Where have all the frogs gone? Biologists have examined a rogue's gallery of possible culprits. A leading suspect is an infective fungus

Natural History, June, 2004 by James P. Collins

Think of an outdoor place where you like to walk. Take a moment and picture what you expect to see: familiar trees and flowers, perhaps singing robins or squawking jays. Think of your favorites, the plants and animals you look for, the pleasure and even reassurance that seeing them brings. What if, the next time you went for that walk, you found that half of your favorites were missing; that still fewer were around the next time; and that, by the third trip, everything you treasured most had disappeared? It would be painfully sad, of course, but wouldn't it seem odd, as well? If all the squirrels, say, or house sparrows in the eastern United States were to suddenly disappear, the first questions on everyone's lips would be: What happened? Why are they gone?

Unfortunately, for biologists studying the Earth's biodiversity, discovering that a familiar organism is suddenly gone is an all-too-familiar experience. Sometimes the explanation is easy. The unmistakable marks of a chain saw on tree stumps provide obvious clues. But more often the answer is not so clear-cut.

By profession I am a herpetologist, a biologist specializing in reptiles and amphibians. In the late 1980s, my colleagues and I began reporting that in familiar amphibian haunts the numbers of flogs and salamanders were declining. By the mid-1990s we were hearing reports that species were going extinct in only a few years; the search for the answer to our question--why are they gone?--was becoming paramount.

Actually, our search became a quest for answers (plural!): the reality in the science, as in any good mystery, turned out to be complicated. In fact, the full story of the decline and extinction of amphibian species remains unknown. But the dimensions of the problem are easier to appreciate if the leading explanations are split into two major categories, the historical and the recent.

Historical explanations point to such causes as competition with exotic, introduced species, or predation by the same; to the harvesting of wild animals for food or pets; and to changes in patterns of land use. Those processes account for most of the damage to amphibian populations for much of the twentieth century, and even today. Although the details of how one of these pressures caused a species to disappear may elude biologists, historical stresses often leave clues--some as obvious as the mark of a chainsaw--from which an investigation can begin.

Of course, none of these historical pressures is unique to amphibians. And, in any event, the declines and extinctions in the 1980s and 1990s left few, if any, clues. Perhaps our biggest shocks were the disappearances of species from national parks and nature reserves, where the obvious historical causes did not apply; somehow, habitat protection, perhaps the best way to ensure a species' survival, was failing to protect some amphibians.

When the standard historical explanations could not solve the mystery, we began to consider the possible role of recent change. Three leading suspects have emerged: global change, particularly global warming and increased ultraviolet radiation; toxic chemicals in the environment; and emerging--in some sense, new--infectious diseases. Each suspect has its champions (or, perhaps, each has its accusers), and most likely none is acting alone. What's more, some suspected causes probably have accomplices that we don't yet even know about. To crack the mystery of the disappearing flogs, the herpetologists' "detective squad" must look at all possibilities.

To appreciate what kinds of stresses must be considered, take the case of the California red-legged frog (Rana aurora draytonii), as documented by Mark R. Jennings, a herpetologist at the National Biological Service in San Simeon, California, and Marc P. Hayes, a herpetologist at the Washington Department of Fish and Wildlife in Olympia. During the great California gold rush of 1849, thousands of forty-niners made fortunes mining gold. Food, though, was so scarce that even a rich man could have a hard time finding something to eat; a chicken egg could sell for fifty cents. So people turned to California's native species for food.

The red-legged frog was among the animals collected, but populations could not sustain the hunting indefinitely. By the end of the nineteenth century commercial frog harvesting had crashed: after a peak harvest of twenty-five tons (about 120,000 frogs) in 1895, by 1907 California produced quantities too small to bother reporting.

Even as populations of the red-legged frog collapsed, consumer demand for frogs remained high. Between 1900 and 1935 entrepreneurs created "frog farms" in California's Central Valley. But because native frog populations were largely gone, the producers imported bullfrogs (Rana catesbeiana) from the eastern U.S. Although the commercial enterprises eventually failed, the bullfrogs thrived, and the animals left over from the abandoned commercial operations established feral populations. Large, invasive, and voracious as adults, bullfrogs often out-compete--and often eat--native amphibians. Since the 1930s they have replaced red-legged frogs in many habitats. What's more, even as the Central Valley became a major source of edible frogs, Californians were converting wetlands to farmland on a massive scale. The triple blow of uncontrolled harvesting, an aggressive exotic species, and the loss of habitat nearly created a knock-out punch for red-legged frogs.