Trait-mediated community assembly and species responses to environmental gradients in alpine plant communities across contrasting regional species pools

The relative importance of deterministic and neutral processes in shaping assembly of communities remains controversial, partly due to inconsistencies between theoretical, empirical, and experimental studies. We investigate the interplay between local (productivity) and regional (size of species pool) assembly mechanisms in communities of phytoplankton and zooplankton in 72 experimental microcosms. Local environmental conditions were manipulated by varying the level of nutrients in the water (ambient, low, high). The size of regional species pool colonizing each microcosm was manipulated by mixing phytoplankton and zooplankton species from different numbers of source ponds (n = 2, 4, 8 and 16). Our results show that local communities assembled differently depending on the numbers of sources available for colonization. Microcosms with larger species pools supported greater numbers of species. In contrast, the effects of productivity led to different results across trophic groups. Phytoplankton communities were, on average, more diverse on more productive treatments, while zooplankton communities were more diverse under less productive treatments. Phytoplankton and zooplankton communities responded to both sources of variation, although the size of species pool was a better predictor of communities’ composition than the local effects of productivity. These results reinforce the view that community assembly is influenced by the interplay of both local and regional drivers but that the relative importance of these factors varies with trophic groups.

Summary The regional species pool is a set of species available in a region and ecologically suitable for growing in the particular environment occupied by a local community. As species pools are largely influenced by evolutionary processes such as the conservation of ecological niches within lineages we hypothesize that the size of the regional species pool increases with the variety of distinct phylogenetic lineages represented in a local community. We contrast this with hypotheses invoking diversification of individual lineages within environments or stochastic present‐day assembly of local communities. We calculated phylogenetic distinctness for a local community as the number of nodes separating two species averaged over all pairwise comparisons across a phylogenetic topology of a regional flora. We calculated the size of the regional species pool for a local community as the number of species in the regional flora that share the ecological niche position of the species constituting the local community. Analysing field‐layer communities across a wide range of environments, we indeed found that local communities composed of phylogenetically highly distinct species recruit from larger species pools than communities of low phylogenetic distinctness. Accounting for the presence of two particularly diversifying lineages (Poaceae and Cyperaceae) confirmed these results. These results help us to understand how the species pool was assembled throughout evolution in different types of environments (immigration vs. in situ radiation of individual lineages). The phylogenetic approach is of large practical value to infer the size of the regional species pool because phylogenies have become available for many groups of species worldwide, while knowledge of the species’ ecological requirements or habitat affiliation (needed for the classical definition of species pools) is often still lacking. Synthesis. We show that the size of the regional species pool can be predicted by the average phylogenetic distinctness between the species present in a local community. This approach contributes to the understanding of the causes of species richness in regional species pools and local communities. The approach is also an important tool for determining the size of the regional species pool when parameters other than species phylogeny are not known.

Two types of species pool are distinguished. The regional species pool is defined as the set of species, occurring in a certain region (here: Estonia) which are capable of coexisting in a target community. The actual species pool is defined as the set of species present in a community. Field data from 14 different vegetation types in Estonia were used. The regional pool was compiled by including from the regional flora (1) all species for which the Ellenberg indicator values did not differ more than 1.5 relative units from the community mean and (2) all indifferent species. The actual pool was compiled by careful field observations. The aim of the paper is to test the validity of two null hypotheses about the species pool. HO 1 postulates that any size of the actual species pool is equally probable in the interval between zero and the size of the regional species pool. HO 2 postulates that any value of species richness per unit area (1 m 2 ) is equally probable in the interval between zero and the size of the actual species pool. To test the strengths of the relationships Monte Carlo modelling was used. It was shown that the relation between variables was stronger than proposed by the null models (P = 0.041 for HO 1 and P = 0.002 for HO 2 ). Consequently, the size of the actual species pool is largely determined by the regional species pool, and the species richness per 1 m 2 is largely determined by the actual pool. The results are discussed in the framework of coexistence theory. The size of the regional pool is determined by evolutionary (speciation) and historical (large-scale migration) processes. The size of the actual pool depends on local-scale migration, which can be a function of isolation, successional stage, local management history, etc.

