Investigating Declining Amphibian Populations
by Michael Hofferber
Copyright © 1994. All rights reserved.
visitors to Yellowstone National Park expect to see elk grazing in its
meadows, moose sauntering through its marshes, and buffalo crossing its
high plateaus. Some even hope to catch a glimpse of the famous grizzly
bear or an endangered grey wolf.
When Dr.Chuck Peterson of Idaho State University visits the park he goes looking for the Western Toad. Together with the graduate students employed to help him, Peterson also searches for Chorus Frogs, Spotted Frogs and Tiger Salamanders.
Since 1991, Peterson has been monitoring amphibians in Yellowstone and Grand Teton National Parks as part of a world-wide effort to determine why so many species of frogs and toads and salamanders are in decline. Of the four species he studies, three appear to have steady populations, but the range and numbers of Western Toads is in serious decline.
"We have eight monitoring sites in the Yellowstone area. The toad used to be found in or near five of these sites, but is now found in only one," Peterson explained. "Forty years ago, the toad was very widespread. Other studies have shown similar declines occurring in Colorado and Eastern Wyoming where the toad is gone from more than 85 percent of its historic range."
The Mountain Yellow-Legged Frogs are in steep decline in the Sierra Nevadas. Australia has lost the Southern Day Frog and the Platypus Frog. Florida may soon lose its Flatwoods Salamander. And the Common Toad is no longer so common in many areas of England and Switzerland.
To find out why these crashes are occurring more than a thousand researchers in 40 regions of the world have been collaborating for the past four years on the Declining Amphibian Populations Task Force of the World Conservation Union. Herpetologists studying these declines, like Peterson in Yellowstone, have been sharing their results with each other through the task force in the hopes of finding answers.
Amphibians are especially sensitive to environmental contaminants. Their early development usually occurs in standing water where pollutants often collect and they respire through their moist skins, allowing toxins easy access to vital organs. In areas where the use of pesticides and herbicides is widespread it is not surprising to find amphibians in decline.
But if toads cannot survive in the wilderness of Yellowstone, where the air is virtually unpolluted and the water naturally pure, then they and many other species are truly in deep trouble.
Wilderness boundaries are no protection against large-scale climate changes or atmospheric disturbances like the deterioration of the ozone layer.
Since amphibians' skins are highly susceptible to damaging ultraviolet light, a thinning of the Earth's protective ozone layer could lead to skin damage and breathing difficulties. Amphibians living at high elevations, or closest to the sun, would likely be the first to be affected and, indeed, many of the mountain species from Queensland to the Ukraine and the Sierra Nevadas are in decline.
No connection between the declines and the ozone depletion has been proven, but Peterson and others have hypothesized a connection. "I believe that it's true that montane populations are being affected by the rising UV levels," Peterson said.
The Declining Amphibian Populations Task Force has found many probable causes for the die-offs and a few smoking guns. Habitat destruction is the mass murderer of amphibian species, most of it man-made. Other culprits include environmental contaminants, the introduction of non-native predatory fish and mammals, and weather-related natural disasters.
"There is evidence of declines in all parts of the world," said Jim Vial, the former international coordinator of the task force. "But the declines are not uniform with any species in all locations."
A status report recently released by the task force found that at least 720 of the world's 4,500 known amphibian species are in some form of serious population decline. Some species are showing population dips that may prove to be part of the species' natural ecology while others are nearing almost certain extinction. In Ecuador, for example, eight amphibian species inhabiting the cloud forests and paramos of the high Andes are in serious decline -- four appear to be recently extinct and four more are faltering.
Because no uniform method of identifying and classifying amphibian populations exists as yet, the task force's findings are unavoidably tentative. But a sense of urgency and concern is widespread among the researchers.
"Immediate measures need to be taken to conserve biodiversity in ecosystems," reads the status report. "If world-wide pollution and habitat destruction continue at present rates it will not be long before amphibian declines are the least of our concerns."
The report calls for ecological studies on global environmental factors such as shifts in acidification, climate, and UV radiation. It also recommends an initiative focused on the lethal and sub-lethal effects of herbicides and insecticides on amphibians.
"It's going to take some long-term study to accomplish all of this," said Vial, the former coordinator of the task force who is now pursuing ecological research in southern Arizona..
Frogs and Toads of the Southeast
Solar Radiation Wiping Out Frogs
Ag Chemicals Wiping Out Frogs
What's the difference between frogs and toads?
"All toads are frogs," says Bill Hamilton, assistant professor of biology at Penn State. "But not all frogs are toads." True toads, to parse it out, are members of the family Bufonidae, which in turn falls under the order of amphibians commonly called frogs, and known officially as Anura, meaning "without a tail." The order includes some 5,000 species, members of which can be found on every continent except Antarctica.
"Frogs are called ‘toads’ when they have a thicker, drier skin," Hamilton adds. A toad’s skin is often covered with bumps and gland openings, which is probably why some people think you can get warts by touching them. Hamilton says that while this is a myth, frogs and toads alike can secrete any number of toxins through their skins.
Having a thick skin permits toads to live away from water longer than most frogs, so you’re more likely to see toads on dry land, he adds. Because they need a different kind of camouflage to live the terrestrial life, toads are often brown in color, which is another way to distinguish them. Toads also prefer to walk where most frogs hop, but that’s where the differences end and the similarities take over. In addition to the obvious physical resemblance, frogs and toads both reproduce and develop in water, both sing and are both are carnivorous. And most importantly, both frogs and toads are under intense extinction stress from human activities.
"Frogs are some of the most fragile, most environmentally vulnerable species on Earth," says Hamilton, which is why they are a good indicator species for the occurrence of habitat degradation. "They are our ‘mining canaries’ for the entire planet," as he puts it. It’s the same permeable skin that sometimes serves in their defense that makes them more susceptible to environmental trauma and pollution. And the fact that across the world frogs are declining significantly in numbers and diversity is cause for great concern, he said. "Their demise could signify major problems for the global ecosystem."
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