Stochasticity-induced stabilization weakens in diverse communities.

Abstract

Environmental stochasticity and the temporal variations of demographic rates associated with it are ubiquitous in nature. The ability of these fluctuations to stabilize a coexistence state of competing populations (sometimes known as the storage effect) is a counterintuitive feature that has aroused much interest. Here we consider the performance of environmental stochasticity as a stabilizer in diverse communities. We review the results of previous studies which suggest that the stabilizing effects of stochasticity weaken as the number of species increases, provide a systematic numerical exploration of the phenomenon and identify the relevant parameter regimes. Of particular importance is the ratio between the dwell time of the environment and the generation time: we show that stochasticity promotes diversity only when this ratio is smaller than the inverse of the fundamental biodiversity parameter. In an opposite regime, when stochasticity impedes coexistence and lowers the species richness, its effect is determined by the ratio between the strength of environmental variations and the rate at which new types are added to the community via speciation, mutation or immigration.