Temperature-dependent interactions explain unexpected responses to environmental warming in communities of competitors

1. Predictions about how species will respond to climate warming are based commonly on eco-physiological models or niche models. One critique of this practice is that it ignores interspecific interactions. 2. We examined how resource competition affects the responses of two ciliate species, Colpidium striatum Stein and Paramecium tetraurelia Sonneborn, to warming in laboratory microcosms. 3. We found that warming had a negative effect on Colpidium abundance and a negligible effect on Paramecium abundance in the absence of interspecific competition. When Colpidium and Paramecium competed, however, Paramecium coexisted with Colpidium at low and high temperatures, and was competitively excluded at intermediate temperatures. 4. Temperature-dependent strength of the interaction between Colpidium and Paramecium may explain the unexpected responses of Paramecium, which were inconsistent with its response to temperature in the absence of Colpidium. 5. Our results provide further support for the important role of species interactions in understanding how species respond to environmental warming, and suggest that the complex interplay between species interactions and climate may make it extremely difficult to predict accurately how species embedded in complex communities will respond to climate warming.