The storage effect is most commonly understood as a mechanism of species coexistence. Key to coexistence by the storage effect is coupling between the competition that species experience and their responses to changes in the physical environment over time. This coupling is measured as covariance between environment and competition (covEC). Coexistence is promoted when this covariance is density-dependent, being strong and positive when a species is absolutely and relatively abundant, but weaker or negative when a species is rare while its competitors are not. Such density dependence of covEC naturally arises when a favorable physical environment promotes resource consumption, and rapid resource depletion occurs. We asked whether slow resource depletion might lead to an opposite form of density dependence such that covEC decreases with density instead of increasing. To investigate this possibility we used a consumer-resource model with a fluctuating environment, and varied the rate of resource depletion while keeping other factors constant. We considered both seasonal and aseasonal environmental variation.
Results/Conclusions
Our model simulations reproduced the usual outcome of species coexistence in both seasonal and aseasonal environments when resource depletion was rapid, but in the case of a seasonal environment, slow resource depletion led to strong negative storage effects due to the predicted reversal of the density dependence of covEC, which was stronger at low density than at high density. These negative storage effects lead to contingent competitive exclusion whereby any species unlucky enough to fall to low density, while its competitors remain abundant, experiences negative average population growth and ultimate competitive exclusion. This phenomenon also implies a priority effect: the first species to colonize a locality excludes other species regardless of species identity. An unexpected outcome was multiple stable states. Intermediate resource speeds led to positive storage effects with a priority effect whereby the first species to colonize has a higher long-term density than the second species. Thus, both species coexist but their relative average long-term densities is dependent on their colonization order. In an aseasonal environment, these effects are much weaker, but our simulations suggest that they are still possible. For a better understanding of species coexistence both theoretically and empirically, these findings emphasize the need for more in depth study of the factors affecting the coupling between competition and the environmental factors, with implications for all stable coexistence mechanisms, not just the storage effect.