BOULDER, Colo., Oct. 20 (UPI) -- The phrase "canary in the coal mine" has endured a lot of use these days as a convenient symbol for a harbinger of doom.
Even a cursory Internet search reveals it has been applied to, among other things, the treatment of Jews as a portent of oppression, the banning of firearms or abortion as presaging the suppression of civil rights, and the warming of the Arctic as a precursor of climate change; not to mention fish poisoned by mercury emissions in New Hampshire, regional healthcare disparities, acid rain in the Great Smoky Mountains National Park, and even former security adviser Richard Clarke's warnings about al-Qaida.
That's a lot of responsibility pinned on a brightly colored, inoffensive bird about the size of a salt shaker.
Amid all this metaphorical construction, however, one thing is certain -- no one wants to be the canary.
The phrase arose from reality. Years ago, coal miners really did carry caged canaries underground. The birds were hypersensitive to the toxic gases that regularly accumulated in the mine shafts. If the birds died -- as they often did -- the miners were given sufficient warning to escape.
Today, in the realm of endangered species, amphibians are the canaries. According to the Global Amphibian Assessment, the taxonomic class appears to fit the "C" word to a T -- they are dying off in huge numbers.
"Amphibians are one of nature's best indicators of overall environmental health," Russell A. Mittermeier, president of Conservation International, said recently in a statement. "Their catastrophic decline serves as a warning that we are in a period of significant environmental degradation."
More than 500 scientists from 60 nations share that assessment. They have found that of the 5,743 known amphibian species, 1,856 -- 32 percent -- are currently threatened with extinction. The data are insufficient to determine the status of nearly 1,300 more species, though they also are thought to be in trouble. An additional 122 species already may have gone extinct.
For nearly 20 years, it has been clear something bad has been happening to amphibians -- frogs, toads, salamanders and such. The cause of the problem, or problems, has been elusive, however, and the search for the source of the declines amounts to a major scientific detective story.
Because many amphibians respirate through their skins, it has been feared they are particularly sensitive to pollutants. A 2001 study by the U.S. Geological Survey in California, for instance, implicated airborne agricultural pesticides as one of the possible causes of amphibian declines in the state.
Gary Fellers, a USGS scientist stationed at Point Reyes National Seashore in California, found the decline of frogs in Sierra Nevada Mountains appears to be related to the use of pesticides in the state's rich, agricultural Central Valley. Chemical residues apparently are borne by the prevailing winds and deposited in the mountains.
In the Great Plains, native male leopard frogs appear to have been "feminized" by absorbing the common herbicide Atrazine, according to a study conducted by biologists at the University of California, Berkeley.
There have been frog deformities observed in the Great Lakes, probably caused by parasitic flatworms, although some scientists think increased ultraviolet solar radiation -- caused by damage to Earth's ozone layer -- is the problem.
So far, there appear to be many different factors implicated in local population declines, anomalies or extinctions, but as yet, no one has been able to explain the global reach of the phenomenon.
Some biologists have argued that perhaps there is nothing to worry about. Historical records show amphibian populations sometimes have undergone wide, natural swings. So perhaps the current die-off merely represents a periodic cycle. This theory was thoroughly exploded, however, by J. Alan Pounds of the Monteverde Cloud Forest Preserve and Tropical Science Center in Costa Rica in 1996. Pounds found the current declines were too sharp and synchronous to be caused by natural cycles.
So scientists remain unsure what is causing the problem -- although evidence from Australia, the United States and Central and South America has begun to implicate a new suspect.
Chytrid fungus exists on the skin of amphibians and usually lies there harmlessly, but certain environmental stresses can cause the fungus to become fatal to the animals.
"The emerging scientific consensus is that chytrid fungus is pretty heavily involved," Simon Stuart told UPI's Blue Planet. Stuart is senior director on biodiversity assessment for the IUCN -- The World Conservation Union -- as well as Conservation International and NatureServe. He also is the lead author of a paper that appeared in Oct. 15 issue of the journal Science detailing the amphibian decline.
"When the fungus strikes in a lethal way," he told Blue Planet, "it seems to be linked to extreme drought periods. Some of these are probably climate change related."
The paper by Stuart and colleagues does not definitively attribute the loss of amphibian populations to any single cause.
"Although many declines are due to habitat loss and overutilization," the authors wrote, "other, unidentified processes threaten 48 percent of rapidly declining species and are driving species most quickly to extinction."
If global warming-induced drought is indeed the trigger, the prospects are not good.
"There is clear evidence in the paleoclimatic record that even in some of these relatively wet areas of the tropics, you can get prolonged dry spells," Jonathan Overpeck, director of the Institute for the Study of Planet Earth at the University of Arizona in Tucson.
"Coupled with global warming accelerating the hydrologic cycle, with more periods of excess and low rainfall, it is plausible to expect to see an increase in the incidence of drought in the low latitudes," Overpeck told Blue Planet.
Should amphibians, then, be added to that long list of coal-mine canary metaphors as presenting silent, fatal warnings?
"Amphibians are a group of species of very narrow ecological tolerances," Stuart explained. "So if things are going wrong in the world, one would expect that amphibians would be ones to warn us about it. I fear that what happens to amphibians today could be a prophecy of what happens to other species and even people in the future."
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Blue Planet is a weekly series by UPI examining the human relationship to the environment. E-mail [email protected]
