COMPLEX REASONS WHY AMPHIBIANS ARE VANISHING

by Richard Monastersky

Last summer, a newsletter called Froglog issued instructions for biologists to follow when encountering a dead or dying amphibian. The guidelines call for picking up the specimen (using disposable gloves) and placing it in a plastic container filled with a preserving chemical. "If the specimen is badly decomposed, COLLECT IT ANYWAY," exhorts the newsletter, which stresses the need for any additional data, no matter how rancid.

The same sense of urgency runs throughout the small universe of herpetology, whose practitioners have watched their research subjects vanish at an alarming rate in recent years. A rough global estimate by an international team last year found a decline of 60 to 70 percent in the populations of frogs, salamanders, and their kin over the past few decades. Some areas have suffered even more severely, and 20 species are presumed extinct.

Since 1989, when they first grew aware of the problem, herpetologists have struggled to decipher what is killing off amphibians. The list of suspects has ballooned to more than 25, starting with the most important problem of habitat loss. But amphibians have also suffered in habitats that have remained intact, and the lineup of potential perpetrators includes disease, pollution, and even over-harvesting for use as pets, food, and other purposes.

Recent studies, however, have demonstrated that different culprits have ganged up to assault amphibians in unanticipated ways. "It doesn't just come down to one thing," says Timothy R. Halliday, a professor of biology at Open University, in Milton Keynes, England, and the international director of the Declining Amphibian Populations Task Force. A detailed investigation of deaths in Oregon suggests that global warming has exerted a subtle but profound effect-killing off toads there through a complex series of changes. And new laboratory experiments indicate that pesticides can harm amphibians much more than scientists had once thought.

Taken together, the new studies demonstrate the complex ways that environmental changes can cause damage. "I would say there's no consensus on there being any one immediate cause," says J. Alan Pounds, a researcher at the Monteverde Cloud Forest Preserve in Costa Rica. "But the obvious ultimate cause is going to be Homo sapiens. It's several manifestations of human degradation [of the environment]."

It was luck, along with diligent work, that enabled researchers to uncover the climate link in Oregon. Biologists from Oregon State University have been collecting data in the Cascade Mountains since the early 1980s to study breeding of the western toad, a fist-sized species. Every spring, those toads lay their strings of ebony eggs by the millions in high-altitude ponds. Normally, the eggs hatch into tadpoles within a week or two.

But by the late 1980s, some disturbing signs had started to appear. Large numbers of the eggs-eventually between 80 and 100 percent of them-were getting stalled in their development and never hatching.

Joseph M. Kiesecker, now an assistant professor of biology at Pennsylvania State University at University Park, and Andrew R. Blaustein, a professor of zoology at Oregon State, found that a fungus called Saprolegnia ferax was infecting and killing many of the eggs. The scientists also found that the fungus wasn't working alone. Their experiments revealed that exposure to ultraviolet light made the eggs more susceptible to fungus attack. By 1995, the two had developed the hypothesis that thinning of the ozone shield in the stratosphere was letting in extra ultraviolet light, making the eggs more vulnerable to infection.

Then the weather turned-and altered their theory. Dry years in the early 1990s gave way to a few years of plentiful snowfall, which melted in spring and raised the water level in the ponds. That change presented a natural experiment for Kiesecker and Blaustein. As it happened, they and their colleagues had been measuring water levels in the ponds for years.

The team set up a study to determine how water level affects the strength of ultraviolet radiation reaching the eggs and the rates of fungal infection. Deeper water, they found, provides an effective shield against the harmful radiation. Ultraviolet light reaching embryos in 20 inches of water measured 43.5 percent weaker than the light hitting eggs in just four inches of water.

That's the difference between life and death for thousands of eggs. At the deeper level, more than 75 percent of the eggs survived to hatch, but less than 45 percent of the eggs hatched at the shallower level, report Kiesecker and his colleagues in Nature. They blame climate change, rather than ozone loss, as the ultimate cause of the egg deaths.

The study reaches beyond the immediate confines of the Cascades, however, because weather patterns there are connected to broad changes in the Pacific Ocean. The Oregon mountains tend to get less snowfall during years when the tropical Pacific warms up in a so-called El Nino event. And the tropical Pacific has been unusually warm since 1975, spurring climate changes all over the globe.

Although meteorologists debate whether the increase of El Ninos is permanent or just cyclical, evidence suggests that ocean temperatures in the tropical Pacific will continue to climb over the next century as greenhouse warming takes hold.

In this way, the amphibian deaths in the Cascades fit into a pattern affecting the entire world. "We've got a very tight mechanism" that explains the deaths, says Kiesecker. "That is very important because it allows us to understand how to translate global climate change into alterations of local interactions that ultimately result in [amphibian] declines."

