Ozone Hole Blamed for Frog Decline

In research that for the first time suggests that the thinning of the Earth’s ozone layer directly harms wild animals, scientists reported Monday that frog eggs are being killed by ultraviolet radiation in the Pacific Northwest.

Oregon State University’s study of frogs in the Cascade Mountains could help solve the mystery of why many of the world’s frogs and toads seem to be vanishing, especially at high altitudes.

The most alarming aspect of the research, some scientists say, is that it indicates UV radiation is damaging wildlife in heavily populated North America, where until recently it was believed that the protective ozone shield was relatively intact compared with that of remote polar regions.

“The implications are much broader than just frogs,” said Oregon State zoologist Andrew R. Blaustein, the lead researcher in the four-year study. “We are looking at natural levels of UV and it’s killing these eggs. If it’s killing frog eggs, it is probably also having an effect on the plants and invertebrates and fish. The implications are pretty broad to other organisms, including humans.”

Since the mid-1980s, wildlife researchers throughout the world have wrestled with the startling disappearances of a large number of amphibian species. The trend has been especially puzzling because many of the declining species live in national parks and other fairly undisturbed, high elevations not prone to the usual culprits--pollution and destruction of habitat--that harm wildlife.

The Oregon study, published in the Proceedings of the National Academy of Sciences, shows that the hatching success of frog eggs in the mountains correlated strongly with their exposure to natural amounts of UV-B radiation. The species of frogs that had the worst hatch rates are the same ones that previous research had shown are suffering severe population declines.

The Oregon State researchers, who began their work in 1990, monitored more than 1,000 frog eggs in the central Cascade Mountains.

The amphibian species that are most susceptible lay their eggs out in the open in shallow waters that receive strong sunlight, while those that lay their eggs under tree canopies or in deep water seem to fare better, according to the study.

“This is a general warning,” said David Wake, a biologist who is director of the Museum of Vertebrate Zoology at UC Berkeley. “I think what we see with the frogs is a group of organisms laying their eggs in the full sunlight and they are right in the front lines of what is happening (to the ozone layer).”

The findings raise a disturbing question concerning human health: If the relatively weak wintertime UV rays in the Pacific Northwest were severe enough to harm frogs, are they also bad enough to cause skin cancer and cataracts? Biologists have long linked UV radiation to those human health problems, but the level of exposure that causes damage is unknown.

Biologists have long known from lab tests that UV radiation alters DNA and tissue of animals and humans, and they recently reported that it is harming phytoplankton, a vital part of the food chain, in the waters of the Antarctic, where the ozone layer is the thinnest. But this study is believed to be the first scientific evidence of natural radiation injuring animals in the wild.

“There is not a great deal of research being done on the biological effects of UV light, and I think that is crazy,” said Ralph Cicerone, a UC Irvine atmospheric chemist who has done world-renowned research on the ozone layer.

Until the Oregon State study, “the only experiments we’ve seen have been reasonably crude. They were done by subjecting the plant or animal to really gross amounts of UV-B and generally under controlled lab conditions, not in the field. This experiment sounds more realistic to me, since it uses natural UV light,” Cicerone said.

In the study, eggs laid by the wild frogs were placed into 72 cages, some with filters that let all sunlight inside except the harmful UV-B radiation. The cages were then placed in the same spot where the frogs laid the eggs. From 20% to 25% more eggs under the filters hatched successfully than those that were unshielded, according to the report.

Three species were studied, the Pacific tree frog, whose population appears to be fairly stable, and the Western toad and the Cascades frog, two species that have suffered severe declines. All three inhabit California.

In addition, the researchers conducted molecular work on 10 species. Frogs with low levels of a natural enzyme, called photolyase, that helps repair DNA damage caused by UV radiation had the worst rate of egg hatchings.

For years, many scientists have wondered if ultraviolet light might be a leading cause of the frog disappearances. But they were skeptical because they did not believe thinning of the ozone layer over the western United States was severe enough.

Then in November, Canadian scientists reported that a decade of measurements has shown that the amount of harmful UV radiation that reaches the ground in Toronto during winter has increased at a rate of 5% per year. Cicerone said ozone depletion reached a record high in North America in the winter and spring of 1993.

“Most of the information we have is from the polar regions,” Blaustein said. “That’s the scary part. We’re now at the highly populated areas. . . . I think we should definitely study the impact of UV on other organisms.”

Blaustein said when he began his work four years ago, he thought he would find no UV problem in Oregon. “We were shocked at the results. Every single experiment was replicated several times at several different sites,” he said.

The ozone layer is believed to be thinning from use of chlorofluorocarbons, or CFCs, industrial solvents and coolants that are gradually being phased out worldwide. In the sun’s intense rays of the upper atmosphere, CFCs release chlorine atoms that destroy the ozone shield.

Tom McElroy, a Canadian government physicist who co-authored the Toronto radiation research, said the frogs may be vulnerable because they lay their eggs at the same time of year--winter and early spring--that UV radiation was increasing the most in North America, according to his study.

The Oregon State researchers suspect amphibians may be more vulnerable to the damage than mammals because of their unique physical attributes.

“Their skin is not protected by hair or feathers; their eggs lack hard outer shells, thus exposing them to the environment,” the study says.

Wake, who reviewed the Oregon State research for the National Academy of Sciences, called the work “quite careful” and said it could explain why some frogs living in the same areas are disappearing while others--less susceptible to UV rays--are maintaining healthy populations.

Differences in UV rays might also explain why some regions, especially the Southeast portion of the United States, seem unaffected, Wake said.

Scientists have suspected a whole host of culprits in the frog problem: acid rain, pesticides, stocking of exotic fish and changes in water temperature. Some researchers say the declines may just be a natural cycle.

“UV is high on my culprit list,” Wake said, “but it is not alone.”