Concern has been mounting for years over the depletion of the ozone layer -- the atmospheric shield that helps block harmful ultraviolet (UV) radiation from reaching the Earth's surface. At the same time amphibian populations have been declining, and many have been turning up with unusual malformations, such as missing or extra limbs.
A number of causes have been suggested to explain the malformations, including exposure to chemicals and parasites. Only recently have researchers been examining the potential connection to UV radiation to determine if it is coincidence or something more.
Until now, most of the research has focused on exposing frogs to UV radiation in the laboratory, providing little information about how these findings translate to natural habitats. "We really wanted to fill the gap between the findings of other laboratory research and what might happen in natural environments," said Steve Diamond, Ph.D., an environmental toxicologist at the United States Environmental Protection Agency in Duluth, Minn., and an author on all three papers.
In the first study, Diamond and his colleagues kept frog eggs in small outdoor containers while exposing them to varying degrees of UV radiation -- from 25 to 100 percent of natural sunlight. As the eggs developed, the researchers observed hatching success, tadpole survival and the presence of malformations. They found that the frequency of malformations increased with increasing UV radiation, with half of the frogs experiencing malformations at 63.5 percent of the intensity of natural sunlight.
This supplements data from a previously published paper, which reported that 100 percent sunlight reduces survival by an average of 50 percent. In the course of these experiments, the researchers also determined that a specific region of the UV spectrum, known as UVB, appears to cause the malformations.
In the real world, however, frogs rarely experience 100 percent of natural sunlight. A variety of environmental factors conspire to reduce the levels of UV radiation entering wetlands, including ozone levels, cloud cover and UVB-absorbing dissolved organic carbon (DOC) in water. Accordingly, in the second study, the researchers measured DOC levels in wetlands in Wisconsin and Minnesota and found that the top five to 20 centimeters of wetlands absorb as much as 99 percent of UVB.
To complete the picture, the third study involved a survey of 26 wetlands in the same region to estimate the specific level of risk of frogs living in these environments. Using a combination of computer models, historical weather records and DOC measurements, they concluded that UVB posed a risk to amphibians living in three of the 26 wetlands.
While these findings suggest that most frogs are not currently at risk for UVB effects in this area of the country, the possibility of effects on amphibians in general should not be completely ignored, according to Diamond. Continued reduction of ozone and other global climate change effects may increase UV exposure in wetlands, suggesting that the potential risk to amphibians should continue to be studied.
Diamond's team and a group of researchers from the National Parks are presently evaluating UVB levels across landscapes to compare them with the occurrence or absence of amphibians. "Those results," Diamond said, "combined with the risk assessment presented in these three manuscripts, will add significantly to our understanding of the relationship between UVB levels and amphibian declines or malformations."