Explaining why species diversity varies within and among plant communities remains a formidable unresolved question in ecology. The limited success of many species interaction models in explaining species diversity patterns has led to investigation of the potential constraints imposed on local diversity by the species pool in the surrounding area (Ricklefs 1987; Cornell & Lawton 1992; Leibold et al. 2004). Species pools can influence local species interactions by determining the number and identity of species available to interact and by rescuing species from extinction through immigration (van der Putten et al. 2009). Mounting evidence suggests that the size and composition of the regional species pool influence plant diversity (Tilman 1997; Zobel 1997; cf. Foster 2001). These species pool effects appear to interact with local abiotic and biotic factors to control observed diversity patterns in many systems, but particularly in grasslands (Zobel et al. 2000; Foster 2001; Houseman & Gross 2006). The growing acknowledgement of the importance of local species interactions and immigration has led to the development of metacommunity theory that specifically addresses how the outcome of local species interactions is modified by migration between communities and shows how local and regional scale processes influence diversity (Leibold et al. 2004).
An important element of local–regional or metacommunity models of diversity is identifying conditions under which species pools have strong or weak effects on diversity. The results of seed-addition experiments in grasslands have shown that the influence of the species pool can vary widely (Turnbull, Crawley & Rees 2000). Foster et al. (2004) proposed that underlying site productivity mediates the importance of the species pool for local diversity, with stronger species pool effects at low to intermediate productivity. Although evidence for this view is growing (Foster et al. 2004; Stevens et al. 2004; Houseman & Gross 2006), there remain substantial differences among these studies. Some studies have found that colonization from augmented species pools is lower in high than intermediate productivity sites (Foster & Dickson 2004; Houseman & Gross 2006), but that sites with low productivity show inconsistent responses (Foster et al. 2004; Houseman & Gross 2006). It is unclear whether this incongruence reflects site-specific differences among studies such as site history, the magnitude of environmental variation found along the resource gradient or the invasibility of the established plant community.
Moreover, while augmenting the species pools has been shown to increase local diversity, it is not clear whether the observed increase in species richness is accounted for by a random subset of the species pool or reflects species sorting patterns driven by trait–environment relationships (Lavorel et al. 1997). Differing effects of species augmentation on local richness across sites that vary in productivity (or some other environmental characteristic) may reflect species sorting based on variation in traits among species in the species pool. If specific traits confer different establishment probabilities depending on environmental conditions, then we would expect to see changes in both the richness and composition of species that colonize sites. In contrast, colonization success may be dependent on stochastic variation during immigration resulting in a random selection from the species pool (Hubbell 2001). Neutral processes could lead to variation in diversity along resource gradients if seedling emergence or mortality rates for all species varied with productivity. Nutrient or moisture limitation at low productivity, and light limitation at high productivity, could decrease emergence or increase seedling mortality relative to intermediate productivity sites. The higher recruitment at intermediate productivity could reduce the number of rare species lost to stochastic extinction leading to peak richness at intermediate productivity. Thus, a unimodal productivity–diversity pattern could occur without any relationship between species identity (or traits) and productivity. Despite the growing interest in local–regional or metacommunity concepts as drivers of diversity, there is little experimental evidence as to whether neutral or niche-based colonization processes determine the observed diversity patterns across productivity gradients.
In this study, we test two hypotheses as to what determines productivity–diversity patterns in grasslands: that (i) the strength and shape of these patterns are dependent on the species pool and (ii) such patterns reflect species sorting associated with species traits. A test of the first hypothesis requires experiments across multiple sites with similar environmental gradients using the same species pool to determine if there are generalities in the strength of the local–regional diversity interaction related to productivity. We report here the results of such a cross-site experiment designed to examine this relationship in four successional grasslands that had substantial within-site variation in productivity and species composition and therefore to investigate the generality of the colonization–productivity relationship (e.g. the unimodal pattern; Houseman & Gross 2006). Second, we tested whether the species that colonized these sites represented a random selection from the species pool or species sorting in relation to productivity. Evidence for species sorting along productivity gradients has primarily come from observational studies (Whittaker 1967; Weiher, Clarke & Keddy 1998; Ackerly & Cornwell 2007), but it is unclear whether the distributional patterns reflect trait–environment sorting patterns or are a consequence of differences in dispersal limitation along the environmental gradient. Finally, we tested whether any of the observed species sorting patterns in our field experiment were related to species traits, which were independently measured in a greenhouse study. Although these experiments cannot reveal the extent to which metacommunities operate in this system, they provide a critical test of whether colonization processes – which are an inherent part of metacommunity dynamics – reflect niche-based or neutral processes.