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Frogs are indicator species whose global population declines are warnings of the state of the environment. Their permeable skin and dual lives in water and on land make them more vulnerable to environmental degradation. Disease, acidification and pollution are partially to blame for frog deaths and mutations, but the rise in ultraviolet radiation attributed to the thinning of the ozone layer also may be a contributing factor.
People wear sunscreen when outside for prolonged periods to protect their skin from ultraviolet radiation, which can cause cancer. Frogs don't have this ability, and their permeable skin is not otherwise protected by hair or feathers. Their eggs have no shells and are susceptible to damaging radiation. Increased UV-B rays associated with the thinning ozone layer kill frogs and their eggs, with the most threatened species living in high elevations where they are more likely to be exposed to more concentrated rays. Researchers found eggs hidden in the shade were more likely to survive, and were able to increase survival rates for eggs left in the open by shielding them with a filter that blocked UV light.
The Danger to DNA
Radiation alters the structure of frogs' DNA. The traditional double-helix strand bonds to itself, ultimately preventing replication and resulting in deformities or death. Frogs have a limited ability to repair the damage using an enzyme called photolyase. However, the thinning ozone layer means they are exposed to far more radiation than would occur naturally. As a result, many frogs don't have enough photolyase to keep up with the damage and repair it in time to enable survival.
A Harrowing Hypothesis
Researchers at the University of Oregon studied the affects of UV radiation on frogs with skepticism. They hypothesized that if UV radiation were a cause of amphibian population declines, those frogs whose eggs were more exposed to sunlight would have higher amounts of the protective enzyme photolyase than frogs whose eggs were laid in protected areas. At the conclusion of their studies, even the researchers admitted surprise. Frogs whose eggs were laid in shallow, uncovered water showed nearly a hundred times greater photolyase activity than those whose eggs were hidden under leaves or in shade. The researchers concluded increased UV radiation brought about by a thinning ozone layer contributes to global amphibian decline. Species living in high altitudes who lay their eggs out in the open are the most at risk.
Elsewhere in the United States, frogs in particular habitats had missing legs or other deformities from birth. These sites typically were constructed rather than naturally occurring, with little aquatic vegetation or shading to diffuse the UV rays filtering into the ponds. While chemical runoff from industrial sites and other pollution may be partially to blame, scientists were able to replicate similar deformities in frogs under laboratory conditions. Those frogs were directly exposed to UV radiation similar to the environmental exposure in their habitats. Although light filtering through water in a pond environment isn't direct, the scientists were able to prove a link between the pond's level of UV penetration and the percentage of malformed frogs in that pond.
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