Fungi play vital regulatory roles in terrestrial ecosystems. Local community assembly mechanisms, including deterministic and stochastic processes, as well as the size of regional species pools (gamma diversity), typically influence overall soil microbial community beta diversity patterns. However, there is limited evidence supporting their direct and indirect effects on beta diversity of different soil fungal functional groups in forest ecosystems. To address this gap, we collected 1606 soil samples from a 25-ha subtropical forest plot in southern China. Our goal was to determine the direct effects and indirect effects of regional species pools on the beta diversity of soil fungi, specifically arbuscular mycorrhizal (AM), ectomycorrhizal (EcM), plant-pathogenic, and saprotrophic fungi. We quantified the effects of soil properties, mycorrhizal tree abundances, and topographical factors on soil fungal diversity. The beta diversity of plant-pathogenic fungi was predominantly influenced by the size of the species pool. In contrast, the beta diversity of EcM fungi was primarily driven indirectly through community assembly processes. Neither of them had significant effects on the beta diversity of AM and saprotrophic fungi. Our results highlight that the direct and indirect effects of species pools on the beta diversity of soil functional groups of fungi can significantly differ even within a relatively small area. They also demonstrate the independent and combined effects of various factors in regulating the diversities of soil functional groups of fungi. Consequently, it is crucial to study the fungal community not only as a whole but also by considering different functional groups within the community.

The causes of linear relationships between local species richness and the size of the actual species pool in closed subalpine meadow communities and open plant communities of the alpine stony substrate (the Greater Caucasus Mountains) were analyzed using a computer simulation model. The results demonstrated that this relationship is insufficient evidence for the variation of local species richness among communities is wholly or partly determined by regional processes (the species-pool hypothesis). A relatively proportional ratio between these variables can also arise where local species richness and the size of the species pool both depend on local processes, or where local species richness is determined by local factors alone while the size of the species pool is determined by both local and regional factors.

1 Species-rich semi-natural grasslands in Europe developed during prehistoric times and have endured due to human activity. At the same time, intensive grassland management or changes in land use may result in species extinction. As a consequence, plant diversity in semi-natural calcareous grasslands may be related to both historical and current human population density. 2 We hypothesize that current vascular plant diversity in semi-natural calcareous grasslands is positively correlated with the Late Iron Age (c. 800-1000 years ago) density of human settlements (indicated by Late Iron Age fortresses and villages) due to enhancement of grassland extent and species dispersal, and negatively correlated with current human population density due to habitat loss and deterioration. 3 We described the size of the community vascular plant species pool, species richness per 1 m(2) and the relative richness (richness divided by the size of the species pool) in 45 thin soil, calcareous (alvar) grasslands in Estonia. In addition to historical and current human population density we considered simultaneously the effects of grassland area, connectivity to other alvar grasslands, elevation above sea level (indicating grassland age), soil pH, soil N, soil P, soil depth, soil depth heterogeneity, geographical east-west gradient, precipitation and spatial autocorrelation. 4 Both the size of the community species pool and the species richness are significantly correlated with the Late Iron Age human population density. In addition, species richness was unimodally related to the current human population density. The relative richness (species 'packing density') was highest in the intermediate current human population densities, indicative of moderate land-use intensity. 5 Community species pool size decreased non-linearly with increasing soil N, and was highest at intermediate elevation. Small-scale richness was greater when sites were well connected and when the elevation was intermediate. Spatial autocorrelation was also significant for both species pool size and small-scale richness. 6 In summary, human land-use legacy from prehistoric times is an important aspect in plant ecology, which could be an important contributor to the current variation in biodiversity. (Less)

Modelling vegetation dynamics in managed grasslands: Responses to drivers depend on species richness

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.

Understanding the structure of and spatial variability in the species composition of ecological communities is at the heart of biogeography. In particular, there has been recent controversy about possible latitudinal trends in compositional heterogeneity across localities (β‐diversity). A gradient in the size of the regional species pool alone can be expected to impose a parallel gradient on β‐diversity, but whether β‐diversity also varies independently of the size of the species pool remains unclear. A recently suggested methodological approach to correct latitudinal β‐diversity gradients for the species pool effect is based on randomization null models that remove the effects of gradients in α‐ and γ‐diversity on β‐diversity. However, the randomization process imposes constraints on the variability of α‐diversity, which in turn force γ‐ and β‐diversity to become interdependent, such that any change in one is mirrored in the other. We argue that simple null model approaches are inadequate to discern whether correlations between α‐, β‐ and γ‐diversity reflect processes of ecological interest or merely differences in the size of the species pool among localities. We demonstrate that this kind of Narcissus effect may also apply to other metrics of spatial or phylogenetic species distribution. We highlight that Narcissus effects may lead to artificially high rejection rates for the focal pattern (Type II errors) and caution that these errors have not received sufficient attention in the ecological literature.

