Stochastic Community Assembly Causes Higher Biodiversity in More Productive Environments

Biodiversity and Productivity When data are analyzed at relatively large spatial scales, biodiversity generally increases with productivity, but the pattern at smaller scales is more variable. Chase (p. 1388, published online 27 May) presents results from a 7-year experiment in artificial ponds. β-diversity (the variation in species composition from site to site) in animal species was persistently higher at higher productivity among otherwise homogeneous environments in this controlled experimental venue. This pattern appeared to result from shifts in the relative importance of deterministic versus stochastic community assembly mechanisms along the productivity gradient. Thus, variation in the mechanism of community assembly might be an important process determining the relationship between biodiversity and productivity. A long-term experiment in ponds addresses the influence of primary productivity on broad-scale patterns of biodiversity. Net primary productivity is a principal driver of biodiversity; large-scale regions with higher productivity generally have more species. This pattern emerges because β-diversity (compositional variation across local sites) increases with productivity, but the mechanisms underlying this phenomenon are unknown. Using data from a long-term experiment in replicate ponds, I show that higher β-diversity at higher productivity resulted from a stronger role for stochastic relative to deterministic assembly processes with increasing productivity. This shift in the relative importance of stochasticity was most consistent with the hypothesis of more intense priority effects leading to multiple stable equilibria at higher productivity. Thus, shifts in community assembly mechanisms across a productivity gradient may underlie one of the most prominent biodiversity gradients on the planet.