Evaluating the impact of a fungal pathogen on the Cascades frog (Rana cascadae) in the mountains of northern California

Background/Question/Methods

The fungal pathogen Batrachochytrium dendrobatidis (Bd) is associated with declines and extinctions of amphibians worldwide. In the western United States, frogs and toads in montane habitats have proven to be particularly susceptible. We focus on the effects of Bd on the Cascades frog (Rana cascadae) and other native amphibians in the mountains of northern California and ask: 1) What biotic and abiotic factors are associated with Bd prevalence? 2) How does Bd affect R. cascadae populations in the field? In 2008 and 2009, we conducted a survey for Bd throughout the California range of R. cascadae, sampling amphibians at over 110 sites. To provide detailed information on the effects of Bd on R. cascadae populations, we intensively sampled 15 populations throughout the 2009 and 2010 summer seasons. We individually marked a subset of the animals in each population, allowing us to follow infection dynamics at both the individual and the population level.

Results/Conclusions

The 2008-2009 survey indicated that Bd is widespread throughout the region, occurring in amphibians at 67% of all sites surveyed. The pathogen was detected in 26% of post-metamorphic R. cascadae and 1.5% larvae. In addition, the pathogen was detected in three other common amphibians – Western Toad (Bufo boreas), Pacific Chorus Frog (Pseudacris regilla), and Rough-skinned Newt (Taricha torosa). The survey also indicated that the prevalence of Bd in post-metamorphic R. cascadae was greatest at high-elevation sites early in the season. Furthermore, seasonal changes in prevalence depended on life stage, with prevalence increasing over the course of the season in subadults, while tending to decrease in adults. Preliminary results from intensively-sampled populations suggest that declining populations have low proportions of juveniles and subadults relative to stable or growing populations. We suggest that younger, smaller individuals are less effective at clearing the pathogen, and are therefore more susceptible to the disease that it causes, leading to low recruitment and population declines.