Although it is well-established that parasites can have strong impacts on their hosts, less is known about how these effects compare with other biotic factors like competition and predation. Parasitism might also interact with these other factors, leading to synergistic effects on host population dynamics and life history characteristics. We conducted two parallel experiments to determine whether and how a trematode parasite (Echinostoma trivolvis) interacts with competition and predation to influence infection rates and life history characteristics in toad tadpoles (Bufo americanus). We used a total of 72 mesocosms and a randomized factorial design.  In both experiments, exposure to parasites was crossed with tadpole density (15 or 30 tadpoles per 40 L container) and the perceived risk of predation (presence or absence of a caged newt, Notophthalmus viridescens). In the first experiment, we measured susceptibility to infection by exposing individual tadpoles to a set number of echinostome cercariae. We postulated that tadpoles exposed to predator cues and high competition would have higher susceptibility to infection due to trade-offs between the immune system, anti-predator defenses and competition. In the second experiment, we exposed tadpoles to continuous infection risk from infected snails until metamorphosis. We postulated that competition would extend the larval period and predation would accelerate it, leading to increased and decreased exposure to parasites, respectively. We also postulated that tadpoles would accelerate development in response to high infection risk, as a way of reducing exposure to parasites. 

Results/Conclusions

The results indicated that parasitism interacted with competition and predation in toad tadpoles and had similar effects on tadpole development, growth and survival.