterns of food web structure and composition in the complex aquatic food web found in the northern pitcher plant Sarracenia purpurea. We used empirical data to determine regional species pools and estimate dispersal probabilities, simulated local food-web dynamics, dispersed species from regional pools into local food webs at rates based on the assumptions of each metacommunity model, and tested their relative fits to empirical data on food-web structure. The species-sorting and patch-dynamics models most accurately reproduced nine food web properties, suggesting that local-scale interactions were important in structuring Sarracenia food webs. However, differences in dispersal abilities were also important in models that accurately reproduced empirical food web properties. Although the models were tested using pitcher-plant food webs, the approach we have developed can be applied to any well-resolved food web for which data are available from multiple locations. [ABSTRACT FROM AUTHOR]

Community assembly rules theory attempt to understand the processes that determine the composition of local communities from a regional species pool. Nestedness and negative co-occurrence are two of the most commonly reported meta-community patterns, but almost exclusively from terrestrial and freshwater ecosystems. Here we analyzed the structure of species coexistence in six datasets containing presence/absence data for 120 marine benthic gastropod species in 249 sampling units on the Uruguayan continental shelf and Río de la Plata estuary. The ecological features of this system, such as the idiosyncratic nature of the biogeographic and oceanographic realms, are clearly different from those observed in other systems previously targeted by studies on coexistence structure. Community patterns were evaluated using null models and four structure indices. The existence of patterns in community assembly, and in particular segregated co-occurrence, was verified only when analyzing the number of checkerboard units (CH index). This indicates more mutually exclusive species pairs than expected by chance. Nestedness, on the other hand, was not detected in any dataset. Storage and rescue effects related to overall high immigration and low local extinction rates are plausible mechanisms to account for the general pattern of random species coexistence, while the segregated co-occurrence pattern depicted by the CH index may be related to differential habitat requirements within species pairs. Our study highlights the importance of analyzing metacommunity structures in alternative biological, environmental, and historical contexts in order to advance on the construction of a general ecological theory, relating patterns with the processes dominating in particular ecosystems. [ABSTRACT FROM AUTHOR]

Recent research has proposed a scale-dependence to relationships between native diversity and exotic invasions. At fine spatial scales, native–exotic richness relationships should be negative as higher native richness confers resistance to invasion. At broad scales, relationships should be positive if natives and exotics respond similarly to extrinsic factors. Yet few studies have examined both native and exotic richness patterns across gradients of human influence, where impacts could affect native and exotic species differently. We examined native–exotic richness relationships and extrinsic drivers of plant species richness and distributions across an urban development gradient in remnant oak savanna patches. In sharp contrast to most reported results, we found a negative relationship at the regional scale, and no relationship at the local scale. The negative regional-scale relationship was best explained by extrinsic factors, surrounding road density and climate, affecting natives and exotics in opposite ways, rather than a direct effect of native on exotic richness, or vice versa. Models of individual species distributions also support the result that road density and climate have largely opposite effects on native and exotic species, although simple life history traits (life form, dispersal mode) do not predict which habitat characteristics are important for particular species. Roads likely influence distributions and species richness by increasing both exotic propagule pressure and disturbance to native species. Climate may partially explain the negative relationship due to differing climatic preferences within the native and exotic species pools. As gradients of human influence are increasingly common, negative broad-scale native–exotic richness relationships may be frequent in such landscapes. [ABSTRACT FROM AUTHOR]

