How temporal environmental stochasticity affects species richness: Destabilization, neutralization and the storage effect

Temporal environmental stochasticity (TES), along with the variations of demographic rates associated with it, is ubiquitous in nature. Here we study the effect of TES on the species richness of diverse communities. In such communities the biodiversity at equilibrium reflects the balance between the rate at which new types are added (via migration, mutation or speciation) and the rate of extinction. We analyze a few generic models in which the speciation rate is fixed and TES affects the rate of extinction, and identify three different mechanisms. First, TES increases abundance variations and shortens extinction times, thus decreasing the species richness (destabilizing effect). Second, TES blurs the time-independent fitness differences between species, making the dynamics more symmetric and thereby increasing the diversity (neutralizing effect). Third, the storage effect allows TES to facilitate the invasion of inferior species, again contributing to the species richness. The stabilizing effect of storage declines significantly in diverse communities and it can overcome the destabilizing effect of TES only when environmental fluctuations are rapid enough.