Complex causes of amphibian population declines

Amphibian populations have suffered widespread declines and extinctions in recent decades. Although climatic changes, increased exposure to ultraviolet-B (UV-B) radiation and increased prevalence of disease have all been implicated at particular localities(1-6), the importance of global environmental change remains unclear. Here we report that pathogen outbreaks in amphibian populations in the western USA are linked to climate-induced changes in UV-B exposure. Using long-term observational data and a field experiment, we examine patterns among interannual variability in precipitation, UV-B exposure and infection by a pathogenic oomycete, Saprolegnia ferax. Our findings indicate that climate-induced reductions in water depth at oviposition sites have caused high mortality of embryos by increasing their exposure to UV-B radiation and, consequently, their vulnerability to infection(1). Precipitation, and thus water depth/UV-B exposure, is strongly linked to El Nino/Southern Oscillation cycles, underscoring the role of large-scale climatic patterns involving the tropical Pacific(7). Elevated sea-surface temperatures in this region since the mid-1970s, which have affected the climate over much of the world(8), could be the precursor for pathogen-mediated amphibian declines in many regions(1,3,4,9).