Recent theoretical developments involving community assembly on the one hand, and invasion biology on the other, suggest a gradual convergence in thought between what have been two largely separate theoretical initiatives. The term "invasibility" emerged in the field of invasion ecology to describe the susceptibility of environments to invasion by species from other regions of the world. Although Elton did not use the term "invasibility" in his pioneering book (1958), he did employ the concept, referring to an ecosystem's "vulnerability to invasion". Given its original definition, the concept of invasibility has been limited in its scope and use, with rather little application to the larger field of community ecology. However, our assessment and usage of the concept (Davis et al. 2000, 2001) has prompted us to consider invasibility as a more general condition of all environments. This expanded perspective of invasibility has caused us to reconsider some fundamental questions and issues regarding community assembly and species diversity as well as recent discussions involving the notion of metacommunities (Leibold and Miller 2004, Leibold et al. 2004). By metacommunity, we mean a set of local communities that are linked by dispersal of multiple, and potentially interacting, species (Leibold et al. 2004). Recent theoretical efforts to characterize community assembly processes have reemphasized understanding the importance of interactions between local and regional processes (Levine 2000, Hubbell 2001, Tilman 2004, Foster and Dickson 2004, Jiang and Morin 2004, Steiner and Leibold 2004, Leibold et al. 2004). There is general agreement that the diversity of the regional species pool and the extent of dispersal of the species from this pool throughout the region are the principal regional processes involved. However, investigators have emphasized the importance of different local conditions and processes, including productivity (Jiang and Morin 2004, Steiner and Leibold 2004), demographic stochasticity (Tilman 2004), ecosystem size (Fukami 2004), biotic limitation of diversity (Tilman 2004), and even extent of tree lean, the latter which affects colonization success of epiphyes (Snäll et al. 2005). We propose that the notion of invasibility can serve as a unifying concept in these discussions and thereby can facilitate current efforts to develop a more comprehensive and realistic theory of community assembly and metacommunity dynamics. We define "invasibility" as the susceptibility of an environment to the colonization and establishment of individuals from species not currently part of the resident community. By establishment, we mean that the persistence of colonizing individuals is due primarily to their ability to sustain themselves by accessing resources in their new environment, e.g. as opposed to surviving on resources imported from their original environment. Although a new species often subsequently spreads throughout its new environment, we believe that colonization and establishment are sufficient criteria to define invasibility, since a species can persist at a site indefinitely without spread, or even recruitment from reproduction, as long as individual colonizers are able to establish and persist long enough for other colonists to replace them before they die. Although practical obstacles will often make it difficult to measure invasibility, conceptually, the quantification of invasibility is straightforward. For example, invasibility can be quantified as the probability of establishment per arriving propagule (Davis et al. 2000). (Formally, invasibility describes only a community's potential for being colonized. Whether that potential is realized is dependent on the presence and abundance of propagules.) Ultimately, a community's invasibility varies not only in time, but from species to species (and even from genotype to genotype within a species). At a particular moment in time, a community might be readily invasible to one species but not to another. Hypothetical changes in invasibility (I) of an environment over time to a particular species (a). Maximum invasibility (1.0) occurs when every arriving propagule successfully establishes. Since establishment success of arriving propagules is normally very low, the magnitude of the invasibility path shown for the hypothetical species has been exaggerated for illustrative purposes. The invasibility at a particular point (x) during the time period is indicated with an arrow. The invasibility of the environment to this species (Ia) over the time period shown (0–t) can be quantified as: . Whether or not a species is a long-term resident in the region or has been recently introduced to the regional species pool, the ability of colonizers to become established in a new community depends on the existence of available resources (Davis et al. 2000) and other site attributes of the new environment, such as the presence or absence of particular predators and pathogens (Shea and Chesson 2002) and the extent to which the physical conditions of the original environment match those of the new environment (Kolar and Lodge 2002). Community invasibility, then, is a general phenomenon, applying to all species and all communities, and represents a composite of local processes affecting community assembly. A central controversy in community ecology for the past forty years has been whether communities are mostly saturated with species or whether local community diversity is limited primarily by the richness of the regional propagule pool (MacArthur 1965, Ricklefs 1987, Cornell and Lawton 1992, Lawton 1999). This debate, like so many in ecology, can be traced back to Darwin (1859), who believed that competition limited diversity and that the earth was largely saturated with species: "The extinction of old forms is almost the inevitable consequence of the production of new forms." (Darwin 1859). The debate over the relative importance of local or regional processes in community assembly intersects with the diversity-invasibility controversy. The diversity-invasibility hypothesis, first proposed by Charles Elton (1958), holds that most available niches will already be occupied in species-rich communities and that thus these communities will be more resistant to invasion than species-poor communities, which are believed to contain more unoccupied niches. Many ecologists since have agreed with Elton (e.g. Tilman 1999, Knops et al. 1999, Naeem et al. 2000) while others have suggested that species rich communities sometimes may actually be more invasible (Lonsdale 1999, Stohlgren et al. 1999). Recent assessments have emphasized the role that spatial scale likely plays in the diversity-invasibility relationship (Levine 2000, Tilman 2004, Jiang and Morin 2004, Steiner and Leibold 2004), while others have questioned whether the relationship exists at all, other than as a statistical artefact (Fridley et al. 2004, Herben et al. 2004). Although we have participated in the diversity-invasibility debate (Davis et al. 2000, 2001), we now believe that the debate has been misdirected since Elton first proposed the diversity-invasibility hypothesis. The original, and hitherto uncontested, assumption of the diversity-invasibility hypothesis is that diversity (D) is the independent variable and invasibility (I) is the dependent variable. Thus, for more than forty years, ecologists have been debating the equation I=f(D). However, perhaps all along we should have been debating D=f(I). We believe that invasibility, not diversity, is the more fundamental essence of a community, and that diversity does not give rise to invasibility, but rather emerges from it. In other words, we believe that invasibility, a condition that represents the integration of many local processes, is one of the two major drivers of diversity at the local level, the other being regional processes involving dispersal from the regional species pool (Fig. 2). The proposed dispersal-invasibility model of metacommunity dynamics, showing that local patterns of diversity result from the interacting dual effects of invasibility, an attribute of a local environment or community, and the diversity of, and dispersal from, the regional species pool. The diagram shows that invasibility of community A (INVA) is a composite attribute, influenced by both physical and biological conditions, events, and processes operating at the local scale. Invasibility of communities is expected to vary over time due to changes in the local conditions, events and processes that, together, define invasibility. The regional species pool represents the species richness of the metacommunity and is made up of all the species residing throughout all the individual communities. As described in the text, in some circumstances, local invasibility can have a feedback effect on the richness of the regional species pool (feedback indicated by the dashed arrows between individual communities and the regional species pool). With this shift in perspective, invasibility is seen as a dynamic property of communities that is more fundamental than species diversity because it precedes species