Print PageThe presence of both parasites and predators can lead to complex, non-additive effects beyond those present when only one type of natural enemy is present. Many parasites affect traits which influence predation rates, and predators can affect prey traits that lead to increased rates of parasite infection. Additionally, the ability of a host species to tolerate a set infection load may change in the presence predator stress, resulting in an additional potential synergistic effect. In an experiment, I examined how nonconsumptive and lethal effects of predators in combination with echinostome (Trematoda: Digenea) parasites affect larval green frogs (Rana clamitans). I performed a 2 x 3 factorial mesocosm experiment varying the presence of echinostomes (3 infected or uninfected Planorbella trivolvis snails) and predators (none, caged or free Anax junius dragonfly larvae).
Results/Conclusions
Parasites and free Anax predators both negatively affected larval green frog survival. Nonlethal predators did not affect survival, but both predator treatments strongly affected green frog behavior. Nonlethal predators and parasites did not have an interactive effect, despite evidence for such an effect in small-scale aquarium experiments in previous studies. However, lethal predators and parasites affected mortality synergistically, likely due to a combination of 1)the effects of parasite infection on predation risk and 2)the effects of predation on density and thus transmission rates. This result suggests that complex interactions among these natural enemies may have important consequences for a shared population of victims.
