Frog population is not leaping
Au contraire, amphibian numbers have been declining for decades
If your neighborhood resounds with a frog spring chorus, count it a blessing. The much publicized worldwide decline of frogs and salamanders has been going on far longer than ecologists have imagined.
A study of 936 populations worldwide shows it has been underway for 40 years at an average rate of 4 to 5 percent a year. Some of the losses have been deep and sudden. Reporting these findings in Nature last week, University of Ottawa ecologist Jeff Houlahan and colleagues note that less than 7 percent of the populations studied actually went extinct. Nevertheless, they conclude that even surviving populations "have undergone strong declines."
Such a finding reinforces ecologists' suspicions that something may be going wrong in Earth's environment. But they are baffled in trying to understand what is happening to frogs and salamanders, let alone what their troubles imply for humans. When scientists pin down a likely cause of the decline, they also discover other factors that muddy the issue. Human destruction of amphibian habitats looms large. Yet many declines are in seemingly undisturbed areas.
Zoologist Andrew Blaustein from Oregon State University at Corvallis told an American Association for the Advancement of Science symposium held in Washington last February that scientists "can say for sure" that certain causes are involved. They include ultraviolet sunlight, pathogens, pollution, and introduced predators, as well as habitat loss. These interact with each other, and with such general environmental influences as climate change, to affect amphibians in subtle ways. Dr. Blaustein said this presents researchers with "an incredibly complex problem" to solve. He added that "there is no end in sight" for their research.
They are pursuing that work with a sense of urgency. Population biologist Cynthia Carey with the University of Colorado at Boulder points out that "there's not much time left to identify and fix the problem before Earth loses most of its amphibians." That means extinctions of entire species, not just local population losses. Australia has already lost 14 species. Costa Rica's golden toad is gone. Five species are seriously endangered in the American Pacific Northwest. Oregon's Cascades frog is suffering massive egg losses.
Scientists have identified a fungus as a major cause of amphibian wipe-outs around the world. A virus also attacks these animals. This puzzles ecologists such as James Collins with Arizona State University at Tempe. He notes that the virus is part of a widely distributed group known in fish and insects. The fungus is from the chytrid group usually associated with decaying plant matter. There is no obvious reason why these microbes now should attack amphibians so virulently. Dr. Collins told the Washington symposium that "we've known about the chytrids for a long time, but never as pathogens of amphibians until the last 18 months."
Dr. Carey worked with National Oceanic and Atmospheric Administration climatologist Michael Alexander to look for a link to climate change. They studied three regions - Queensland, Australia, Costa Rica and Panama, and Colorado mountains - where the fungus and other pathogens have caused well-documented die-offs. They found no correlation between the attacks of the pathogens and unusual weather. Carey says they concluded that "we don't know anything about what sorts of environmental factors could tip the balance between pathogens and amphibian immune defenses."
Researchers are back to square one in trying to understand the amphibians' plight. Collins says "the nature of [their] science itself is going to have to change." It will have to include insights from a wide range of physical and biological fields. He explains that the researchers "have to do a better job of integrating humans" into their analyses. "This means that ecology, as a hard natural science, will also need to ... [be] integrated with social sciences and probably the humanities."
Collins is leading a three-year international project organized along this line. Called the "Host-Pathogen Biology and the Global Decline of Amphibians," it involves 24 scientists from a wide range of disciplines.
Meanwhile, scientists have reached no consensus as to what the amphibians' troubles imply for humans. Mike Lannoo from Ball State University in Muncie, Ind., says there's "no clear demonstrated link between amphibian declines and human disease." But he notes that "some people" consider it to be "an indicator of poor ecological health."
(c) Copyright 2000. The Christian Science Publishing Society