diversity. Invasibility exists and can be measured (at least theoretically) even in completely unpopulated environments. Although there would be no competition for resources from resident species in such cases (since no species are present), invasibility still exists as a measurable attribute of the environment, and would be affected by the absolute levels of resources present in the environment and by the extent to which the physical environment, including the disturbance regime, compromises the colonists' ability to access those resources (Fig. 2). Thus, invasibility is not a peripheral feature of a community relevant only to a particular subset of species and ecological processes, but describes a general and fundamental condition of all environments. As shown in Fig. 2, the invasibility of an environment is influenced by the interaction of biological and physical processes operating at the local scale. Physical conditions include basic life constraints, such as temperature, water availability (for terrestrial organisms), O2 or CO2 levels (for aquatic organisms), and presence or absence of a necessary substrate, e.g. soil, rocky crevices, etc. Food web interactions, both within and between trophic levels, can either increase or decrease the invasibility of an environment for a particular species, or group of similar species, depending on the nature of the interactions. Facilitative effects of species often involve modifying physical conditions, events, and/or processes, such as increasing gross resource levels (e.g. legumes), ameliorating harsh physical conditions (e.g. nurse plants), and introducing disturbances (e.g. burrowing animals), but they also may provide benefits such as pollination and increased ability to access resources (e.g. mychorrizal fungi). While each of the individual physical and biological processes plays a role, ultimately it is the integrated sum of the processes, the environment's invasibility, that is the local driver of diversity. The primary effect of an environment's invasibility on local diversity is as a filter of incoming propagules. A more invasible environment means that more of the dispersing propagules will be able to become established, thereby increasing diversity whenever the newly established propagules represent a new species. If invasibility represents the accessibility of an environment to all prospective colonizers, then species-rich communities must be, or have been in the past, quite invasible, at least periodically. Unless a community's high diversity is due primarily to in situ speciation, colonization by new species must have been a common occurrence at some point in its history. Logically, it cannot be any other way. The highly invasible nature of species-rich grasslands is not a new discovery, but has been known for some time. Grubb (1976) noted that much of the diversity of species-rich limestone grasslands consisted of annuals, biennials and short-lived perennials that only persisted in the system by continual regeneration from seed. Van der Maarel and Sykes (1993) pointed out that high rates of turnover of species and individuals were typical of limestone grasslands in Sweden. Later, they showed that this was also true for species-rich grasslands on other continents (Sykes et al. 1994). Stampfli and Zeiter (2004) found similar high turnover and rates in their study of a species-rich semi-natural meadow in Switzerland. Further evidence that species-rich limestone grasslands are not strongly structured by interspecific competition are findings that most species appear to be distributed at random relative to each other (Pearce 1987, Mahdi and Law 1987, Mahdi et al. 1989, Campbell et al. 1991). We agree with Leibold et al. (2004) that invasibility at the local level can generate some feedback to the species pool at the regional scale (Fig. 2), although we believe this feedback is likely quite small, at least for metacommunities consisting of a large number of local communities, for the following reasons. An environment with low invasibility will support a community comparatively low in species richness, meaning that species not residing in this community must reside in other local communities in order to remain a part of the regional species pool. Thus, environments with low invasibility are supporting a smaller proportion of the regional species pool than highly invasible, and hence more species-rich, environments. As long as there are many species-rich environments, it is unlikely that one, or a few, low-invasibility environments will reduce the regional species pool. However, as the proportion of low-invasibility environments increases, colonization events throughout the metacommunity will not be able to keep pace with local extinction rates of some species, resulting in the regional extinction of some species, and hence a decline in the richness of the regional species pool. Lawton (1999) described a one-dimensional continuum of communities, ranging from what he referred to as Type I communities, the diversity of which seemed to be determined primarily by regional processes, e.g. diversity of the regional propagule pool, to Type II communities, which seemed to be governed more by local processes, e.g. species interactions and habitat suitability. The perspective we are presenting allows us to consider invasibility (local processes) and diversity of the regional species pool (regional processes) as two largely independent variables that can be presented orthogonally to construct a simple two-dimensional graphical representation (Fig. 3) of the dispersal-invasibility model of metacommunity dynamics presented in Fig. 2. In this visual framework, differences in local diversity are seen to arise from differences in the richness of regional species pools and the invasibility of the respective local environments. For example, Region A (Fig. 3) characterizes environments with high invasibility that encounter rich regional species pools. Examples of this environment type are tropical rain forests and coral reefs. Both environments experience periodic disturbances that facilitate the introduction of new species and the persistence of resident species (Sale 1977, Connell 1978, Hubbell 2001), and the species diversity of the regional species pool is very high in both cases. Distribution of different community types and environments shown as a function of local invasibility and the regional species pool using a graphical representation of the diversity-invasibility model presented in Fig. 2. Region B (Fig. 3) characterizes environments with high invasibility, but diversity is limited by a comparatively poor regional species pool. Temperate environments and many islands represent this region type. For example, temperate forests also experience frequent disturbances, including fire, wind, and insect outbreaks, however the diversity of these environments is limited by the comparatively small number of tree species in the temperate regional pool. Whatever the ultimate cause(s) for regional differences in the diversity of species pools, the simple graphical representation of the proposed dispersal-invasibility model shows that diversity differences among similar environments from different regions of the world should be due primarily to differences in the richness of the respective regional species pools. Assuming similar environments in a single region encounter a similar species pool, differences in diversity among similar environments within a single region should be due primarily to differences in invasibility of the environments. Diversity can be suppressed by low levels of invasibility in the face of adequate, or even rich, species pools (Region C in Fig. 3) in several ways. Abundant resources may be available at a site in an absolute sense, but completely, or nearly completely, already sequestered by the residents. For example, over-harvesting of herbivorous reef fish has eliminated, or sharply reduced the extent of, grazing by fish in many reef ecosystems throughout the world, and is believed to have contributed to the recent domination of algae in these reefs (Stimson et al. 1996, McClannahan 1997). Even though these reefs most likely still encounter propagule pools rich in coral species due to the pelagic dispersal patterns of coral larvae (Karlson and Cornell 2002), the algal dominated reefs have become quite resistant to coral colonization since the algae have coopted virtually all available space, the key limiting resource in these environments. A terrestrial example of Region C (Fig. 3) is the species-rich limestone grasslands of northern Europe, the diversity of which can be drastically reduced by the invasion of the rhizomatous grass Brachypodiumpinnatum (Bobbink and Willems 1987, 1991, Hurst and John 1999). Even small patches of Brachypodium are markedly less diverse despite exposure to a diverse seed rain from surrounding species-rich grassland. Another way that diversity can be suppressed by low levels of invasibility even in the face of rich propagule pools (Region C, Fig. 3) is if a site is resource-poor in absolute terms. In such cases, even if few resources are sequestered