This study examines the recent evolutionary and behavioral response of prey in response to varying predator regimes. Classic density-mediated trophic cascades occur when predator density reduces, through consumption, prey density which in turn decreases the capacity of those prey to reduce the densities of their resources. Our central hypothesis is that the evolution of prey feeding traits in response to predation will have ecological consequences, specifically, that it will amplify the strength of a trophic cascade. Using mosquitofish (Gambusia affinis), a widespread freshwater fish, we ask whether predation by largemouth bass (Micropterus salmoides) and bluegill sunfish (Lepomis macrochirus) results in evolution of mosquitofish morphology and behavior and what the ecological consequences of this evolution is. We tested this by measuring the abundance of zooplankton, mosquitofish, and predatory fish in 25 ponds located in California. We collected wild fish from a subset of these ponds and assessed their boldness, measured as time to exit an enclosure. Last, we reared F1 fish from 12 population where each population was split into two groups and reared in the presence or absence of predator cues. We then conducted feeding trials with each population to assess feeding rates in the presence or absence of predators.
Results/Conclusions
Mosquitofish abundance was significantly higher in ponds without predators, this matches what is predicted by classic trophic cascades. Bluegill and MVC populations showed an increase in mean boldness while bass and no predator populations did not differ in mean boldness. Each predator treatment differed in their behavioral response to increased competition with bluegill populations significantly increasing boldness as competition increased. In bluegill and MVC populations we observed an increase in male boldness and a decrease in female boldness, there was no difference between male and female behavior in bass and no predator populations. We found that feeding rates of F1 mosquitofish from both predator and no predator populations were reduced in the immediate presence of predators, this reduction is highest in predator populations. Mosquitofish have been observed to have differing behavioral responses due to varying predator regimes which has the potential to impact their prey source, zooplankton, and amplify a density driven trophic cascades. This study allows us to evaluate how evolutionary history, rearing environment, and immediate environment impact feeding rates.