Monteverde's Pounds says that the new study is "the first time that someone has demonstrated a link between large-scale climate change and high mortality of amphibians in a particular population."

Other studies, including one by Pounds, had hinted at such a link, but they were unable to demonstrate the specific chain of cause and effect through which climate change kills amphibians. He says it is unlikely that the exact same connection can explain the bulk of the amphibian declines elsewhere because many threatened species lay their eggs underground or beneath the canopy of trees, where they are shielded from ultraviolet light. "The real take-home message is that climate may be leading to amphibian declines through a number of different mechanisms," says Pounds.

And climate is not acting alone. A study reported in the February 27 Proceedings of the National Academy of Sciences points an accusing finger at pesticides. In that paper, Rick A. Relyea, an assistant professor of biology at the University of Pittsburgh, and Nathan Mills, a graduate student at the University of Missouri at Columbia, examined a common pesticide called Sevin, used extensively in agriculture and also sprayed in 31 million gardens across the United States. Earlier research on this and other pesticides had tested how high doses affected amphibians in lab studies that lasted only one to four days.

Such short-duration tests, however, cannot mimic conditions in the real world, says Mr. Relyea. He and Mills examined how lower doses of Sevin affected the tadpoles of gray treefrogs over a longer span of time, up to 16 days. And they added one more complication-the stress that most animals face from predators. To make the tests more realistic, the researchers put a caged salamander in the tanks with the tadpoles. The salamander could not reach the tadpoles, but it emitted chemicals that they sensed as a threat.

The pesticide killed off tadpoles even without the predator there. But the presence of the salamander made the chemical two to four times more deadly, report the scientists. In tanks without pesticide, the presence of the salamander did not kill any tadpoles.

Aventis CropScience, the company that produces Sevin, failed to return repeated telephone calls seeking comment.

The experiments point out just how lethal pesticides can be in a more natural situation, says Relyea. "We're using really low amounts, and yet we find we can kill up to 98 percent of the tadpoles, which is incredible to me to think that we have a 'safe' pesticide.... No one would have guessed that such a low level of pesticide would have that effect on an animal."

He is now starting to test other species to see if minute concentrations of Sevin devastate them as well. "We suspect that the case with gray treefrogs is just the tip of the iceberg," says Relyea. "I think that the evidence is coming in that [pesticide use] is a lot bigger problem than we would have suspected."

That could help explain amphibian declines in regions where researchers have lacked any clear leads. "You could have places where winds are bringing pesticides into pretty pristine habitats and they're killing amphibians even though the habitat doesn't look like it's been harmed by people," he says.

This scenario seems to be playing out in the Sierra Nevada mountains of California, where frog populations have declined precipitously even in national parks. A study due in Environmental Toxicology and Chemistry suggests that winds are carrying dangerous doses of agricultural chemicals into the mountains. "We have demonstrated that pesticides and herbicides are causing neurological disruptions in frogs even at the highest elevations of the Sierra Nevada," says Gary M. Fellers, a biologist with the U.S. Geological Survey at Point Reyes National Seashore.

The pollution problem will only worsen in the future. A study published in the journal Science [April 13, 2001] projects a 2.7-fold increase in pesticide use over the next half-century, as the global population swells.

Although amphibians may seem remote from the lives of most people, the discovery about pesticides raises a disturbing question closer to home. The compound tested by Relyea has been deemed safe for mammals, including humans, at low doses. But the new information raises questions about whether stress would make the pesticides more harmful to people, just as it had with amphibians. No studies have answered that question, he says.

The news about pesticides and climate change, though, should not overshadow other factors hurting amphibians. "Most people would agree that habitat loss is by far the dominant factor," says Relyea. "And it is the dominant factor in the declines of most animals-that we are taking their habitat away."

There is a growing feeling among herpetologists that what they see among amphibians reaches far wider. In the past, scientists and the news media have held up amphibians as "canaries in a coal mine"-particularly sensitive creatures that may be suffering problems before they become evident in other animals. But amphibian experts now question that assumption.

"What are the chances that there's something unusual going on with amphibians?" asks David B. Wake, a professor of integrative biology at Berkeley. "There are all kinds of organisms out there." In fact, populations of small reptiles have dwindled in some of the same places where amphibians have suffered.

As part of an effort to resolve these issues, the World Conservation Union [has embarked] on a global assessment of the 5,200 known amphibian species, documenting how each is faring. The irony is that the list of known species has swelled in recent years because the crisis focused researchers' attention on this group. "At the same time as we're losing amphibians," says Wake, "all this activity on amphibians is leading to more discoveries."

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Richard Monastersky is a senior writer with the Chronicle of Higher Education.

Where Have All the Frogs Gone? Chronicle of Higher Education, April 20, 2001. © 2001 by the Chronicle of Higher Education.