by residents, and hence most are available to colonizers, the amount of available resources is still insufficient to support most new colonizers. For example, even if many plant species dispersed to an environment with sterile soils (whether historically nutrient poor or impoverished due to human activity), few would encounter sufficient resources to permit the species to establish successfully. Very high disturbance rates can also lead to low levels of invasibility. Although high disturbance rates would presumably free up considerable resources for both colonizers and residents, relatively few species would be sufficiently disturbance-tolerant to be able to colonize and persist in these environments and thereby take advantage of the abundant resources available. Annually cultivated agricultural lands are an example of this. The most species-poor communities are communities characterized by low levels of invasibility located in regions with poor regional species pools (Region D, Fig. 3). Examples of such environments are high-latitude sites in which successful colonization and persistence is limited by the harsh physical conditions and often low absolute levels of resources, and which encounter depauperate species pools. Remote rocky islands are another example of Region D communities, their limited regional species pools a product of their remoteness, and the low invasibility limited by the low absolute levels of resources. Several recent theoretical studies of community assembly have investigated some of the issues we have presented here. Tilman (2004) proposed an elaboration of classical competition theory, which he termed "stochastic niche theory", in which he emphasized the importance of the stochasticity of colonization and the interaction between the independent processes of "recruitment limitation" and "biotic limitation of diversity" in explaining patterns of invasion and community assembly. Like us, Tilman emphasized the essential interaction of regional and local processes in determining local patterns of diversity. However, his stochastic niche theory is still based in the traditional approach that conceives invasibility as the dependent variable, with diversity affecting invasibility via competition. Steiner and Leibold (2004) presented a theoretical model designed to provide insights as to why productivity-diversity relationships are usually unimodal at the local scale but monotonically increasing at larger spatial scales. Their model showed that high productivity should result in both high invasibility and high species turnover at the local level, which when combined with stochastic dispersal processes, would tend to produce different species compositions among different communities, resulting in high beta diversity, and the resulting montotonic increase of diversity with increasing spatial scale. Jiang and Morin (2004) created a productivity gradient in aquatic mesocosms stocked and invaded with different species of microbes that provided support for the that in productivity can produce the relationship between diversity and invasibility that is so often at larger scales. Steiner and model and Jiang and on a we believe is often with the invasibility of a site (Davis et al. 2000). Steiner and model and Jiang and findings represent a of the more general of local invasibility we are presenting that of invasibility being a composite (and attribute of an environment, which is with conditions, events, and processes in to Leibold et al. (2004) the new concept of metacommunity as a way to community ecology, the local patterns of community are affected by the larger regional species pools and local community processes may back and the larger scale regional In their Leibold et al. described that have both theoretical and on species and in whether or not individual communities vary in their attributes and to different species and whether or not the species involving dispersal and The model we are all of these For example, Fig. shows that communities not vary in their attributes and is the as the communities not vary in their invasibility. Although we that invasibility should be a of diversity, rather than a our model does for feedback of community on invasibility. The large and diverse that web effects can have on invasibility (Fig. means that invasibility can be affected by the presence or absence of particular species that have a such as habitat and competition. et al. to such species as However, of this feedback is not the as the notion of effects between invasibility and diversity. for very the only all may be in which diversity has sometimes been found to invasibility et al. 1999, Naeem et al. but et al. 2004, Herben et al. 2004), we believe the relationship between invasibility and diversity is and not D=f(I). is very that ecologists are able to describe basic processes to and the general We believe that the simple two-dimensional model we have proposed an to and the general on the processes that In we believe the model can be to the effects on local and regional patterns of diversity of both human and can be in of their effects on regional species pools and the invasibility of particular local communities. The only we might suggest for these discussions is a more term for perhaps or of the For example, while increasing the availability of often in the domination of a small number of species that reduce diversity by to reduce invasibility and 2004, et al. (Fig. nutrient e.g. poor and can many species from a site they and et al. and 1999, and also a in invasibility and diversity (Fig. that a community's disturbance can reduce community invasibility, and thereby its diversity, if disturbance rates or increase or decrease and resource availability (Fig. the other hand, some disturbances, such as the of can increase an environment's invasibility by an disturbance (Fig. in patterns of local diversity that can be expected to result from and other due to local changes in invasibility and regional changes in the diversity of the species pool. The magnitude of the effect of these changes on local patterns of diversity for a particular and even the of the diversity will be influenced by site conditions and the species as well as by the and extent of the and other The and of species by has increased the diversity of species pools in many regions of the world 2001, et al. which has in in the diversity of many local communities within those regions and et al. (Fig. The presence of new species in the regional pool and their colonization and establishment in individual communities can either increase or decrease the invasibility of those communities (Fig. in the way that the community's invasibility is increased or by the presence of species. For example, the new species may resources thereby invasibility and community diversity (Bobbink and Willems 1987, 1991, Hurst and John or the new species may harsh physical conditions, thereby the introduction other species, including species, resulting in an increase in the diversity of the local environment 2004, and 2005). Invasibility and regional species pools, and hence patterns of diversity, are also to changes in et al. 2004). in of of events, and other processes are likely to the of successful as patterns of resource availability and physical Thus, depending on the particular changes a community due to its invasibility may either increase or decrease (Fig. In any changes in the of would be expected to be by of species, thereby a community's regional species pool (Fig. In a some environments would be expected to experience an increase in their regional species pools, e.g. environments, species pools might be expected to decline in regions that become more The notion that ecological communities are via and that this dispersal species of the respective communities is not new (MacArthur and Ricklefs However, there is in our understanding of local and regional processes a more model of metacommunity dynamics (Leibold et al. 2004). at least different have been proposed dynamics, species and the model (Leibold et al. 2004). of these are highly for example, on such as are in all other than species of these represent first in a comprehensive theory of is now is a more comprehensive and one that would be able to the key and of and that would provide a much more realistic of metacommunity dynamics (Leibold et al. 2004). Recent theoretical developments involving community assembly on the one hand, and invasion biology on the other, suggest a gradual convergence in thought in what have been two largely separate theoretical initiatives. We the presented will this we believe that the concept of invasibility, with the dispersal-invasibility model (Fig. 2), that we have presented can serve as an in which to and the conditions, events, and processes that the patterns of diversity we within and between local communities in different regions throughout the world. In we believe that the simple graphical representation of the model (Fig. thought and among can be by ecologists to and the general of ecological processes that and the way in which human can these processes (Fig. in this were by by the and the

