Coexistence Mediated by Recruitment Fluctuations: A Field Guide to the Storage Effect

Abstract

For most species, a changeable environment creates a situation in which recruitment varies considerably from one breeding season to the next. If adults survive well, an occasional favorable recruitment can sustain population numbers over long periods. In effect, the gains made in favorable periods are stored in the adult population. Storage is particularly important when the species is at low densities, because then the potential population growth rate is very high if a favorable period occurs. Our past work showed that the storage mechanism could lead to coexistence of two species in lottery competition for space, as long as generations overlapped and there was sufficient variation in recruitment (Chesson and Warner 1981). This was true even if one species had an average competitive advantage. The storage model also operates when more than two species are competing, when resources renew independently of population sizes, and when not all the resource is used. It also operates in simple Lotka-Volterra systems in which adults do not compete directly with juveniles. The field ecologist is faced with the more practical problem of determining whether the storage mechanism is operating in a particular system. Species with relatively long lives and high fecundities are most likely to enjoy the benefits of the storage effect. Environments that theoretically elicit these life history characteristics are relatively benign and permanent for established adults, but are such that births and/or juvenile survivorship vary widely. Trees and many marine organisms are examples of species with the proper life histories, and storage may be important in maintaining the high diversity of these communities. The storage mechanism is capable of independently maintaining species coexistence, and we provide some suggestions on how to distinguish qualitatively the operation of storage from alternative mechanisms. We expect, however, that storage will make some positive contribution toward species persistence in nearly all communities, and we give a method for estimating empirically how large that contribution is.