Studies dispute ultraviolet effect on amphibian population declines

For several years it has been widely believed that increased ultraviolet-B radiation because of thinning of atmospheric ozone was a major culprit in deforming amphibian offspring and dwindling populations.

Now two new studies cast serious doubt on that assumption, and the lead author of one says the belief could have had negative impacts on efforts to save amphibians.

“All of the concentration on UV might have misdirected our conservation and research priorities,” said Wendy Palen, a University of Washington zoology doctoral student.

UV-B, which causes a number of damaging photochemical changes within cells, makes up less than 1 percent of the ultraviolet radiation reaching the Earth’s surface. Previous laboratory and field experiments have shown that it causes deformities and increased mortality in amphibian embryos. That finding led some scientists to contend the Earth’s thinning ozone layer, which protects the planet from some harmful radiation, might have contributed to declining amphibian populations.

However, Palen said, the previous experiments, both in the laboratory and in the field, dealt with individuals or small groups in carefully prepared settings. A critical limitation of those studies has been in understanding whether results of small-scale experiments are representative of the range of environments amphibians experience in nature.

The new study examined UV-B levels in natural amphibian breeding habitats in Washington and Oregon and found that the amount of dissolved organic matter in the water actually protects most amphibian embryos from harmful levels of UV-B radiation.

The study is published in the November issue of the journal Ecology. Besides Palen, authors are Daniel Schindler, a UW associate zoology professor and Palen’s doctoral adviser; Michael Adams, Christopher Pearl and R. Bruce Bury of the U.S. Geological Survey; and Stephen Diamond of the U.S. Environmental Protection Agency. The USGS, the EPA and the National Park Service’s Canon National Parks Science Scholars Program helped pay for the research.

The same edition of Ecology includes a second, corroborating study, by USGS researchers P. Stephen Corn and Erin Muths, that examines breeding of the boreal chorus frog at a Colorado pond.

The research by Palen’s group used amphibian population and breeding information scientists collected at hundreds of Pacific Northwest sites from 1997 through 2000. The focus was narrowed to 136 sites that yielded the best information, and then water samples were collected from each of those sites, spread throughout the Olympic Mountains and the Cascade Range in Washington and Oregon.

The water samples were analyzed for dissolved organic matter – produced primarily from terrestrial vegetation and soils – that is leeched into lakes and streams by rain or melting snow. The research concluded that 85 percent of the amphibian habitats sampled are protected from harmful levels of UV-B by dissolved organics.

Using the water-sample approach allowed the scientists to bring the small scale at which experiments are done into line with the much larger scale at which conditions could affect entire populations.

Most of the previous research that implicated UV-B in amphibian mortality and deformity has been done in the Northwest, but the effects were thought to occur elsewhere as well. Now, Palen said, it appears that scientists need to keep looking for causes, and that they could include a number of factors that vary across geographic regions.

For instance, she noted that the mountain yellow-legged frog in California’s Sierra Nevada has been declared endangered, but the reason has nothing to do with UV-B. Instead, the frog population has declined because of the introduction of non-native trout species that feed on frog eggs, juveniles and adults.

Such evidence suggests that the causes of amphibian population declines might be specific to each area – and the solutions might be too, Palen said.

In addition, the new study emphasizes how intertwined aquatic ecosystems are with those on land, she said, and that interdependence must be taken into account as scientists try to find ways to protect and foster amphibian and other aquatic species.

For more information, contact Palen at (206) 616-3798 or wpalen@u.washington.edu, or Schindler at (206) 616-6724 or deschind@u.washington.edu