Species interactions mediate thermal evolution.

Understanding whether populations and communities can evolve fast enough to keep up with ongoing climate change is one of the most pressing issues in biology today. A growing number of studies have documented rapid evolutionary responses to warming, suggesting that populations may be able to persist despite temperature increases. The challenge now is to better understand how species interactions, which are ubiquitous in nature, mediate these population responses to warming. Here, we use laboratory natural selection experiments in a freshwater community to test hypotheses related to how thermal evolution of Daphnia pulex to two selection temperatures (12 and 18°C) is mediated by rapid thermal evolution of its algal resource (Scenedesmus obliquus) or by the presence of the zooplankton predator Chaoborus americanus. We found that cold-evolved algae (a high-quality resource) facilitated the evolution of increased thermal plasticity in Daphnia populations selected at 12°C, for both body size and per capita growth rates (r). Conversely, warm-evolved algae facilitated the evolution of increased r thermal plasticity for Daphnia selected at 18°C. Lastly, we found that the effect of selection temperature on evolved Daphnia body size was more pronounced when Daphnia were also reared with predators. These data demonstrate that trait evolution of a focal population to the thermal environment can be affected by both bottom-up and top-down species interactions and that rapid temperature evolution of a resource can have cascading effects on consumer thermal evolution. Our study highlights the importance of incorporating species interactions when estimating ecological and evolutionary responses of populations and communities to ongoing temperature warming.