Interactions between Echinostome Parasites and Larval Anurans across Ecological Contexts and Scales.

Abstract

Parasites influence population dynamics and community structure, yet historically have received relatively little attention. Most host-parasite studies focus on these interactions in isolation, despite the likelihood that outcomes depend on the presence of other species (i.e., community context). In this dissertation, I examine the context dependence of trematode parasite (Digenea: Echinostomatidae) interactions with larval frog hosts and the implications for amphibian population dynamics and community structure. First, I address the dependence of host-parasite interactions on predator presence. Like many animals, tadpoles frequently encounter both natural enemies simultaneously, and I show how the presence of echinostomes and odonate predators leads to non-additive effects on important host traits such as physiology, behavior, and development. I further demonstrate that both enemies synergistically affect survival because of such trait effects, i.e., parasite avoidance behavior (higher activity) increases predation rates by enhancing tadpole visibility. These results suggest a potentially general tradeoff between responding to predation risk and parasitism. Second, I address the dependence of host-parasite interactions on the presence of host competitors and resource levels. I show that, at least under some circumstances (i.e., poor growth conditions or high densities), competition exacerbates both infection levels and the negative effects of echinostomes on tadpole growth and survival. I also show that higher resources increase infection levels, due to enhanced tadpole growth rates and size-dependent infection rates. Third, I scale this approach up to a multi-host context, which provides evolutionary insights and suggests important implications for community structure. I show that faster developing and spring-breeding species experience relatively greater infection levels and post-infection mortality, and species with greater variation in habitat use tend to exhibit more plastic behavioral responses to parasites. Finally, I examine parasitism in natural ponds, finding that both biotic and abiotic factors affect echinostome distributions and abundances, and echinostome abundances influence larval frog survival. Overall, my findings emphasize the importance of considering ecological context in understanding and predicting host-parasite interactions, and the consequences to population dynamics and community structure. Further, since echinostome abundance appears associated with human activities and disease has contributed to recent global amphibian declines, my findings have significant conservation implications.