When do initial conditions, which reflect the assembly history of a community, affect the final community state? Comparative field studies and recent theory suggest that initial conditions matter at high productivity, because uninvasible alternate stable states and assembly cycles are more likely at high productivity. However, this prediction and the mechanisms behind it have not been tested experimentally. An alternative hypothesis is that initial conditions are relatively unimportant, and that communities generally are comprised of species with appropriate traits, which might vary with productivity. I assembled communities of protists and rotifers in laboratory microcosms from a species-rich, trophically-diverse species pool using all possible combinations of two initial conditions and four productivity levels. After communities approached their final states, invasions by the species that initially failed to persist tested the invasibility of those final states and tested for assembly cycles. I also examined how local (within-microcosm) diversity and regional diversity (total species richness of all microcosms of a given productivity level) varied with productivity. Comparative field work has used such scale-dependent diversity–productivity relationships as evidence for effects of assembly history. Productivity had a modest effect on final pre-invasion species composition, while initial conditions had a very weak effect. Most invasions failed, and the frequency of successful invasions and of post-invasion extinctions did not vary with productivity. Instead, species that were present most frequently pre-invasion were also the most successful invaders, and the least-likely species to go extinct post-invasion. Local and regional richness did not vary substantially with productivity. Overall, the results suggest that final communities are predictably comprised of species with appropriate traits, and are not an unpredictable outcome of initial conditions. [ABSTRACT FROM AUTHOR]

The Evolutionary species pool hypothesis (ESPH) predicts that historically more common habitats will be richer in species because they have had greater opportunity for the evolution of suitably adapted species. We explored the relationship between mire species richness and pH, an important environmental variable in mires, in two regions that differ in habitat pH distribution: the West Carpathians and Bulgaria. Mire habitats in both the West Carpathians and Bulgaria demonstrate support for the ESPH prediction that habitats with more common pH values host more species. We also explored the influence of habitat history by examining the distribution of generalists and specialists along gradients of habitat pH, using extensive community-level vegetation data from European mires in these two regions. We found a striking pattern with the distribution of pH-specialists having three distinct peaks in both regions, whereas the total species pool peaked in near neutral pH habitats in both regions. Because the peaks in specialist richness do not correspond to regional pH distribution patterns, we hypothesize that historical explanations may be important, and that habitats currently rich in pH-specialists may have historically acted as pleniglacial refugia for many mire species. Our findings support the general predictions of the ESPH, but further suggest that historical processes such as patterns of glacial refugia, may significantly influence contemporary species distributions and the diversity of plant species in mire habitats. [ABSTRACT FROM AUTHOR]

Empirical studies across a wide range of taxa show that the slopes of species–time relationships often decline as average species richness increases, indicating that more diverse communities have greater temporal stability in species composition. I explored potential explanations for this observation using two simple model formulations for species temporal dynamics. In the Abiotic model, species turnover is governed by the degree of heterogeneity in the environment and the range of species’ tolerances. In this case, more variable conditions lead to lower species richness and higher turnover, but only if the distribution of species’ niche widths and the size of the species pool are independent of the degree of environmental variability. The Biotic model represents direct effects of diversity on turnover through positive or negative feedbacks between diversity and species’ colonization and extinction rates. Declining turnover with increasing richness occurred when higher diversity either facilitated colonization by new species or reduced extinction rates of extant species. Both models could produce the observed pattern of declining turnover at higher diversity under some circumstances, however the conditions for this outcome in the Abiotic model were restrictive and potentially unrealistic. The models provide a process-based framework for understanding the connection between diversity and species turnover through time. [ABSTRACT FROM AUTHOR]

The role of density compensation (the decline of species density with increasing diversity), in the context of biodiversity and ecosystem functioning, has not been explicitly explored. I used aquatic microbial communities containing bacterivorous consumers (protozoans and rotifers) to investigate whether competition can lead to density compensation and whether density compensation can contribute to the relationship between biodiversity and ecosystem functioning. The experiment employed a nested design in which the consumer diversity gradient (0, 1, 2 or 4 species) was constructed by drawing all possible species or species combinations at each diversity level from a five-species pool. All consumer species coexisted but there was little evidence for overyielding or species dominance, suggesting weak complementarity and sampling effects. Rather, increasing number of consumer species resulted in community-wide density compensation, such that aggregate consumer biomass was unaffected by consumer diversity. Whereas culturable bacterial density declined as consumer diversity increased, total bacterial density showed no discernible response to changes in consumer diversity, a result probably due in part to heterogeneity in bacterial edibility. This study demonstrates the potential for density compensation to shape the relationship between biodiversity and ecosystem functioning. [ABSTRACT FROM AUTHOR]

