Ultraviolet (UV) radiation is emitted by the sun, and levels of UV radiation have increased slightly over historic levels as a result of thinning of the Earth's protective ozone layer. Although the chemicals responsible for ozone depletion are being phased out, thinning of the ozone layer is expected to continue for at least another decade.
Increasing UV radiation is of concern as a potential cause of amphibian declines because many observed declines are occurring at high elevations where UV exposure may be elevated compared to lower elevations. Indeed, field and laboratory experiments in which different amphibian life stages (e.g., eggs, tadpoles) are exposed to different intensities of UV radiation suggest that survival of eggs and tadpoles of several amphibian species can be reduced by exposure to UV radiation.
Aquatic habitats in the Sierra Nevada receive high doses of UV radiation due to their high elevation and extraordinary water clarity. However, recent research indicates that the distribution of mountain yellow-legged frogs in the Sierra Nevada is not correlated with UV exposure (Adams et al. 2001). In addition, experiments with Sierran frogs and toads, including mountain yellow-legged frogs, indicate that current levels of UV radiation do not cause reduced survival (Vredenburg 2002). Therefore, it is unlikely that UV radiation is responsible for the dramatic decline of the mountain yellow-legged frog.