Host food resource supplementation increases echinostome infection in larval anurans.

Host-parasite interactions are often influenced by environmental factors through multiple mechanisms. For example, changes in host food resources may affect multiple host traits (e.g., body size, behavior, immunocompetence), which may increase or decrease infection levels and the impact of parasites on host fitness. We often lack an understanding of which traits are most important for parasite transmission and fitness effects, posing challenges to predicting consequences of changing environmental conditions (e.g., eutrophication). Here, I examined the effects of food resources and host traits experimentally in a larval frog (Rana clamitans Latreille, 1801)-trematode parasite (Echinostoma revolutum Looss, 1899) system. I hypothesized that higher food resources reduce parasite infection and parasite effects on host growth and survival, due to increased host investment in parasite defenses, which I tested in a laboratory experiment. Contrary to my hypothesis, the results indicated that increased food levels enhanced infection in hosts, while the effect of parasites on survival did not depend on host food resources. A potential explanation for the positive effect of food level on infection was size-dependent infection rates (i.e., higher food levels increased infection through increased host growth), which is supported by a positive relationship between host body size and infection. These findings emphasize the complex relationship between host food resources and parasitism and the importance of environmental context and host traits (i.e., body size) in mediating interactions with parasites. The results also have relevance for conservation in light of rising anthropogenic impacts on aquatic systems and recent amphibian declines.