In this study we analyse with an experiment how the spatial area from which the species pool is sampled, i.e. local vs regional, can have functional consequences for ecosystem processes and structure. Particularly, we analysed how major stocks and processes respond to a change in basal productivity and how this depends on the source of the species pool. The experimental microcosms used in this experiment were developed by inoculating nutrient solution with samples of rockpool sediment containing resting stages. Here we show that communities developed from sediment samples of single origin (local communities) exhibit important differences in ecosystem structure and their response to a productivity gradient compared to systems developed from mixed samples of multiple and spatially dispersed origin (regional communities). The difference between local and regional communities was most pronounced for zooplankton which had much lower abundances in the local communities compared to the regional communities resulting in cascading effects on phytoplankton. We demonstrate that the spatial extend from which the species pool is sampled can affect ecosystem structure and functioning comparable in magnitude to changes in the basal productivity of the system. We discuss with a conceptual model how the regional species pool is important for sustaining diversity in functional groups, and how higher trophic levels are more sensitive to changes in the regional species pool. [ABSTRACT FROM AUTHOR]

Concerns about the impact of species diversity loss has heightened the importance of determining what limits local diversity and the relative roles of the available species pool and local interactions. Recent theory suggests that local diversity may exhibit linear or asymptotic responses to regional variation in species pools depending resource supply rates. We tested this idea by augmenting species pools in three sites along a natural productivity gradient (a surrogate for resource supply) in a mid-successional old-field. Augmentation of the species pool, enhanced local diversity more at medium than low productivity sites in this field and seed addition had no effect at high productivity, suggesting that diversity was saturated in high resource areas of this field. These results suggest that resource supply rates may mediate species pool-richness relationships. Species additions into cleared plots demonstrated that the observed differences in species recruitment along this gradient were largely driven by abiotic filtering at low and competitive filtering at high productivity. Furthermore, we observed that augmentation of the species pool shifted the productivity–diversity relationship from a negative linear to unimodal suggesting that species pools may influence reported productivity–diversity patterns. [ABSTRACT FROM AUTHOR]

A general model of linearized species interactions, essentially Lotka–Volterra theory, applied to questions of biodiversity has previously been shown to be a powerful tool for understanding local species–abundance patterns and community responses to environmental change for a single trophic level. Here this approach is extended to predict community composition and responses to environmental changes in trophically structured systems. We show how resource and consumer species richness and their relative abundances vary with the means and variances in enrichment level and strengths of intra- and interspecific interactions. Also demonstrated are the responses of local resource and consumer species richness to the global species pools at both trophic levels, as well as the covariation with net resource productivity. These predictions for resource and consumer specific responses to changes in environmental enrichment and global biodiversity are directly testable. [ABSTRACT FROM AUTHOR]

Nestedness has received considerable attention in community ecology and conservation biology from both theoretical and empirical perspectives. This has lead to the creation of various metrics and null models to analyze nested subsets, all of which rely on the random placement of species to assess significance. However, if immigration and extinction are the processes that underlie species distributions on island systems, then null models might be better determined on the basis of randomly placed individuals. Consequently, we examined the effects of species–abundance distributions (uniform, dominance–decay, random–assortment, and dominance–preemption), island–size distributions (uniform and linear decrease), and total abundances (128, 256, 512, 1024, 2048, 4096 and 8192) on the degree of nestedness and its significance. Generally, matrices of species presence and absence created from the random placement of individuals were nested significantly according to null models based on the random placement of species. Island size and abundance had less of an effect on nestedness in systems dominated by only a few species than in systems in which abundances were distributed more evenly. Stochastic processes, such as the random placement of individuals, predispose systems to evince patterns of nestedness at the species level, which may account, in part, for the ubiquity of nestedness in nature. [ABSTRACT FROM AUTHOR]

