The frog is represented in the Sierra Nevada by three or four populations. Its decline is attributed to many factors, including introduced species of fish such as trout, livestock grazing, chytrid fungus, and probably pesticides, drought, and ultraviolet radiation. Recreational activities in streams have also had an impact. Introduced fish species Trout were introduced to lakes and streams throughout the Sierra Nevada in the late 1800s to increase recreational fishing in the area. 90% of these Sierra Nevada lakes and streams did not previously have fish in them, although the trout were regionally present. The fish feed on tadpoles, a main prey item. The fish feed on tadpoles, a main prey item. Some of these fish types that prey on tadpoles include the brown, golden, and rainbow trout, as well as the brook char. The introduced trout have changed the distribution of several native species in the local ecosystems. Most of the mountain yellow-legged frog populations did not evolve to adapt to the trout. The trout caused populations to isolate, restricted the amount of available habitats, and increased the chances of extinction. After the removal of fish from several lakes, the frog reappeared and its populations increased. These frogs then began to disperse to other suitable habitats nearby. Pesticides The decline of the frog from its historic range has been associated with pesticide drift from agricultural areas. Frogs that have been reintroduced to water bodies cleared of fish have failed to survive, and analysis has isolated pesticides in their tissues. Pesticides are considered by some authorities to be a greater threat to the frog than the trout. The relative roles that pesticides and introduced fish play in frog declines are still debated, and the loss of R. muscosa in its former range has probably been influenced by multiple factors. Chytridiomycosis This species is one of many amphibians affected by the fungal disease chytridiomycosis. Ample research has explored the biology of the fungus and how to prevent related amphibian declines. The fungus attacks keratinized areas of a frog's body. Tadpoles are not severely affected because only their jaw sheaths and tooth rows are heavily keratinized. Infection in a tadpole can be identified by changes in the pigmentation of these parts. Adults have keratin-rich skin and suffer worse infections. In studies, healthy adult frogs exposed to infected frogs for at least two weeks developed the disease. Transmission takes longer in tadpoles, generally