California's Central Valley is one of the most intensively farmed areas of the world, with more than 150 million pounds (90 million kilograms) of pesticide active ingredients applied to crops every year. The Sierra Nevada is located immediately east and downwind of the Central Valley, and there is increasing concern that pesticides applied in the Central Valley may be drifting eastward into the Sierra Nevada at sufficient concentrations to impact amphibians, including the mountain yellow-legged frog. Contaminants could impact amphibians directly if concentrations reach toxic levels, or indirectly by compromising amphibian immune systems and reducing their ability to fight off disease.
Pesticides and mountain yellow-legged frog declines
In California, winds generally blow through the Central Valley and then eastward across the Sierra Nevada, and detectable (but very low) concentrations of several agricultural chemicals have been detected in the Sierra Nevada, including in mountain yellow-legged frogs from high elevations (Fellers et al. 2004). Consistent with the hypothesis that pesticides are negatively affecting amphibians, recent studies have reported that the probability of extinction for mountain yellow-legged frog populations is positively correlated with the amount of agricultural land use upwind (Davidson et al. 2002) and the amount of pesticides applied upwind (Davidson 2004; Davidson and Knapp 2007). In addition, numerous reintroductions of Rana muscosa into historically-occupied habitat in the southwestern Sierra Nevada (Tablelands area of Sequoia National Park) have failed, perhaps due to the relatively high concentrations of pesticides characteristic of this area due to its proximity to the southern San Joaquin Valley (Fellers et al. 2007).
A shortcoming of all of these studies is the inability to distinguish between effects caused by pesticides and those caused by chytridiomycosis (details on chytridiomycosis are provided in the Threats > Disease page). The commonly-reported pattern of mountain yellow-legged frog disappearances in the western Sierra Nevada close to the Central Valley and their continued existence in more eastern localities has generally been attributed to the exposure of western Sierra Nevada populations to higher pesticide concentrations. However, this pattern is also entirely consistent with the amphibian chytrid fungus (Batrachochytrium dendrobatidis) spreading across the Sierra Nevada from west to east, and several observations suggest the overriding role of chytridiomycosis relative to pesticides in causing the decline of the mountain yellow-legged frog. The most important of these is the recent B. dendrobatidis-caused die-off of hundreds of mountain yellow-legged frog populations in areas of the Sierra Nevada that are remote from the Central Valley and that are subjected to only very low pesticide concentrations.
In summary, pesticides may be having negative effects on R. muscosa and R. sierrae, but available data are still far from conclusive. Additional research is critically needed in several areas, including (1) measurement of pesticide concentrations across the Sierra Nevada to allow direct comparison of pesticide levels with patterns of frog decline, (2) evaluation of whether exposure to low concentrations of pesticides makes mountain yellow-legged frogs more vulnerable to chytridiomycosis, and (3) toxicity tests on mountain yellow-legged frogs to determine their sensitivity to low levels of pesticides.