Parasites and predators can strongly affect the structure and composition of animal communities. In addition to directly causing mortality, both types of natural enemies can affect traits, such as growth or behavior, which may influence interactions with other species. Such trait-mediated indirect interactions can have important consequences for communities. Larval frog communities frequently experience the dual threats of parasitism and predation; each can affect survival and traits, which may facilitate indirect effects. To examine these effects, we performed 3x2x2 factorial experiment in mesocosms (1300L cattle tanks). Using an additive design with 3 levels of competition, we examined whether larval Anax (odonate) predators and echinostome (trematode) parasites mediate competition between two competitors: large (LG) and small (SG) green frog (Rana clamitans) tadpoles. The experiment excluded consumptive effects of predators by using caged predators, which prevented direct predation but produced chemical cues. Smaller green frogs tend to experience higher predation risk and a lower tolerance of echinostome infection, which influences their phenotypic response to each natural enemy. Based on these differences in susceptibility and phenotypic responses to parasites and predators, I expected that both types of natural enemies would lessen the competitive effect of SG on LG, potentially synergistically.
Results/Conclusions
As expected, predators had an indirect positive effect on the growth of LG, mediated through effects on SG traits. Contrary to our expectations, however, LG exposed to high levels of SG competition experienced lower growth in the presence of parasites compared to controls. This result suggests that the combination of parasitism and competition negatively affected the growth of LG, overwhelming any positive effect mediated through the effects of the parasites on SG traits. Also, surprisingly, at lower levels of competition, both LG and SG increased growth in the presence of parasites relative to controls. In the absence of strongly limited resources, both groups of tadpoles may have increased foraging, potentially to compensate for costs of infection. The effects of predators and echinostomes thus depend strongly on community context, potentially leading to counterintuitive results. Further investigation into such dependencies will be essential to understand how these interactions occur in natural communities.