Effects of Anthropogenic Environmental Stressors on Amphibian Immune Defenses and Host-Pathogen Interactions Across Temperate and Tropical Systems

As infectious diseases, climate change, and anthropogenic habitat alterations are projected to continue negatively impacting global biodiversity, we need to understand how physiological responses to these ecological stressors are impacting host health. Physiological stress responses are governed by glucocorticoids, which are responsible for homeostasis, resource allocation, and remobilization of energy stores, but can also directly and indirectly impact host immune function. Amphibians are the most threatened vertebrate taxon, in part because of climate change and habitat alteration, but also because of fungal pathogen Batrachochytrium dendrobatidis (Bd). Bd infects amphibian skin, resulting in chytridiomycosis and a disturbance of the host skin bacteriome. I used multiple amphibian species from both neotropical and temperate systems to investigate my hypothesis that the primary amphibian glucocorticoid, corticosterone (CORT), would be associated with anthropogenic habitat alteration, Bd infection, host bacteriome, and host body condition. To non-invasively quantify CORT, I used a novel skin swabbing method to collect dermal (mucosal) glucocorticoids and validated the collection and extraction method with ELISAs. I predicted that higher CORT levels would be found in habitats with higher levels of anthropogenic alteration and that higher CORT would result in higher Bd pathogen loads, disturbance of the skin bacteriome, and lower host body condition. In Boana faber (Hylidae; Brazil Atlantic Forest (BAF)), we found that probiotic bacteriome manipulation treatments were associated with lower CORT levels and that forest fragmentation and host bacteriome manipulations impacted body condition. Our results revealed differing disease and CORT dynamics in two BAF bromeligenous anurans, but we found some evidence that CORT impacts both Bd infection and skin bacteriome composition. In Notophthalmus viridescens (Eastern newts; Birmingham, AL) we found no association between CORT and Bd, and marginal support for a negative association between CORT and body condition. Overall, this research indicates that dermal CORT may indeed play a role in disease, skin bacteriome composition, and body condition, but that more research is necessary.