Natural selection is an important ecological process that can drive evolutionary change. However, the causes of selection remain largely unexplored. Predators are important selective agents in many natural populations, and among communities variation in predator density is common. Variation in predator density could result in variation in the strength of predator-prey interactions, and thus the strength of selection predators exert on their prey. However, selection could also become uncoupled from the strength of predator-prey interactions when the strength of the interaction becomes too strong. We tested the hypothesis that the strength of selection varies with the strength of predator-prey ecological interactions. We predicted that as predator density increased, the strength of natural selection on prey phenotypes likewise increased. However, when predator density was extremely high, we predicted that selection on prey phenotype would be stochastic. To test this hypothesis, we manipulated the density of predators (bluegill sunfish, Lepomis microchirus) while holding the density of prey (orange bluet; Enallagma signatum) constant in lake mesocosms. We measured growth and mortality of the prey in response to predator density. We also quantified natural selection on prey activity levels, because it is a phenotype known to be under strong selection by fish predators.
Results/Conclusions
Contrary to our predictions, there was no relationship between predator density and selection on prey phenotype. Sixty-two percent of prey survived; however, survival did not vary with predator density. Likewise, growth and activity of damselflies were predation risk invariant. We conclude that there is not a relationship between fish density and damselfly activity phenotype and suggest that other factors, such intraspecific competition or predator behavior, may interact with selection by predators to shape the relationship with prey phenotype.