SummaryUnderstanding variation in community composition across space, or beta diversity, is of longstanding interest in ecology, yet the determinants of beta diversity remain poorly known. In part, this results from a lack of manipulative tests of hypothesized drivers. The size of species pools is one putative driver, but few studies have provided a direct test of this mechanism through manipulation of clearly defined species pools independent of local communities. Furthermore, we know little about underlying mechanisms, such as enhanced species sorting, or whether a species pool size‐beta diversity relationship is scale‐dependent or modified by environmental conditions.Here, we evaluate 29 prairie plant communities restored from bare soil with known species pools (seed mixes) to address those questions. To address the generality of beta diversity drivers across scales, we investigated how the size of species pools during restoration influenced beta diversity in the plant community at two scales: among prairies and within prairies (among plots).Among a group of prairies sown with larger species pools, among‐site beta diversity was greater than among a group of prairies assembled from smaller pools, but not because of enhanced species sorting. We found an interaction between species pool size and an environmental filter, whereby beta diversity was higher among prairies restored with species‐rich seed mixes, but only when soil moisture was also high. We detected neither greater beta diversity nor stronger species sorting among plots within prairies sown with species‐rich mixes.Synthesis. This work provides what is to our knowledge the first large‐scale manipulative test of how species pool size influences beta diversity. We found higher beta diversity among restored prairies sown with species‐rich seed mixes, but little evidence for species sorting as a causal mechanism. Our results, based on manipulated real‐world communities, provide an important link between previous theoretical and observational studies and small‐scale experimental approaches. Of applied importance, our findings show that by creating communities of high beta diversity, ecological restoration can counteract widespread anthropogenic biotic homogenization.

