Decline and localized extirpation of the foothill yellow-legged frog (Rana boylii) in the presence of the fungal pathogen, Batrachochytrium dendrobatidis: Contemporary and historical perspectives

The introduction and spread of novel pathogens is an increasingly important contributor to biodiversity loss, and Batrachochytrium dendrobatidis (Bd), the causative agent of the deadly amphibian disease chytridiomycosis, has led to the declines or extinctions of many amphibians globally. Repatriation programs are increasing in response to species extirpations; however, a prerequisite to these programs is an understanding of the causative factors in declines. The foothill yellow-legged frog (Rana boylii) was enigmatically extirpated from the southern California portion of its range between the late 1960s and the mid-1970s. Little is known about the cause of this rapid, localized extinction event, R. boylii’s Bd susceptibility, the impact of Bd on extant R. boylii populations, or the history and current status of Bd in amphibian populations in southern California. I tested the hypothesis that Bd could have played an important role in the rapid extirpation of R. boylii from southern California through field surveys, laboratory experiments, interviews, and museum specimen sampling. I found that where the species is extant in central California, it is lethally susceptible to chytridiomycosis, and that infection is related to the presence of non-native American bullfrogs. I optimized methods to detect Bd DNA from formalin-fixed museum specimens using real-time quantitative polymerase chain reaction (qPCR), and found that both DNA extraction method and the pathogen load of frogs prior to formalin fixation are important predictors of successful Bd DNA detection. Finally, using an interdisciplinary approach that combined museum specimen sampling, interviews, and field notes, I examined the historical prevalence of Bd and abundance of R. boylii in southern California, and the timing of R. boylii’s extirpation. I found that Bd prevalence increased in the mid-to-late 20th century and coincided with R. boylii’s rapid decline from the region. This work reveals new insights into the current and historical role of chytridiomycosis in California amphibian declines and can serve to inform future R. boylii reintroduction efforts