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Amphibian Deaths: Environment's 'Canary in a Coal Mine' Amphibians are dying all over the world, taking many species to extinction. So far, no one really knows why.  By Michael Lindemann Start Date: 2/25/00 Amphibians were the first vertebrate animals to inhabit the land of ancient Earth. The class Amphibia, including frogs, toads and salamanders, has existed for more than 300 million years and is found almost everywhere on Earth except in the arctic zones and the driest deserts. But in the last few decades, scientists have noted that something is killing off amphibians in alarming numbers, again almost everywhere in the world. In less than 20 years, several dozen species in such widely separated places as Australia, Central America and the U.S. Rocky Mountains are known to have disappeared or are nearing extinction, while many more could be endangered. But finding out why has turned out to be more difficult than scientists had expected. At the recent annual meeting of the American Association for the Advancement of Science (AAAS), held Feb. 17 to Feb. 22 in Washington, D.C., amphibian decline was a hot topic. What scientists can now agree on, judging from presentations delivered at that conference, is that no single factor explains all the deaths. Apparently a constellation of factors is at work, and behind the causes now visible there may lie other causes still unseen. According to Andrew Blaustein, a professor of zoology at Oregon State University, some of the known causal factors are "rising levels of UV-B radiation in sunlight, pathogens, pollutants, habitat destruction, introduced predators and most recently, crop fertilizers." Blaustein and others agree that the complexity of amphibian decline helps to illustrate how ecological changes may have synergistic effects that compound previously minor problems. A single change such as increased UV-B radiation, or higher nitrate levels in agricultural run-off, by itself might not be enough to cause death or deformity. But put such changes together and you have far more serious impacts, Blaustein says. He points to 14 species of amphibians that have disappeared from Australia in recent years; five species of amphibians in the U.S. Pacific Northwest that are new candidates for the endangered species list; the extinction of the golden toad in Costa Rica; and massive egg mortalities of the Cascades frog in Oregon. Amphibian declines have been measured in Europe, South America, Asia, and Africa; and they are occurring even in some of the most pristine environments on the planet, places where human encroachment is still considered minimal. "This is an incredibly complex problem, and there's no end in sight," Blaustein says. Not long ago, a U.S. research team announced with considerable fanfare that gross deformities in some species of frogs -- usually marked by missing or extra legs -- had been traced to a parasitic flatworm known as a trematode or fluke. This information was touted as an indication that human-induced pollution could not be blamed for the plight of the world's amphibians. But it's not that simple, Blaustein and others argue. "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," Blaustein says. The possibility exists, therefore, that an explosion in the trematode population is overwhelming the natural defenses of susceptible frogs; and the increase in trematodes is part of a complex set of changes that might have been begun with increased nitrogen run-off from heavily fertilized U.S. farms. Other scientists also suspect such intricate and, thus far, poorly understood linkages in the strange phenomenon of amphibian deaths. But what this illustrates, they say, is 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. Elizabeth Davidson, a pathologist at Arizona State University, does research on a lethal salamander virus. She had heard about massive numbers of tiger salamanders dying in Arizona's San Rafael Valley from fellow ASU biologist Jim Collins, who was studying the salamanders' ecology. Davidson and research assistant James Jancovich, curious about what was killing the animals, accompanied Collins to the San Rafael Valley during a dieoff in 1995. They found ponds full of dead salamanders apparently killed by a bacterial infection called "red leg" disease. Assuming that the bacteria were to blame for the deaths, Davidson and Jancovich took samples back to the laboratory for further study. What they would find later would open a minefield of unanswered questions. In the laboratory, they had no trouble isolating the bacterium causing red leg and plenty of other bacteria. "But we were isolating the same bacteria from healthy animals, too. That was one of my first clues that we were barking up the wrong tree," says Jancovich. If healthy animals harbored the same bacteria, why weren't they dying? Davidson and Jancovich began to suspect that the bacteria were simply opportunists preying on already-sick animals, and that a different agent was the real culprit. Davidson sought the help of colleague Frank Morado, a marine pathologist at the National Oceanographic and Atmospheric Administration in Seattle. The damage Morado saw in the infected tissue reminded him of a similar infection he had recently seen in salmon and crustaceans. The salamanders, he told Davidson, showed all the hallmarks of attack by a virus. Sensing they were on the right track, Davidson and Jancovich put tissue samples under an electron microscope. And they found the virus -- huge amounts of it. Theirs is a type of iridovirus, a group that typically infects insects and other invertebrates, as well as fish and frogs. The name derives from the iridescent appearance that virus-laden insects acquire in the late stages of infection. At death, 75% of an insect's body weight may be virus, enough to refract light. Since its identification in the San Rafael Valley, the salamander virus has now been found to be responsible for major dieoffs of several subspecies of tiger salamanders, including an endangered subspecies, at two other sites in Arizona. Other closely related viruses have caused salamander deaths in Utah, North Dakota, Wyoming, and distant Saskatchewan. How the virus is spread from one site to another is still an unsolved mystery. Ponds in a single region may be too distant for salamanders to make the trek, virus in tow. And a promenade from Arizona to Saskatchewan seems out of the question. Professor Cynthia Carey of the University of Colorado at Boulder has been looking for links between climate change and amphibian mortality. So far, she says, those links are not evident, but she believes the recent rapid spread of disease among many amphibian species may be indirectly influenced by climate and environmental factors. "There has been a lot of deforestation, lake and pond draining, pesticide use and predator introductions in areas of amphibian die-offs. But one of the bizarre things about the declines is that many amphibians are dying in relatively undisturbed areas where man-made environmental degradation is not obvious," Carey says. In 1998, the fungus Batrachochytrium dendrobatidis was identified as a major cause of global amphibian deaths. The fungus appears to sicken amphibians by infecting their skin, which the animals use for respiration, ion and water transport and defense against water-borne pathogens. But researchers do not yet know enough about fungal interactions with amphibian immune systems to understand how the fungus kills the creatures, Carey says. The chytrid fungus group to which B. dendrobatitis belongs is a parasite that feeds primarily on algae, higher plants and small invertebrates. Key questions still unanswered are when it began infecting amphibians, whether it has spread around the world in recent decades, and if its virulence fluctuates, causing corresponding fatalities in amphibian species. Carey, one of the first U.S. researchers to tie amphibian die-offs to disease, says this fungus is killing amphibians in Australia, Costa Rica, Panama and the Western United States. "It infects and kills populations very rapidly, taking out all the breeding adults in a very short time," she says. She and other researchers hope to determine if the same chytrid fungus is travelling around the world, if specific local environmental factors cause local populations of the fungus to become virulent, and how specific climatic conditions may affect the susceptibility of amphibians to such infections, she said. "We need to move on this problem quickly," said Carey. "There is not much time left to identify and fix the problem before Earth loses most of its amphibians." For some time, researchers have been referring to dying amphibians as the "canary in the coal mine," an early warning sign of environmental danger. But while deaths increase and species disappear, the underlying cause is frustratingly elusive. If there is a silver lining in this crisis, it is that scientists around the world are becoming aware that complex interactions and constellations of small changes in the environment can result in huge and deadly effects. If this awareness spreads to governments and the general population, the amphibians may not have died in vain.
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