Ultraviolet (UV) light can zap amphibian eggs and might be responsible for plummeting populations of frogs and toads, some researchers have found. But a new study shows that many amphibian breeding sites in the northwest United States are too murky for UV light to shine through and cause harm. The controversial finding suggests that UV alone cannot explain the widespread decline of amphibians in the region.
Amphibian populations have been declining worldwide, probably for about 40 years. And since 1995, deformed frogs have been found across North America. Scientists attributed both phenomena to pesticides, parasites, and increasing exposure--due to ozone loss--to a type of ultraviolet radiation called UV-B. Experiments indicated that in some breeding sites, shading amphibian eggs from UV-B increased survival (ScienceNOW, 5 April 2001).
To find out if UV-B could be causing widespread decline, zoologist Wendy Palen of the University of Washington, Seattle, and colleagues surveyed 136 suitable breeding sites in the northwestern United States. They took water samples from each site and determined the clarity of the water by measuring how much light it absorbed. Using what Palen calls a "worst-case scenario" of high UV-B levels and an egg lying only 10 centimeters below the water's surface, they extrapolated how much UV-B the egg would be exposed to at each site. For 85% of the sites, exposure would not be severe enough to harm the eggs, the authors report in the December issue of Ecology.
Zoologist Andrew Blaustein of Oregon State University, Corvallis, isn't convinced. He stands by his earlier results that UV-B can harm amphibian development. Moreover, he points out, amphibians are vulnerable after hatching, when they leave the water.
Palen says one reason for the apparent discrepancy with previous studies might be that the previous work was done in sites with particularly clear water, which affords little protection from UV-B, she says. Both agree the whole truth has yet to emerge. "No one has really identified any single cause for total population declines," Blaustein says.