Assessing the threat of two deadly fungal pathogens (Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans) to amphibian biodiversity and the impacts of human-mediated movement of an invasive carrier species and climate change

Batrachochytrium dendrobatidis (Bd), a fungal pathogen that causes chytridiomycosis in amphibians, has infected >500 species and caused declines and extinctions in >200 species. Recently, a second deadly fungal pathogen that also causes chytridiomycosis, Batrachochytrium salamandrivorans (Bsal) was discovered. The presence of these lethal pathogens in international trade threatens amphibian diversity. In this dissertation, I use a novel modeling approach to predict disease risk from Bd and/or Bsal to amphibian populations in North America and Asia by incorporating pathogen habitat suitability, host availability, the potential presence of an invasive carrier species, and pathogen invasion history. First I identify Bsal threat to North American salamanders to be greatest in the Southeast US, the West Coast, and highlands of Mexico. I then investigate the compounded risk of Bd and Bsal in North America and find highest relative risk in those same areas and in the Sierra Nevada Mountains and the northern Rocky Mountains. However, when I incorporate the historical context of Bd into the model by treating the eastern US as the native range of Bd, I find invasive Bd risk to be highest in the mountain ranges of the western US and Mexico, which could influence how amphibians in these areas respond to Bsal. I also uncover a pattern that suggests the translocation of the American bullfrog (Rana catesbeiana) to western North America may have facilitated Bd spread. Last, I investigate the threat of Bd in Asia and find that mountain forests in China, Southeast Asia, and the Pacific Islands have the highest relative Bd risk, and the potential presence of R. catesbeiana and amphibian richness may have a strong influence on Bd risk. I also predict that climate change may lead to a 3-23% decrease in total area of Bd risk in Asia by 2070. These findings provide helpful guidance for conservation planning and management by further highlighting the role of R. catesbeiana in Bd spread and identifying areas at highest risk to prioritize for surveillance, monitoring, and species susceptibility studies needed to enhance our understanding of the pathogen invasion history, disease dynamics, and potential interactions of Bd and Bsal