Effects of multiple stressors on the dynamics of a fungal pathogen associated with global amphibian declines

Within the biological sciences there is increasing interest in understanding how ecological context alters host-pathogen interactions. Incidences of emerging infectious diseases (EIDs) are increasing around the globe with devastating impacts on many taxonomic groups. This trend is thought to result, in part, from increased frequency or intensity of interactions with stressors that facilitate the spread of a pathogen or increase its virulence. The mechanisms underlying these interactions are poorly understood. \ud This thesis explored how the stress of predators, competitors, and the insecticide malathion alter interactions between the fungal pathogen (Batrachochytrium dendrobatidis, Bd) and two of its amphibian hosts (Rana sylvatica and R. pipiens). Malathion is a widely-used insecticide that has been implicated amphibian declines. I examined how these interactions altered traits associated with within-host interactions (immune responses) and between-host interactions (life history, survival and behavior). \ud More than 200 amphibian species suffer from the EID caused by Bd, and the results of this study suggest that natural environmental stressors can alter patterns of infection. Sublethal exposure to predators caused decreases in releases of antimicrobial peptides (AMPs) from the skin. AMPs are an important defense against infection. The extent of this effect depended on the timing of exposure to predators as well as the length of exposure. In addition, AMPs were also more concentrated in high competition environments. This may be an adaptive response to increased risk of infection by pathogens with density-dependent transmission. Predation would also be expected to reduce disease incidence. Tadpoles infected with Bd were more active than uninfected and resistant tadpoles, suggesting that they would be more susceptible to predation by visually-cued predators. In contrast, resistant tadpoles were the least active, making them most likely to avoid detection by predators. While the pesticide malathion had no effect on susceptibility to Bd infection or antimicrobial peptide releases, a separate study showed that it and other pesticides in the same class (acetylcholine esterase inhibitors) can alter survival and life history trajectories of amphibians through direct toxic effects and indirectly through trait- and density-mediated trophic cascades. Overall these results suggest that environmental context can have complex effects on host-pathogen interactions.\u