University of California Water Resources Center

Declines in amphibian populations have been reported throughout the world in recent years. Chytridiomycosis, a disease of amphibians caused by the chytrid fungus, Batrachochytrium dendrobatidis, is one of a number of factors that have been shown to contribute to these population declines. B. dendrobatidis is associated with rapid population declines and local extinctions of populations of mountain yellow-legged frog, Rana muscosa, in some areas of the California Sierra Nevada mountains, however in other areas R. muscosa populations are persisting in the presence of this fungal pathogen. We conducted field surveys and laboratory experiments to investigate several factors that may explain these different population-level outcomes of B. dendrobatidis. Our field surveys revealed that R. muscosa at persistent sites are carrying very low fungal loads, which lead to sub-clinical infections. At these sites, B. dendrobatidis is having no detectable effect on adult R. muscosa survival (although subadults may be experiencing high disease-induced mortality), and some adults are apparently losing the infection. At sites experiencing frog population crashes, a large fraction of R. muscosa adult are carrying extremely high fungal loads, and very few infected adults are surviving over the course of a summer, or between years. We found no consistent differences in temperature profiles between the two types of sites. Our laboratory experiments detected no difference in transmission rate or virulence of fungal strains from persistent versus die-off sites. We are investigating differences in susceptibility of frogs from the two types of sites in an experiment currently underway.