Local species richness frequently is linearly related to the richness of the regional species pool from which the local community was presumably assembled. What, if anything, does this pattern imply about the relative importance of species interactions and dispersal as determinants of local species richness? Two recent papers by Hugueny and Cornell and He et al. propose that the classical island biogeography model of MacArthur and Wilson can help answer this question, by serving as a null model of the relationship between local (island) and regional (mainland) species richness in the absence of local species interactions. The two models make very different predictions, despite being derived from apparently-similar assumptions. Here we reinterpret these two models and show that their contrasting predictions can be regarded as arising from different, implicit assumptions about how species abundances vary with species richness on the mainland. We derive a more general island biogeography model of local–regional richness relationships that explicitly incorporates mainland species abundance and subsumes the two previous models as limiting cases. The new model predicts that the local–regional richness relationship can range from nearly linear to strongly curvilinear, depending on how species abundances on the mainland vary with mainland richness, as well as on rates of immigration to and extinction from islands. Local species interactions are not necessary for producing curvilinear local–regional richness relationships. We discuss the implications of our new model for the interpretation of local–regional richness relationships. [ABSTRACT FROM AUTHOR]

We propose a conceptual model to explain the variation in species richness in local communities and in build-up of regional species pools over time. The idea is that the opportunity for new species to enter a community (its invasibility) determines the present richness of that community as well as the long-term build-up of a species pool by speciation and migration. We propose that a community's invasibility is determined by the turnover rate of reproductive genets in the community, which we call the ‘community-level birth rate’. The faster the turn-over, the more species will accumulate per unit time and per unit community size (number of genets) at a given per-birth rate of immigration and speciation. Spatially discrete communities inhabiting similar environments sum up to metacommunities, whose inhabitant species constitute the regional species pool. We propose that the size of a regional species pool is determined by the aggregate community-level birth rate, the size of the metacommunity through time and age of the metacommunity. Thus, the novel contribution is our proposal of a direct effect of local environment on the build-up rate of species pools. The relative importance of immigrating species and neospecies originating locally will change with the temporal and spatial scale under consideration. We propose that the diversification rate specific to evolutionary lineages and the build-up rate of species pools are two sides of the same coin, and that they are both depending on mean generation time. The proposed model offers a reconciliation of two contrasting paradigms in current community ecology, viz. one focussing on present-time ecological processes and one focussing on historical events governing the size of species pools which in turn determines local richness. [ABSTRACT FROM AUTHOR]

We studied the effect of productivity on competition intensity and the relationship between competition intensity and community species richness, using a removal experiment with the perennial plant Solidago virgaurea. The experiment was conducted in 16 different communities from two geographically distant areas (western Estonia and northern Norway). The results were compared with the results of previous experiments with Anthoxanthum odoratum from the same areas. Removal of neighbors had a positive effect on the biomass of both Solidago and Anthoxanthum, and this response was stronger in communities with higher productivity. Thus, the corrected index of relative competition intensity, CRCI, increased with increasing community productivity. Species richness was negatively correlated with CRCI in Estonia but not in Norway and not in the case of the pooled material. The results suggest that competitive exclusion operates at least in these communities which species pool is large. Our results indicate that the relationship between competition intensity and productivity is non-linear. In our data, competition prevails in communities where living plant biomass exceeds 200 g m−2, whereas in less productive communities, competition remains undetected and direct plant–plant relationships might at times be even mutualistic. Moreover, we found that the relationship between competition intensity and productivity is strongly dependent on regional differences and is intimately connected to a concordant variation in the intensity of grazing. The least productive communities both in Estonia and in Norway are characterized by intensive grazing, which reduces importance of competition. Hence, the contrasting results corroborates the predictions of the hypothesis of exploitation ecosystems, predicting that trophic dynamics account for the relationship between competition intensity and primary productivity. [ABSTRACT FROM AUTHOR]

