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Where Have All the Frogs Gone?
Biologists have examined a rogues’ gallery of possible culprits.
A leading suspect is an infective fungus.
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 frogs 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.
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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 frogs, 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.
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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.
The case history of the red-legged frog is likely quite common. In 2001 the U.N. Food and Agriculture Organization (FAO) issued a report titled “The World Market for Frog Legs,” based on data collected from 1987 to 1998. According to that report, wild stocks supply almost 95 percent of worldwide demand for frog legs and frog products. Worldwide, the FAO estimates, at least 5,200 tons of frogs—more than 200 times the peak harvest of red-legged frogs—were collected annually worldwide from 1987 until 1997. Given the history of the red-legged frog, those harvests are so large that at least some local populations are almost undoubtedly being depleted. Many countries on the FAO list of heavy producers are also rapidly developing countries (just as California was in the nineteenth century), which entails rapid changes in land-use patterns. Species such as the North American bullfrog are being introduced in new regions, such as South America. Clearly, the causes of historical amphibian declines have not disappeared.
In spite of the worldwide scope of the problem epitomized by the red-legged frog, those threats still do not explain the rapid disappearance of species from undisturbed and protected areas. Biologists have been forced instead to focus on the ways amphibians might be particularly harmed by recent environmental changes. Many amphibian species, for instance, have both terrestrial and aquatic stages, making them susceptible to stresses on land as well as in the water. Permeable skin and eggs without a shell also increase their susceptibility. The most likely new threats, to which natural selection in amphibian populations has had little time to react, include: changes in global climate at an unprecedented rate, the introduction of novel toxic chemicals, and emerging infectious diseases to which the amphibians have never before been exposed.
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In the tropical cloud forests of Costa Rica and Panama, at altitudes between about 5,000 to 9,000 feet, global warming poses a clear threat to the local ecosystem. Changing climate alters the patterns of temperature, mist, and rainfall, causing cloudbanks to form at increasing elevations, and compressing the possible range for cloud forests. Such a clear causal relation makes the cloud forests excellent natural laboratories for studying the effects of global warming on amphibians. The amphibians living there are often supported by the water in the clouds. And sure enough, frog populations in cloud forests have significantly declined, and a number of species have become extinct.
Nevertheless, rising temperatures per se don’t seem to be killing the amphibians. What is happening instead, according to investigators such as Allen Pounds of the Tropical Science Center in San José, Costa Rica, and his colleagues, is that by narrowing the range of cloud forests, global warming could be forcing populations of amphibians to live together so densely that they become more susceptible to stresses such as disease-causing pathogens. Global warming is also making the cloud-forest habitat so attractive to species from lower elevations that they are invading and could threaten the amphibian communities.
Of course, global temperatures have fluctuated throughout the evolutionary history of amphibians, but that fact does not address the unprecedented speed with which temperatures and moisture patterns are predicted to change, and the possibility that amphibians won’t be able to keep up with the shifting locations of their cloud-forest habitat. Even if the present amphibian declines are not caused by global warming, the magnitude of the predicted changes will likely threaten all amphibians.
What about the role of ultraviolet radiation? Studies by Andrew R. Blaustein at Oregon State University in Corvallis and his students have demonstrated how increased UV radiation has diminished the hatching success of amphibians in the Pacific Northwest. Blaustein has noted the effect was particularly strong when other stressors were present, such as global warming or changes in precipitation patterns. So far investigators have not been able to show that places where amphibians are declining are exposed to increased UV radiation, but UV exposure could still be part of a complex set of interacting stresses that are causing declines.
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As for the other environmental stresses, laboratory research has long demonstrated that pollutants such as pesticides and herbicides can kill or debilitate amphibian larvae and adults. For example, Tyrone B. Hayes of the University of California, Berkeley, and his students have shown that minute levels of atrazine, a widely used herbicide, cause individual leopard frogs (Rana pipiens) to develop both ovaries and testes. These accidental hermaphrodites cannot reproduce. Although no one knows how such deformed frogs affect a population, there is no reason to expect the effect to be positive.
There is one threat, however, that might connect a number of recent losses of amphibians: emerging infectious diseases. Several features distinguish an emerging disease from an established one: it may be newly recognized by biologists; it may be a known disease that has recently appeared in a new population; or it may be an established disease that is rapidly increasing in incidence, virulence, or geographic range. AIDS, SARS, and hepatitis C are all diseases that have emerged in human populations in the past fifty years or so. Amphibians, of course, don’t suffer from those diseases, but they do suffer from others. The two major suspected emerging pathogens of amphibians are a fungus, Batrachochytridium dendrobatidis, and a class of viruses known as iridoviruses.
