Many studies of community ecology focus on interactions between groups of free-living species, however, within these groups there are often a large number of interacting cryptic parasite species. Amphibians serve as hosts to many parasite taxa, including the trematode flatworm, Echinostoma sp., a macroparasite with relatively low pathogenicity, and the amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), a microparasite with high pathogenicity. These parasites occur simultaneously across the Midwestern landscape, yet their combined effects are largely unknown, particularly during physiologically stressful times such as metamorphosis and overwintering. The objective of this study was to determine the individual and combined effects of Echinostoma sp. exposure during larval development and Bd exposure at metamorphosis on growth and survival through overwintering using Blanchard’s cricket frog (Acris crepitans blanchardi), a species declining in parts of its range. We tested the hypothesis that infection by Echinostoma sp. and Bd will have net negative effects on metamorphosis and overwintering. To test this hypothesis, we manipulated trematode exposure in mesocosm communities by exposing a subset of developing tadpoles to wild caught ramshorn snails (Helisoma trivolvis) infected with Echinostoma sp. Following metamorphosis, a subset of frogs was exposed to Bd and reared terrestrially in the laboratory through overwintering.
Results/Conclusions
Trematode and Bd infections both altered amphibian development and survival, supporting our hypothesis. Trematode exposure decreased survival and mass at metamorphosis, and increased time to metamorphosis. Bd reduced post-overwintering survival by 70% relative to the control. The combined effect of these parasites resulted in the lowest growth among juveniles exposed to both parasites relative to the juveniles exposed to one or no parasites. During post-experiment necropsies, no trematodes were found in any frogs that survived overwintering, indicating the possibility of larval anti-parasite behavior or parasite clearance, despite apparent effects on responses measured. Both trematodes and Bd decreased survival during critical times of development, highlighting the need for long-term studies with these parasites. Reductions in size and survival mediated by exposure to parasites could potentially influence population viability. It is critical to monitor the effects of parasites through terrestrial life stages, particularly overwintering, as our results emphasize the potential importance of overwintering in modulating the outcomes of parasite infections among temperate species.