Coexistence by recruitment variation: How large is the contribution of relative nonlinearity?

Background/Question/Methods

Recruitment variation is believed to be an important mechanism of species coexistence in variable environments. This mechanism, however, is a composite of two other mechanisms: the storage effect, and relative nonlinearity. The storage effect is a formalization of the concept of temporal environmental niche differentiation.  It is well-understood theoretically and has been tested in natural systems such as forests and annual plant communities. Relative nonlinearity results from species differences in nonlinear responses to fluctuations in competition. Such differences arise whenever species differ in life-history traits such as mean longevity. While the storage effect is assumed to be the stronger mechanism, no studies have compared the relative magnitudes of the storage effect and relative nonlinearity in the same system, and so there has been no rigorous justification for the neglect of relative nonlinearity in theoretical and empirical studies of coexistence by recruitment variation. Our research uses analytical modeling and simulation tools to quantify the magnitude of these two mechanisms for iteroparous organisms coexisting by recruitment variation.

Results/Conclusions

Our results show that the strength of relative nonlinearity increases with species differences in longevity, and is larger with larger environmental variation. However, the storage effect is also larger with larger environmental variation, and in most cases, is much stronger than relative nonlinearity. Nevertheless, there are circumstances where relatively nonlinearity is predicted to be stronger than the storage effect. We find this is outcome in two-species cases where species have strong differences in their sensitivities to environmental fluctuations, as well as having different longevities. Indeed, it is possible for the storage effect to make no contribution to stabilization of coexistence, leaving relative nonlinearity as the only stabilizing mechanism. However, this outcome occurs only in a narrow range of circumstances. We conclude that under most circumstances, relative nonlinearity contributes little to the overall stabilizing effect and the neglect of relative nonlinearity in previous work is justified in those cases. Nevertheless, relative nonlinearity has the potential to make substantial contributions to species coexistence when species differ in their sensitivities to environmental variation.