Disentangling regional and local drivers of species richness in communities is a long-term focus of ecology. Regional species pools affect local communities by providing their constituent species. Additionally, the amount and variety of resources enhance diversity locally. Here, we investigated whether the same ecological factor (vegetation complexity) shapes both regional and local species richness and thus drives local diversity both indirectly (via pool size) and directly (via facilitating the coexistence of species). We studied passerine birds of woodlands and forests in eastern Australia. We quantified regional species pool size and sampled local bird communities at 63 transects spanning 3,000 km. We estimated canopy height both regionally using satellite imagery and locally using vegetation sampling in the field. We studied how species pool size changed with regional canopy height and water availability, and how local species richness changed with pool size and local canopy height. Local species richness increased with both local canopy height and the size of the regional species pool. Pool size, in turn, increased with regional canopy height, which itself increased with water availability. Moreover, local species richness expressed as a proportion of the regional pool also increased with local canopy height. In sum, vegetation complexity indexed by canopy height had a doubly positive effect on local species richness: indirectly by promoting a large regional species pool and directly by facilitating the coexistence of disproportionately many species locally. Regional pools were larger in tall forests probably due to the legacy of extensive moist forests that once covered most of Australia, thus providing a sizeable potential for speciation, diversification, and species persistence. Local species richness was greater in tall, more productive forests with more vegetation layers likely due to more and varied resources (i.e., more potential niches), allowing the coexistence of more individuals and species of consumers.

