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2008). For example, despite its aquatic life cycle, Bd has been found on fully terrestrial species of amphibians that never enter water, and the role of zoospores in these forms is uncertain. No resting stage has been found, and no alternative hosts are known. Vectors have not been identified. It is relatively easy to rid a healthy frog of the fungus by using standard fungicides (Parker et al., 2002). Yet the fungus is surprisingly virulent. Finally, and importantly, how the fungus causes death is not clear, although it is thought to interfere with oxygen exchange and osmoregulation (Voyles et al., 2007).

With associates, we have been studying frog populations in alpine watersheds within Yosemite, Sequoia, and Kings Canyon national parks for over a decade. We recently showed that yellow-legged frogs are genetically diverse (Vredenburg et al., 2007). Mitochondrial DNA sequence data identified six geographically distinct haplotype clades in the two species of frogs, and we recommended that these clades be used to define conservation goals. Population extinctions, based on historical records, ranged from 91.3% to 98.1% in each of the six clades, so challenges for conservation are daunting. In the last 5 years, we have documented mass die-offs (Fig. 2.3) and the collapse of populations due to chytridiomycosis outbreaks (Rachowicz et al., 2006). Although the mechanism of spread is unknown, it may involve movements of adult frogs among lakes within basins or possibly movements of a common, more vagile, and terrestrial frog, Pseudacris regilla (on which Bd has been detected), ahead of the Rana infection wave. Mammals, birds, or insects also are possible vectors. We have followed movements of R. muscosa and R. sierrae using pit tags and

FIGURE 2.3 Distribution of the critically endangered yellow-legged frogs in California. Chytridiomycosis outbreaks have had devastating effects (Rana muscosa photographed in Sixty Lake Basin, August 15, 2006).

FIGURE 2.3 Distribution of the critically endangered yellow-legged frogs in California. Chytridiomycosis outbreaks have had devastating effects (Rana muscosa photographed in Sixty Lake Basin, August 15, 2006).



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