Toward a mechanistic understanding of thermal niche partitioning.
We develop a theoretical framework to elucidate the mechanistic basis of thermal niche partitioning in ectotherms. Using a food web module of two consumers competing for a biotic resource, we investigate how temperature effects on species' attack and mortality rates scale up to population-level outcomes of coexistence and exclusion. We find that differences between species in their competitive effects ultimately arise from asymmetries generated by the nonlinear nature of the temperature response of mortality: cold-adapted species and thermal specialists limit themselves more strongly than they limit their warm-adapted and generalist competitors. These asymmetries become greater as seasonal temperature fluctuations increase, generating latitudinal variation in coexistence patterns and priority effects. Characterizing species' thermal niches in terms of mechanistic descriptions of trait responses to temperature allows us to make testable predictions about the population-level outcomes of competition based solely on three fundamental - and easily measurable - quantities: attack rate optima, response breadths, and temperature sensitivity of mortality. We validate our framework by testing its predictions with data from an insect host-parasitoid community. Simply by quantifying the three basic quantities, we predict that priority effects cannot occur in this system, which is borne out by population-level experiments showing that the outcome of competition does not depend on initial conditions. More broadly, our framework can predict the conditions under which exotic invasive species can exclude or coexist with native biota as well as the effects of climate warming on competitive communities across latitudinal gradients.