Bounded population sizes, fluctuating selection and the tempo and mode of coexistence.

Existing theory predicts competitors (species or genetic clones) cannot coexist in a fluctuating environment unless relative fitness is negatively frequency-dependent (relative fitness declines as the frequency of a competitor increases). We develop simple theory to show coexistence does not require frequency-dependent selection, and we confirm this prediction by direct experiment. The conditions for coexistence in a fluctuating environment are precisely the same as those for coexistence in a spatially variable environment, conditions that arise naturally whenever population abundances are bounded. Simulations show the likelihood of coexistence increases with environmental uncertainty. The capacity of temporally variable environments to maintain biological diversity is far broader than generally envisaged.