Science Uncertain Why Frogs Are Vanishing

In other parts of the globe, from the rain forests in Central Americato national parks in Canada and from plantations in Sri Lanka to mountain forests in Australia scientists also have logged startling reductions in frog and toad populations.

Now the quest to find the cause -- or causes -- of this apparent phenomenon affecting the frogs and toads of the world has sparked furious debate in the international scientific community. While almost all researchers agree that environmental degradation is the main cause of the problem, they argue greatly over the specific explanation.

Frogs and toads have been disappearing world-wide for the last 15 years, according to biologists. In 1996, the Geneva-based International Union for Conservation of Nature (IUCN) listed 156 amphibian species -- or 25 percent of all amphibians on earth -- as extinct, critical, endangered, or vulnerable to extinction. A similar survey by the Washington-based Nature Conservancy on the status of 20,481 species of plants and animals in the United States found that almost 40 percent of amphibians are in danger of becoming extinct.

In 1991, scientists formed the Declining Amphibian Population Taskforce to look for underlying causes. Virtually every explanation turned into a heated debate. The first problem has been to determine whether the observed changes represented a natural population fluctuation, or an unusual event that might bring to light something important about declining environmental quality.

Some scientists feel that no one knows whether abnormalities and population drops are truly on the rise or if people have just become better about reporting them. "Is it a phenomenon, or is it just because we're looking?" Greg Hellyer, a scientist with the Environmental Protection Agency's New England Regional Laboratory told Science Magazine.

Most scientists, however, agree that habitat destruction or degradation is the leading cause of amphibian declines in most parts of the world. As woods are cleared, roads are built, and wetlands are drained, frog populations become increasingly scarce.

The introduction of exotic predatory fish can also clear streams and ponds of frog eggs.

But this does not explain the deformations or why frog species are disappearing in relatively pristine rainforests in Central America, for example. One of the main explanations that seems to stick out is the role of the global spread of synthetic chemicals in the environment.

The roughly 70,000 synthetic chemicals available on the global market, and the many others that are emitted as by-products of their production or incineration have barely been tested adequately to see if they cause developmental effects, according to Jennifer Mitchell, a researcher with the Washington-based World Watch Institute.

"In just a few decades, synthetic chemicals have reached the remotest ends of the earth," she says. "They run off farmland or city streets into streams and sewers, are carried downstream, circulate in the air, and later fall as rain."

In Sri Lanka, frogs are nearly absent from tea plantations where herbicides are heavily sprayed, according to Ranil Sananayake, a researcher with the Sri Lanka-based environmental group Earthkind. "Conversion to organic (pesticide-free) tea production in this region has contributed greatly to the re-establishment of populations of local frogs," he said.

Recently Swiss researchers reported in the Journal of Environmental Toxicology and Chemistry that exposing developing frogs to the fungicide triphenyltin stunts the growth of tadpoles and retards the sexual development of adult frogs. Hundreds of thousands of pounds of this same fungicide is used by farmers in the American states of Minnesota and Georgia.

Although the deformities are not the same as witnessed by the school children in Minnesota, the researchers say that the fungicide acts differently depending on the species and "may contribute to the ...decline of populations of more-sensitive amphibian species."

A group of scientists at the University of California and the Salk Institute say the frog deformities may be caused by retinoids, a class of chemicals including vitamin A that tell embryo cells how to grow. Various insecticides and industrial pollutants contain retinoids.

Scientists studying the recent disappearance of about 20 species of frogs in Central America have a entirely different explanation. Researchers, like Karen Lips at St. Lawrence University in New York, say a pathogenic protozoa, or disease causing agent, may be infecting the frogs across the region.

Lips and others believe that the lethal protozoan is sweeping across Central America in a "death wave" moving through the mountains, from one range to the next. In the Monteverde Cloud Forest Reserve in northwestern Costa Rica in 1987, for example, biologists saw hundreds of thousands of frogs. Two years later, they only found five animals, Since then, not one has been seen.

Biologists say the disease first broke out in the Monteverde Reserve in the last 1980s and has since moved south and east into Panama and perhaps Nicaragua. Other researchers, however, say a combination of factors may be at work in this region.

The frogs could have been particularly susceptible to the protozoa because the immune systems of the disappearing amphibians may have been weakened by pesticides or herbicides carried up from coastal farms by rain clouds, says Cynthia Carey, a biologist at the University of Colorado.

This ultraviolet radiation, according to Gary Ankley, an ecologist with the U.S. Environmental Protection Agency (EPA), could be damaging amphibian embryo DNA, resulting in abnormalities during development.

Ankley and other scientists have reported that the northern leopard frog embryos develop abnormalities -- including missing limbs -- after being exposed to about 30 percent of natural light for at least 24 days. He admits that the ultraviolet radiation could also be transforming pesticides into chemicals that can interfere with development and cause birth defects.

Recent field experiments in the Oregon Cascade Mountains also seem to confirm that ultraviolet-B could be responsible. Researchers compared the embryos of long-toed salamanders shielded from UV-B radiation by mylar filters to unshielded embryos. They found that 95 percent of the shielded embryos hatched, compared to only 14.5 percent of the unshielded embryos. Even more striking, less than one percent of the surviving shielded salamanders had deformities while almost all of the unshielded salamanders had deformities.

"There has been a great deal of recent attention to the suspected increase in amphibian deformities. However, most reports have been anecdotal, and no experiment in the field under natural conditions had been performed previously," said Joseph M. Kiesecker of Yale University, who presented his findings in February at the annual meeting of the American Association for the Advancement of Science in Philadelphia.

In light of all of these different studies and observations, the one thing on which all scientists seem to agree is that more research needs to be done.

"There is still much to be learned," says Peter Montague, a researcher with the Maryland-based Environmental Research Foundation. But, he warns, even though the search for the cause is not over, the evidence points to increasing awareness that the global environment is out of balance and under attack from many different sources.

When asked if he thought humans would be exempt from such environmental damage, Montague replied, "It seems very unlikely."