Their diurnal habits and their use of relatively simple and exposed alpine habitats make them readily visible and easy to capture. Typically these frogs occurred in large populations, and rarely were they found >2 m from the shores of ponds, lakes, and streams. Censuses throughout the Sierra Nevada began in the early 1990s and intensified in this century. Although most of the frog habitat in this large mountain range is protected in national parks and wilderness areas, yellow-legged frogs are now documented to have disappeared from >90% of their historic range during the last several decades (Vredenburg et al., 2007). The most recent assessment lists them as Critically Endangered (SN Stuart et al., 2004). Factors implicated in the declines include introduced predatory trout (Knapp and Matthews, 2000), disease (Rachowicz et al., 2006), and air pollution (Davidson et al., 2001; Davidson, 2004). Experiments that extirpated introduced trout led to rapid recovery of frog populations (Vredenburg, 2004). Thus, for a time, there was hope that, simply by removing introduced trout, frog populations would persist and eventually spread back into formerly occupied habitat. Curiously, multiple attempts at reintroduction in the more western parts of the range clearly failed (Fellers et al., 2007). Hundreds of dead frogs were encountered at both reintroduction and many other sites in the western part of the range (Rachowicz et al., 2006), and it became apparent that predation was not the only factor affecting the frogs’ survival.

In 2001, chytridiomycosis, a disease of amphibians caused by a newly discovered pathogenic fungus [Batrachochytrium dendrobatidis (Bd)] (Berger et al., 1998) was detected in the Sierra Nevada (Fellers et al., 2001). Subsequently, a retrospective study disclosed that Bd was found on eight frogs (R. muscosa, wrongly identified as R. boylii) collected on the west edge of Sequoia and Kings Canyon national parks in 1975 (Oullet et al., 2005). Infected tadpoles of these species are not killed by Bd. When tadpoles metamorphose, the juveniles became reinfected and usually die (Rachowicz and Vredenburg, 2004). However, tadpoles of yellow-legged frogs in the high Sierra Nevada live for 2 to 4 years, so even if adults and juveniles die, there is a chance that some individuals might survive if they can avoid reinfection after metamorphosis.

The disease is peculiar in many ways (Daszak et al., 2003; Mitchell et al., 2008). Pathogenicity is unusual for chytrid fungi, and Bd is the first chytrid known to infect vertebrates. The pathogen, found only on amphibians, apparently lives on keratin, present in tadpoles on the external mouth parts and in adults in the outer layer of the skin. The life cycle includes a sporangium in the skin, which sheds flagellated zoospores outside of the host. The zoospores then infect a new host or reinfect the original host, establishing new sporangia and completing the asexual life cycle. Sexual reproduction, seen in other chytrids, is unknown in Bd (Morgan et al., 2007). Much remains to be learned about the organism (Mitchell et al.,