The storage effect is not about bet-hedging or population stage-structure

Abstract

The storage effect is a well-known explanation for the coexistence of competing species in temporally varying environments. Like many complex ecological theories, the storage effect is often used as an explanation for observed coexistence on the basis of heuristic understanding, rather than careful application of a detailed model. But, a careful examination of some widely employed heuristic representations of the storage effect shows that they can be misleading. One interpretation of the storage effect states that species coexist by specializing on a small slice of all environmental states, and therefore must have a robust life-stage (e.g., long-lived adults, a seedbank) in order to "wait it out" for a good year. Another more general interpretation states that buffering helps species "wait it out", where buffering means populations are protected from large losses when the environment is poor and competition is high. Here, we show that both of these conventional interpretations are imperfect. Multiple models show that stage-structure, long lifespans, and overlapping generations are not required for the storage effect. Further, a species that experiences buffering necessarily grows poorly when the environment is favorable and competition is high. We review the empirical literature and conclude that species are likely to find themselves in this good-environment/high-competition scenario, and therefore, that buffering tends to hurt individual species. The buffering interpretation of the storage effect can be thought of as conflating a community-level criterion for coexistence with a population-level criterion for persistence; it is like claiming that species can persist by limiting their own growth rates, since the Lotka-Volterra model tells us that intraspecific competition must be greater than interspecific competition. To build a better understanding of the storage effect, we examine how it manifests in particular models and we demonstrate the connections between the mathematical and verbal/textual descriptions of the storage effect. While it is possible to improve one's understanding of the storage effect, the storage effect is a very general phenomenon, and so a simple ecological interpretation (in terms of a small set of life-history characteristics like stage-structure or dormancy) will not be forthcoming.