The fundamental role that parasites play in shaping populations, communities, and ecosystems can vary across environmental gradients. In amphibian-parasite systems, predators can decrease a host’s probability of becoming infected by parasites by reducing parasite density through direct consumption or by acting as alternative hosts. Conversely, amphibian predator-avoidance mechanisms, like decreased activity, can increase a host’s probability of becoming infected. Effects of larval exposure to predators and parasites can carry-over to influence amphibian host responses in terrestrial life stages. We evaluated the potential for predator exposure to alter host-parasite dynamics in northern leopard frogs (Lithobates pipiens) reared as tadpoles in outdoor aquatic mesocosms, and then transferred to terrestrial enclosures for rearing through overwintering. We hypothesized that larval exposure to predators (crayfish [Orconectes rusticus], bluegill sunfish [Lepomis macrochirus], or mosquito fish [Gambusia affinis]) would impact northern leopard frog behavior, development, and survival during larval development through terrestrial life stages in ways that altered the effects of the trematode parasite, Echinostoma sp. All three predators consume tadpoles, but mosquito fish also consume larval parasites, and both fish species can serve as alternative hosts for this parasite, and thus may differentially impact amphibian-trematode systems.
Results/Conclusions
Tadpoles responded to the perceived threat of predators and parasites by reducing their activity, increasing their risk of parasite detection. While parasites negatively impacted tadpole survival when crayfish or no predators were present, the influence of fish on survival was less straightforward. Exposure to bluegill sunfish reduced survival dramatically and obscured any effect of parasites, whereas mosquito fish did not impact survival even in the presence of parasites. By acting as alternative hosts and parasite consumers, mosquito fish may reduce the number of parasites available to infect tadpoles, thereby reducing parasite effects. Parasite exposure increased the length of the larval period and resulted in greater mass at metamorphosis. Presence of bluegill sunfish also increased mass at metamorphosis. The influence of these factors on mass is likely mediated by reduced tadpole density releasing surviving tadpoles from competition. Larval exposure to parasites and predators also interacted to influence growth in the terrestrial environment but not terrestrial survival post-overwintering, which suggests these factors influence terrestrial condition in sublethal ways. This study indicates changes in community composition can impact both host and parasite responses, and thereby, regulate the outcome of infections, highlighting the importance of maintaining existing biodiversity across in aquatic systems.