To explain the ecological processes affecting community assembly is one of the important theoretical and practical issues in ecology research. In recent years, some studies have revealed that local processes alone cannot sufficiently explain the community assembly mechanism, and the assembly process is often related to spatial scales. We constructed seven species pool gradients for null model analyses using 186 species from three tropical cloud forests in Hainan Island, China, and determined 11 functional traits of these species. We calculated Rao quadratic entropy (RaoQ) index values of these functional traits on three spatial scales. In combination with the null model, the effects of species pool size and spatial scale on the community assembly were further explored. We found that the observed values of RaoQ were significantly lower than expected in different species pool sizes and spatial scales, indicating that habitat filtering drove the community assembly process of tropical cloud forests, and the species pool size and spatial scale had no significant effect on the community assembly of tropical cloud forests. The results showed that the standard effect size of RaoQ was higher in larger species pools and smaller scale (5 × 5 m), indicating that the effect of habitat filtering was more significant on larger species pools and smaller scales.KeywordsFunctional diversitySpecies pool sizeSpatial scaleHabitat filteringTropical cloud forest

Questions: To what extent can Shipley et al.'s original maximum entropy model of trait-based community assembly predict relative abundances of species over a large (3000 km2) landscape? How does variation in the species pool affect predictive ability of the model? How might the effects of missing traits be detected? How can non-trait-based processes be incorporated into the model? Location: Central England. Material and Methods: Using 10 traits measured on 506 plant species from 1308 1-m2 plots collected over 3000 km2 in central England, we tested one aspect of Shipley et al.'s original maximum entropy model of “pure” trait-based community assembly (S1), and modified it to represent both a neutral (S2) and a hybrid (S3) scenario of community assembly at the local level. Predictive ability of the three corresponding models was determined with different species pool sizes (30, 60, 100 and 506 species). Statistical significance was tested using a distribution-free permutation test. Results: Predictive ability was high and significantly different from random expectations in S1. Predictive ability was low but significant in S2. Highest predictive ability occurred when both neutral and trait-based processes were included in the model (S3). Increasing the pool size decreased predictive ability, but less so in S3. Incorporating habitat affinity (to indicate missing traits) increased predictive ability. Conclusions: The measured functional traits were significantly related to species relative abundance. Our results both confirm the generality of the original model but also highlight the importance of (i) taking into account neutral processes during assembly of a plant community, and (ii) properly defining the species pool.

The species pool concept has advanced our understanding for how biodiversity is coupled at local and regional scales. However, it remains unclear how species pool size, the number of species available to disperse to a site, influences community assembly across spatial scales. We provide one of the first studies that assesses diversity across scales after experimentally assembling grassland communities from species pools of different sizes. We show that species pool size causes scale-dependent effects on diversity in grasslands undergoing restoration by altering the shape of the species-area relationship (SAR). Specifically, larger species pools increased the slope of the SAR, but not the intercept, suggesting that dispersal from a larger pool causes species to be more spatially aggregated. This increased aggregation appears to be caused by sampling effects due to fewer individuals arriving per species, rather than stronger species sorting across variation in soil moisture. These scale-dependent effects suggest that studies evaluating species pools at a single, small scale may underestimate their effects, thereby contributing to uncertainty about the importance of regional processes for community assembly and their consequences for ecological restoration.