Predator trait evolution alters prey community composition

Predators have potentially strong effects on prey abundance, growth, life histories, and behavior, but the net direction and strength of these effects depend on traits in the predator population. Trait evolution that occurs on ecological timescales is common and may affect species interactions. Using laboratory microcosms of a bacterivorous ciliate (Colpidium sp.) and a diverse community of bacteria, we tested the hypothesis that predator trait evolution in response to temperature variability and nutrient concentration alters the composition of the prey community using a two-phase experiment. The first phase consisted of a selection experiment, in which microcosm communities were randomly assigned to one of eight fully factorial treatments of high or low nutrients, variable or stable temperatures, and with or without Colpidium. The second phase was a reciprocal transplant experiment, in which a subsample of each microcosm community from the selection experiment was transferred to one of four common garden environments, consisting of all combinations of high or low nutrients and variable or stable temperature. Nutrient enrichment directly decreased bacterial species richness and altered the community composition, but also indirectly increased bacterial species richness through decreases in ciliate peak density and, subsequently, ciliate abundance. Moreover, the evolutionary effects of nutrient enrichment on predator traits were more pronounced in some contemporary common garden environments, and undetectable in others. Our findings suggest that historical and contemporary environments are equally important to consider in understanding how trait evolution affects community structure.