Coexistence and evolutionary dynamics mediated by seasonal environmental variation in annual plant communities

It is well established theoretically that competing species may coexist by having different responses to variation over time in the physical environment. Whereas previous theory has focused mostly on year-to-year environmental variation, we investigate how within-year variation can be the basis of species coexistence. We ask also the important but often neglected question of whether the species differences that allow coexistence are compatible with evolutionary processes. We seek the simplest circumstances that permit coexistence based on within-year environmental variation, and then evaluate the robustness of coexistence in the face of evolutionary forces. Our focus is on coexistence of annual plant species living in arid regions. We first consider environmental variation of a very simple structure where a single pulse of rain occurs, and different species have different patterns of growth activity following the rain pulse. We show that coexistence of two species is possible based on the storage effect coexistence mechanism in this simplest of varying environments. We find an exact expression for the magnitude of the storage effect that allows the functioning of the coexistence mechanism to be analyzed. However, in these simplest of circumstances, coexistence in our models is not evolutionarily stable. Increasing the complexity of the environment to two rain pulses leads to evolutionarily stable species coexistence, and a route to diversity via evolutionary branching. This demonstration of the compatibility of a coexistence mechanism with evolutionary processes is an important step in assessing the likely importance of a mechanism in nature.