Theory suggests that temporal fluctuations can promote species coexistence via two independent mechanism: relative nonlinearity and the storage effect. While the storage effect has attracted much attention, relative nonlinearity has been assumed to play a minor role in real ecosystems, partly because of its dependence on either differences in longevity or in the nonlinearity of competing species' functional responses. However, recent empirical studies have demonstrated the potential importance of relative nonlinearity for promoting species coexistence. Here we demonstrate that there is another pathway for relative nonlinearity to promote species coexistence: differences in the speed of adaptation to temporally changing environments. We consider a model of resource competition between two consumer species, and the species with a higher resource requirement (R*) adapt to changing resource levels by rapid evolution or phenotypic plasticity (e.g., dormancy). We assume that there is a trade-off between resource consumption and mortality, and that the rapidly adapting species can adjust in response to resource availability.
Results/Conclusions
Even when the two consumer species have the same nonlinearity of functional response, differences in the speed of adaptation can result in differences in the nonlinearity of species' realized per capita growth rate. The rapidly adapting species with a higher R* can stabilize population dynamics through eco-evolutionary feedback, and has a fitness advantage in a fluctuating environments. Thus, a rapidly adapting species can "consume temporal variance" and coexist with a non-adapting species with a lower R*, even when it has a fitness disadvantage in stable environments. This is also true when the species with a lower R* exhibits some time lag in response to resource availability whereas the other species with a higher R* has no time lag. Resource-dependent dormancy, where consumer species becomes dormant when resources are scarce, results in similar dynamics. As recent studies have revealed that rapid adaptation is pervasive in the wild, relative nonlinearity may play a more important role than generally envisaged.