Parasites and pathogens are among the most common members of most ecosystems and influence community structure, food webs, and even evolution. In recent decades, however, numerous diseases have emerged or re-emerged, frequently owing to changes in the environment of hosts and their parasites. In this study, I explore the direct and indirect causes of one such disease - widespread observations of severe limb deformities in North American amphibian populations. Many of these malformations are caused by a parasite (Ribeiroia ondatrae) that burrows into the developing limbs of larval amphibians. However, reasons for the apparent increase in infection remain unclear. I test the hypothesis that nutrient runoff into aquatic systems associated with agriculture and cattle production is responsible for promoting parasite infection. Ribeiroia has a complex life cycle involving passage among snails, amphibians, and birds. Excess nutrients (eutrophication) are expected to increase the numbers of snails that support the parasite, leading to more parasites in amphibians and therefore more deformities. By studying these relationships in the field, conducting ecological experiments, and using mathematical models, I investigate how increased nutrient inputs - a global problem in lakes, ponds, and rivers - affect the parasite, its hosts, and the level of deformities. Taken together, these approaches may offer new insights into the drivers of amphibian malformations while helping to advance an ecosystem-level approach to the study of disease.
