Biologists became aware of a problem in the 1980s, after reports began to accumulate of dwindling or lost frog populations and unusual deformities in amphibians. Evidence came to light that ultraviolet-B radiation, an iridiovirus. a chytrid fungus and fluke parasites could each damage amphibian populations. And a study released last April showed that one frog species disappeared when lakes in the California Sierras were stocked with non-native trout.