Amphibians are osmotically-challenged due to their highly permeable skin and egg membranes. As such, biologists have mostly ignored their occurrence in saline habitats. The goal of this dissertation was to investigate the ability of amphibians to adapt to inhabit these osmotically-stressful environments. As freshwater resources are increasingly salinized due to a combination of anthropogenic and natural stressors, including road-deicing salt application, coastal storm events, and sea-level rise, it is important to understand the abilities and constraints of sensitive organisms, like amphibians, to persist in these environments. I began by exploring what is known about amphibians inhabiting saline habitats around the world, and reviewed information dating from the 1800s to the present, documenting over 100 species inhabiting inland and coastal environments. I then proceeded to investigate the evolutionary potential for one species in particular, the rough-skinned newt (Taricha granulosa) to adapt to increased salinity in its habitat. I documented the negative effects of salinity (both NaCl and MgCl2 – the two most commonly-used road deicers) on embryonic and larval survival and development through a series of experiments, and explored the importance of both developmental and evolutionary history with a stressor on survival. I investigated the evolutionary potential of salt-naïve populations through examining intrapopulation and interfamily variation in critical early life-history traits, and determined that there is high interfamily variation in the salt tolerances of offspring of individual females within a population, providing the raw material for natural selection and local adaptation. Finally, I discovered a population of newts living in the tidal area of a coastal stream, with a natural source of salinity. Through conducting a series of laboratory salt challenges and assays on osmotic, stress, and immune physiology, I was able to determine that newts in the tidal area appear to be physiologically adapted to increased salinity compared to newts in a freshwater area upstream. All this information suggests that amphibians, while still osmotically-challenged, may also not be helpless in the face of salinization, and populations may be able to locally adapt to habitats impacted by natural and anthropogenic sources of salinity.

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.