Batrachochytrium dendrobatidis (Bd) is a fungal pathogen causing amphibian population declines. Bd has a narrow thermal tolerance and requires moisture to survive. Differences in frog biology, pathogen biology or temperature and moisture conditions may determine population response to disease. Population responses to Bd vary among sites, habitats, species and populations. In the tropics, stream-dwelling species decline to a greater degree than forest species, yet not all stream species decline to extirpation and not all forest species survive. I hypothesized that variation in operative temperature (Te) or body temperature (Tb) might explain differences in host population change documented among sites, seasons, habitats, and species. I sampled three moist-forest Panamanian sites (elevation 375 - 1300 m) during 2.5 months of the 2008 wet season and four different moist-forest sites (elevation 400 - 1300 m) during 3 weeks of the 2008 dry season. I measured Te and Tb of anurans along stream and forest transects. Additional environmental variables such as height, substrate, canopy cover and sunfleck presence were measured concomitantly. I used analysis of covariance to determine whether these factors influenced Te and Tb. I compared frequency distributions of Tb and Te to a Bd thermal growth curve to determine: 1) whether temperatures above Bd's critical thermal maximum were available to frogs, and 2) whether populations of species that have declined occupied habitats more frequently in Bd's optimal thermal range than species that have not. Te and Tb differed among sites, with cooler temperatures at higher elevation. Te was cooler during the dry season yet the presence of sunflecks and open canopy had greater effect on Te during the dry season. Within a site, Te and Tb were not different between habitats. Within a site, Tb did not vary among species. Average Te and Tb for all sites fell within Bd's thermal tolerance range, but the low elevation sites had Tb ranges extending above Bd's critical thermal maximum. Although temperature may explain greater losses at higher elevations, I found no significant difference in operative temperatures between stream and forest habitats at any site which indicates that temperature alone does not explain greater losses of stream anurans. Species that have declined to extirpation elsewhere did not consistently have cooler body temperatures compared to surviving species. Within the Neotropics, moisture, instead of temperature, may explain patterns of Bd prevalence among seasons, habitats, and species.

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For over three decades, amphibian populations have been declining across the globe. Emerging infectious diseases are responsible for some of these declines. Ranaviruses have caused die-offs in wild amphibian populations on 4 continents, in 5 Canadian provinces, and in over 25 U.S. states. In order to understand host-pathogen dynamics, it is critical to establish baseline information on species susceptibility and the effects of natural stressors. The goal of my thesis research was to quantify the effects of anuran development and exposure to invertebrate predators on species-specific susceptibility to ranavirus. My experiments were designed in factorial arrangements, and consisted of exposure to ranavirus during different developmental stages or with and without predator cues in a controlled environment. I found that exposure to invertebrate predator cues did not increase susceptibility to ranavirus for 4 anuran species tested. Susceptibility differed among embryo, hatchling, larval and metamorph stages, but trends differed among species and did not follow predictions based on Xenopus laevis immune function. Low susceptibility during the embryo stage was the only consistent development result among species, perhaps owing to protective qualities of the vitelline membrane or mucoidal capsules surrounding the embryo. Across 7 anuran species tested, mean mortality rates ranged from 5 – 100%, with Lithobates sylvaticus and Scaphiopus holbrookii most susceptible. I found that infection rates and viral load were correlated with mortality rates, thus these variables are good indicators of susceptibility to ranavirus. My results indicate that ranaviruses can cause catastrophic natural mortality in some anuran species, and likely play a significant role in local population dynamics. For highly susceptible species, ranaviruses could cause local extirpations that lead to species declines. More information is needed on the role of natural (e.g., co-infection, competition) and anthropogenic stressors in driving ranavirus epizootic events. I encourage natural resource agencies to initiate ranavirus surveillance programs, especially for rare species and fragmented populations. Future studies should take an immunogenetic approach to identifying mechanisms driving susceptibility. Identifying mechanisms associated with ranavirus emergence is fundamental to developing science-based conservation strategies.