Amphibian Mortality Due To UV...( o the destruction of fragile plant life...)

UV-B radiation also may impair disease defense mechanisms, making amphibians more susceptible to pathogens and parasites that may hamper normal development and increase mortality, Kiesecker said. For example, he found increased mortality associated with a pathogenic fungus (Saprolegnia ferax) infecting some embryos exposed to UV-B, while embryos under mylar filters were not infected. The UV-B may work synergistically with the fungus, said Kiesecker, who reports seeing an outbreak of fungal pathogens in a number of amphibian species in the last 10 years.

Amphibians are ideal species for the study of UV-B exposure, he noted. Many lay their eggs in open, shallow water where exposure to UV-B is high. Typically, a population of 200 breeding pairs of toads, for example, will produce as many as 1 million embryos. Furthermore, amphibian species have varying amounts of an enzyme called photolyase, which is the principal enzyme for repairing UV damage to DNA. Photolyase attacks a major UV photoproduct in DNA -- cyclobutane pyrimidine dimers -- which can cause mutations and cell death if left unchecked.

Kiesecker, a zoologist and postdoctoral fellow, reported that frog and toad species with the greatest photolyase activity had the lowest mortality rates in developing embryos. For example, he and his colleagues noted an increase in embryo mortality of 15 to 20 percent in the Western toad and the Cascade frog -- two species with low levels of photolyase -- while the Pacific tree frog, which has a high photolyase level, is thriving. All three species live in the same habitat in the Cascade Mountains.

The field studies, which were completed in May and June 1997
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Contact: Cynthia Atwood
cynthia.atwood@yale.edu
203 432-1326
Yale University
17-Feb-1998