Predicting how climate change will affect natural systems is possibly the most urgent challenge facing ecologists. Several theories provide frameworks for how temperature effects propagate from the organismal level to shape communities and ecosystems. Ecophysiology and the Metabolic Theory of Ecology (MTE) postulate that community-level performance reflects the summed performance of individual organisms so temperature only impacts density-independent processes. Yet, density-dependent interactions are critical in structuring communities and determining species coexistence. Modern Coexistence Theory (MCT) can provide explicit predictions of how both density-independent and density-dependent demographic rates affect coexistence, but doing so requires additional knowledge of how demographic rates vary across temperature. We combine metabolic models of temperature dependence from MTE with competition theory of MCT to test the hypothesis that temperature dependence of density-independent rates directly predicts competitive outcomes and performance of species in diverse communities. We then experimentally test our hypothesis by quantifying the individual-level metabolic performance, resource acquisition and fecundity of two co-occurring cladoceran species, Daphnia magna and Diaphanosoma brachyurum, over a thermal gradient. We then test the importance of density-dependent mechanisms to the outcome of competition between the two species across temperatures in an experiment. Finally, we determine whether these results scale up to predict the abundances of each of the two species in diverse communities with natural temperature fluctuations.
Results/Conclusions
We find that temperature has profound and consistent effects on performance. Daphnia had a lower thermal optimum (25 to 26.5°C) than its smaller competitor, Diaphanosoma (30 to 32°C), as predicted by the scaling relationships of MTE. Thermal tolerances of each species overlapped considerably, but analyzing species’ temperature-dependent population growth rates with MCT shows that Daphnia consistently outcompetes Diaphanosoma across most of this thermal range. This result was confirmed directly in competition experiments. Despite the predictions of MTE, competition ultimately restricted Diaphanosoma to temperatures outside of Daphnia’s optimum thermal range. The results of this analysis also scaled up to explain the community-level experiments, where Diaphanosoma was found to be abundant only in temperatures above the thermal optima of Daphnia. These results suggest that metabolic rates predict higher-order performance but that summing the density-independent rates of single species will poorly predict performances of multiple species in complex communities subject to density-dependent interactions. This study suggests common approaches that do not account for density-dependent competition may underestimate the impacts of climate changes on species distributions and the composition of communities.