The decline and extinction of frogs and toads in Australia, Central America, and North America are associated with B. dendrobatidis. Analysis shows that thirty-two globally distributed strains of the fungus are closely related, which suggests it emerged only recently. Of the infected amphibian populations, the most susceptible species occur at relatively high elevations in the tropics. They also have large bodies and breed in streams, which is consistent with what is known about the biology of the fungus.
Out of forty-six affected Australian frog species analyzed to date, mainly from eastern Australia, but especially from the northeast, thirteen species appear to have declined, and three are extinct. Of the frog species surveyed in Costa Rica and Panama, many of which spend at least part of their lives in streams at high elevation, three-quarters have declined, and B. dendrobatidis is associated in almost every instance.
The precipitous declines of the Wyoming toad (Bufo baxteri) and the boreal toad (B. boreas) in the U.S. likewise are associated with infection by the fungus. Both species live above 7,500 feet.
Iridoviruses, the second kind of pathogen, make up a large group of viruses that occur around the world. A research group of students and postdoctoral associates at Arizona State University in Tempe,
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People are probably partly to blame for the quick spread of the virus. Tiger salamanders are commonly used as bait in the western U.S., and so they are shipped by the millions from ponds, marshes, and stocktanks in the Great Plains. Those bodies of water can serve as excellent incubators and reservoirs for the virus. Fortunately, though, the virus has not proved to be as harmful to salamanders as B. dendrobatidis has been to frogs and toads.
Ironically, even as some amphibian species are declining or going extinct, others are increasing in numbers and range. In fact, the latter species probably play a major role in the overall pattern of decline. For example, marine, or cane, toads (Bufo marinus) were introduced into Queensland, Australia, from South America in 1935, to eat beetles feeding on sugar cane. As it turned out, they rarely ate the beetles, but they dispersed rapidly, by as much as twenty-five miles a year, and by now they have reached the Northern Territories.
Likewise, American bullfrogs, endemic to the eastern U.S., have become established not only in the western continental U.S., but also in Hawai’i, South America, Mexico, various Caribbean islands, Europe, and Asia. And bullfrogs are just one of a number of amphibian species people have moved, either by accident or for some intended benefit. But the exotic newcomers not only displace native species through competition and predation; they can also carry pathogens such as B. dendrobatidis.
The global decline and extinction of amphibians is not a simple problem. Whatever the causes, the extinctions can be understood in a couple of ways. On the one hand, the history of life is a story of extinction: 99 percent of the species that ever existed are now extinct. On the other hand, extinction, as it is happening now in amphibians, also occurs on a much smaller scale, as one, two, tens, or even dozens of species die out in various locales.
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Time affects how we understand extinction, too. One can only stand in awe before the fact that 99 percent of species have disappeared over thousands or millions of years: the span of time alone is impossible to grasp. But the disappearance of amphibians and other species in perhaps less than a human lifetime is far more shocking and worrying. These extinctions become a problem to solve rather than the natural course of things.
The final irony of the recent amphibian extinctions is that, as the twentieth century gives way to the twenty-first, species are disappearing just when herpetologists are poised to make great progress in describing and understanding them. Throughout the twentieth century the number of amphibian species described by scientists increased each decade. Molecular methods and more research combined to increase greatly the rate of discovery since the 1970s.
The harlequin frogs of the genus Atelopus, which disappeared in the 1990s from the cloud forests of Monteverde, Costa Rica, afford a particularly poignant case. Specimens collected in that region decades earlier and stored in the Museum of Vertebrate Zoology at the University of California, Berkeley, were studied recently by Roberto Ibáñez and César Jaramillo of the Smithsonian Tropical Research Institute in Panama as part of an effort to describe the number of species in the genus. The results suggest that the Monteverde form is an unknown species that had gone extinct in the wild before it was even described.
That frog, of course, was not the first such case. Some 360 million years ago, the forebears of today’s amphibians, creatures that could grow to more than six feet long, walked in great marshes that also harbored ferns the size of oak trees. The amphibians of today are smaller and less numerous. Yet the present generation has its charms—in the sounds of spring and summer evenings
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Biologist James P. Collins devotes much of his professional life to protecting amphibian diversity. A professor at Arizona State University in Tempe, he has spent the past thirty-five years studying the ecology and evolution of frogs and salamanders. Collins directs a team of international scientists in a project investigating emerging diseases that threaten amphibians, and also acts as chair of the Declining Amphibian Populations Task Force, a specialist group within the World Conservation Union (IUCN). For those interested in learning more about the threats to frogs, as well as their evolution, biology, and importance to ecosystems, be sure to visit the newest exhibition at the American Museum of Natural History in New York City: “Frogs: A Chorus of Colors,” which opened May 29 and runs through January 9, 2005.
Copyright © Natural History Magazine, Inc., 2004