Stochastic processes, not species sorting, drive positive effects of species pool richness on beta diversity in prairie restorations

Background/Question/Methods

Beta diversity, differences in community composition across space, is a fundamental property of ecological systems. The processes establishing and maintaining beta diversity, however, remain a puzzle for basic and applied ecologists. Beta diversity can be driven by stochastic processes such as dispersal that lead to uncertain and variable outcomes. It can also be driven by deterministic processes such as species sorting along environmental gradients. High species richness of species pools could enhance both processes. High beta diversity in plant communities is often a goal of restoration practitioners, both because beta diversity is a component of biodiversity and because beta diversity may increase landscape diversity and combat biotic homogenization. Furthermore, restorations present an opportunity to study the mechanisms driving the generation of beta diversity during community assembly. In a survey of thirty prairie restorations in southwest Michigan, we asked whether species richness of seed mixes increased plant community beta diversity. We also asked whether it did so by enhancing species sorting or by increasing stochastic establishment of rare species. 

Results/Conclusions

We found that prairie restorations established with diverse species pools (i.e., seeded with a species rich seed mix) had higher levels of beta diversity (turnover among plots within restorations). Variation in seed mix richness also caused variation in community composition among restorations. However, this higher beta diversity was driven largely by higher species richness at the site scale (gamma diversity) indicating stochastic assembly. Heterogeneity in soil water holding capacity, a potential driver of species sorting, weakly increased beta diversity, but we detected no statistically significant effect of water holding capacity on species sorting. We also detected no interactions between seed mix richness and heterogeneity, again pointing to weak sorting. Instead, seed mix richness appeared to increase diversity by increasing the number of rare species, suggesting a key role of stochastic processes in the development of beta diversity. These results suggest that restoration practitioners can increase alpha, beta, and gamma diversity, and community differentiation among restoration sites, by increasing the species richness of seed mixes. The results also suggest that at least in newly-assembled communities, the positive effects of species pool diversity on compositional turnover at both small (within-site) and larger (among-site) scales may be predominantly driven by stochastic processes.