  The storage effect is a well-known explanation for coexistence 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.
One interpretation states that species coexist by specializing on specific
environmental states, and therefore must have a robust life-stage (e.g.,
long-lived adults, a seed-bank) in order to "wait it out" for favorable
conditions. Here we show that this widely employed interpretation can be
misleading. Multiple models show that stage-structure, long lifespans, and
overlapping generations are neither necessary nor sufficient for the storage
effect. In models where a robust life-stage does engender a storage effect, it
does not do so by preventing stochastic extinction or by improving relative
bet-hedging. A robust life-stage is best understood as one of many ways to
fulfill an abstract condition for the storage effect: an interaction effect of
environment and competition on per capita growth rates. Using a dataset of
annual plants from a Mediterranean grassland in Spain, we show that such
interaction effects occur between water availability and the number of
germinant competitors, leading to storage in the absence of a persistent seed
bank. Empiricists hoping to uncover the storage effect should look for
interaction effects between environmental conditions and competition -- easily
identifiable with multiple regression -- at all stages of a species'
life-cycle.
