Amphibian Deaths Linked to Global Climate Change

Joseph M. Kiesecker
Pennsylvania State University
R01ES11067

Background: One of the major obstacles in studying ecological or health effects of global climate change is establishing links between seemingly disparate meteorological and biological events. This report is the first to suggest global climate change as a cause of amphibian population decline in the western United States. Scientists have been reporting declines in worldwide amphibian populations since the late 1970s. There have been reports in the scientific and lay media about frog deformities as well, but the causes of these events have been elusive.

Advance: Dr. Kiesecker's laboratory has linked extremely dry climatic events and ultraviolet radiation with infection of toad eggs by Saprolegnia ferax, a water-borne mold pathogen. The team has discovered that El Niño induced warming of the Pacific Ocean causes dryer than normal conditions in the Cascade mountains. Dry conditions cause lakes where amphibians reproduce to be shallower than normal, which in turn causes the amphibian eggs to be closer to the water's surface where they are exposed to higher levels of ultraviolet-B radiation (UV-B). UV-B radiation weakens the immune system in the developing embryos making them more susceptible to infection by S. ferax which is the ultimate cause of death. S. ferax generally only presents a risk to organisms that are injured or under stress. Experiments where the depth of the water is controlled produce healthy toads, but Kiesecker reports mortality of up to 100% in shallow conditions.

Implication: This research documents an interesting paradigm in which an environmental toxicant, in this case UV radiation, can weaken an immune system leading to opportunistic infections. The finding serves as a warning that global climate changes are having effects on biodiversity and the health of ecosystems. It also suggests that other population declines may not only be caused by habitat destruction or natural population flux. Finally, the results offer evidence that to avoid potentially high losses of biodiversity, cleaner, more sustainable energy options need to be stressed by policy makers.

Publication: Kiesecker JM, Blaustein AR, Belden LK. Complex causes of amphibian population declines. Nature, 2001 Apr 5;410(6829):681-4.