WASHINGTON, D.C. - People who are looking for a magic bullet that will explain all of the amphibian deaths and declines around the world are going to be disappointed, a leading expert said Friday at the annual meeting of the American Association for the Advancement of Science.

It's now a certainty that there are multiple causes contributing to this problem, said Andrew Blaustein, a professor of zoology at Oregon State University and one of the pioneers in this field of study. But the lack of a single, definite cause does not diminish the seriousness of this alarming ecological phenomenon, he said.

"At this point we can say for sure that there are several causes of amphibian declines, which include rising levels of UV-B radiation in sunlight, pathogens, pollutants, habitat destruction, introduced predators and most recently, crop fertilizers," Blaustein said. "But the overall result is that this group of animals which has been around since the time of the dinosaurs is now in serious decline all over the world. And some of the things that are killing frogs almost certainly have implications for other animal species, including humans."

The multiple causes of amphibian declines, in fact, helps to illustrate how ecological changes may have a synergistic effect to compound problems, Blaustein said. In various instances it might be that UV-B radiation, or pathogens, or high nitrate levels by themselves would not be enough to cause death or deformity.

Put them all together and you have far more serious impacts, he said, such as: The 14 species of amphibians that have disappeared from Australia in recent years. The five species of amphibians in the Pacific Northwest of the United States that are listed as candidates for the endangered species list. The extinction of the golden toad in Costa Rica. Massive egg mortalities of the Cascades frog in Oregon. Amphibian declines in Europe, South America, Asia, Africa. Even problems in the pristine confines of Yosemite National Park.

"This is an incredibly complex problem, a disturbing one, and there's no end in sight," he said. In 1997, Blaustein published a major paper in Proceedings of the National Academy of Sciences which linked ambient but rising levels of UV-B radiation in sunlight to physical deformities in amphibians. This field study found that more than 90 percent of the salamander embryos not shielded from UV-B radiation either died or hatched with deformities, whereas practically all of those protected by special filters survived and were perfectly normal.

In 1998, Blaustein published a study which correlated an increase in UV-B radiation to retinal damage in the Cascades frog. The authors pointed out that the effect of solar UV radiation on the eye and retina are well known in animals and that the risk increases at higher altitudes. In frogs, this could lead to progressive decline in visual ability, impairment of visually guided behaviors, and less successful avoidance of predators. They concluded that increasing terrestrial levels of solar UV radiation represent a serious environmental threat to species across many ecosystems, including humans.

In late 1999, Blaustein published a study in the journal Environmental Toxicology that showed a level of nitrogen-based compounds the EPA says is safe for human drinking water was high enough to kill some species of amphibians. Levels of this type are often found in agricultural areas as a result of using crop fertilizers, the authors said. When exposed to them, some tadpoles and young frogs reduced their feeding activity, swam less vigorously, experienced disequilibrium, developed physical abnormalities, suffered paralysis and eventually died.

And problems such as that, Blaustein said, may go even further.

"The furor that has arisen over frog deformities such as extra legs has been linked to a trematode parasite known as a fluke," Blaustein said. "But these flukes have been around forever and we never observed the level of problem we're now seeing with deformed frogs. One thing we know is that these flukes live part of their life cycle in a snail. Snails eat algae. And higher levels of nitrogen-based fertilizers can cause increased algal growth, increasing the snail populations."

Those types of linkages, he said - intricate, complicated, sometimes even unproven - are starting to crop up more and more in the strange case of declining amphibians. It means that the Earth's ecological systems work in a delicate balance and that seemingly trivial impacts in one area can become magnified as they ripple through the ecosystem, with unintended results or consequences that are difficult to predict and sometimes frightening in their scope.

For some time, researchers have been referring to the dying frogs as the "canary in the coal mine," an early warning sign of environmental danger.

What's less clear, Blaustein said, is exactly what insult, or combination of them, killed these animals, or caused their diseases and deformities.

Or, he added, which species will be the next to fall.

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Andrew Blaustein, 541-737-5356