Effects of Biotic and Abiotic Setting on a Host-Pathogen Relationship: How Environmental and Community Characteristics Influence Infection Prevalence and Intensity of Amphibian Chytrid on California's Central Coast

Author(s): Hemingway, Valentine | Advisor(s): Raimondi, Peter | Abstract: In the face of swift anthropogenic change, it is essential to examine the broad ecological context for species of concern using a variety of approaches in order to understand their interactions in a natural context. Host-pathogen relationships offer a close interaction to examine how each are acted upon by biotic and abiotic conditions. Batrachochytrium dendrobatidis, an emerging infectious disease of amphibians, has been implicated with wholesale loss and marked declines in amphibian species across the globe, thus understanding its dynamics across amphibian hosts and in complex, natural environments is a key area for conservation focus.For this dissertation, I tested the importance of various biotic and abiotic factors in the relationship between B. dendrobatidis and three co-occurring amphibian hosts, as well as across metrics of host physiological health. In Chapter 1, I ranked environmental conditions that favor B. dendrobatidis success in two native and one introduced amphibian tadpole species. I found top models favoring B. dendrobatidis infection included: a) a positive relationship with amphibian community diversity, b) elevated B. dendrobatidis infection in co-occurring infected amphibian species, and c) a varying, but strong relationship with assorted vegetative cover types. In Chapter 2, I asked which factor, Bd infection, amphibian community diversity, and predator diversity, best explained tadpole physiological metrics including, body condition, total white blood cell count, and neutrophil to lymphocyte ratio, for three species of tadpole. The most important factors predicting tadpole body condition varied markedly by species. Tadpole body condition was positively correlated with Bd infection and predator diversity for the introduced Lithobates catesbeiana, negatively correlated with amphibian community diversity for Rana draytonii, and positively correlated with predator diversity index for Pseudacris regilla. While the only factor that impacted neutrophil to lymphocyte ratios was Bd infection in bullfrogs, with white blood cell counts, I again saw a difference in the way the different species responded to the various stressors in their environment. Bd infection was the top driver in elevating white blood cell counts in P. regilla, while only amphibian diversity had this effect for R. draytonii. Predator diversity had a negative effect on white blood cell counts in L. catesbeiana. This diversity of responses is useful from a management perspective, as it may allow conservation practitioners to shift habitat suitability to species of interest. Given the differences in how the amphibians respond, a handful of concrete management recommendations for native amphibians emerge:1. Decrease Bd by decreasing co-occurring species; provide ponds specialized for species in a site, thus decreasing in-pond diversity, but maintaining amphibian diversity across the site2. Increase sunning sites for R. draytonii and P. regilla and maintain habitat complexity3. Create or maintain smaller, exposed, ephemeral ponds for P. regilla 4. Create or maintain larger, shallower, ephemeral ponds for R. draytonii 4. Improve water quality for R. draytonii5. Decrease amphibian species diversity by controlling introduced species, particularly bullfrog tadpoles, adults, and introduced fishMy dissertation demonstrates the importance of taking a broad approach to examine ecological relationships by designing studies across various species, combining perspectives such as pathogen success and metrics of host physiological stress, and taking into account a suite of likely interacting biotic and abiotic factors. Broad studies such as this can help to avoid spurious conservation decisions given limited time and resources to protect rapidly declining species.