Considerable debate has developed over the importance of community biomass and species pools in the regulation of community diversity. Attempts to explain patterns of plant diversity as a function of community biomass or productivity have been only partially successful and, in general, have explained only a fraction of the observed variation in diversity. At the same time, studies that have focused on the importance of species pools have led some to conclude that diversity is primarily regulated in the short term by the size of the species pool rather than by biotic interactions. In this paper, I explore how community biomass and species pools may work in combination to regulate diversity in herbaceous plant communities. To address this problem, I employ a simple model in which the dynamics of species richness are a function of aboveground community biomass and environmentally controlled gradients in species pools. Model results lead to two main predictions about the role of biomass regulation: (1) Seasonal dynamics of richness will tend to follow a regular oscillation, with richness rising to peak values during the early to middle portion of the growing season and then declining during the latter part of the season. (2) Seasonal dieback of aboveground tissues facilitates the long-term maintenance of high levels of richness in the community. The persistence of aboveground tissues and accumulation of litter are especially important in limiting the number of species through the suppression of recruitment. Model results also lead to two main predictions about the role of species pools: (1) The height and position of peak richness relative to community biomass will be influenced by the rate at which the species pool increases as available soil resources increase. (2) Variations in nonresource environmental factors (e.g. soil pH or soil salinity) have the potential to regulate species pools in a way that is uncorrelated with aboveground biomass. Under extreme conditions, such nonresource effects can create a unimodal envelope of biomass–richness values. Available evidence from the literature provides partial support for these predictions, though additional data are needed to provide more convincing tests. [ABSTRACT FROM AUTHOR]

The size of the local species pool (i.e., species surrounding a community capable of dispersal into that community) and other dispersal limitations strongly influence native plant community composition. However, the role that the local species pool plays in determining the invasibility of communities by exotic plants remains to be evaluated. We hypothesized that the richness and abundance of exotic species would be greater in C4-dominated grassland communities if the local species pool included a larger proportion of exotic species. We also predicted that an increase in the exotic species pool would increase the invasibility of sites thought to be resistant to invasion (annually burned grassland). To test these hypotheses, study plots were established within two long-term (>20 yr) fire experiments at a tallgrass prairie preserve in NE Kansas (USA). Study plots were surrounded by either a small pool of exotic species (small species pool (SSP) plots; six species) or a larger exotic species pool (large species pool (LSP) plots; 18 species). We found that richness and absolute cover of exotic species was significantly (P <0.001) lower (∼70 and 90%, respectively) in annually burned compared to unburned plots, regardless of the size of the exotic species pool. As predicted, exotic species richness was higher (P <0.001) for LSP plots (3.9 per 250 m2) than for SSP plots (0.7 per 250 m2); however, absolute cover was unaffected by the size of the exotic species pool. In the absence of fire, plots with a LSP had four times as many exotic species than SSP plots. An increase in the local exotic species pool also increased the invasibility of annually burned grassland. Indeed, richness of exotic plant species in annually burned LSP plots did not differ from unburned plots with a SSP, indicating that a larger pool of exotic species countered the negative effects of fire. These findings have important implications for predicting how the invasion of plant communities may respond to human-induced global changes, such as habitat fragmentation. Community characteristics or factors such as frequent fires in grasslands may impart resistance to invasions by exotic species in large, intact ecosystems. However, when a large pool of exotic species is present, frequent fire may not be sufficient to limit the invasions of exotic plants in fragmented landscapes. [ABSTRACT FROM AUTHOR]

Discusses research on limitations to the richness of productive and oligotrophic plant communities. Studies showing that large-scale processes, both in space and time, may describe the variability in small-scale richness patterns; Results of the author's study, showing that variation in vascular plant species richness was linearly and significantly related to the size of regional species pools; Subdivision of communities into classes based on their limiting factors, including abiotically limited, diaspore limited, and competition limited; Ways in which richness can be increased in these three categories.