Human Impact on Environment Is Not Always
the Culprit in Amphibian Decline
by Stephanie Neal
It whistled through the scientific community like an early warning siren: Ecologists from Oregon to Australia were reporting declines in the population of amphibians.
Fear for the fate of frogs -- and for what it meant for the environment -- began to distress scientists worldwide.
But an ongoing UGA study urges ecologists not to draw hasty conclusions from the apparent signs of amphibian decline.
The 12-year research project at the University's Savannah River Ecology Laboratory indicates that nature, not human impact on the environment, may be behind some of the alarming reports.
In a cover story in the journal Science last August, the research team disclosed that populations of four amphibian species at one South Carolina pond fluctuated widely over a 12-year period -- and for a variety of reasons.
"The study revealed that local environmental conditions, like the amount and seasonal timing of the rainfall, can be very important in influencing breeding patterns of amphibian populations," said Dr. J. Whitfield Gibbons, a senior ecologist at the laboratory and a member of the amphibian research team since its inception.
"The long-term nature of the research has revealed one especially important point, and that is animal populations can fluctuate greatly from year to year," he said. "We must be cautious about identifying trends. Short-term observations of only two or three years can be misleading about the population status of a species. If we had gathered data only in the mid 1980s -- during the long-term drought when amphibian activity was greatly reduced -- our interpretation would have been entirely different."
In recent years, a spate of reports from sites around the globe have chronicled population declines among amphibians.
Some scientists saw the reports as a signal. They theorized that the decline of amphibian species could be a harbinger of the failing health of the Earth.
The theory made scientific sense: Because amphibians live both on land and in water, they're more likely to encounter pollution in the environment. And because their skins are permeable, amphibians might be more susceptible to contaminants than are other animals.
While the Savannah River ecologists don't discount the theory, they urge caution and continued research before drawing conclusions.
"The fact that we didn't observe any declines other than drought-related fluctuations in four species at one pond doesn't mean that populations at other ponds aren't in trouble," said Joseph H.K. Pechmann, a research technician at the ecology lab and a doctoral candidate in zoology at Duke University.
Pechmann said it can be difficult to tell in individual cases whether declines in amphibian populations are occurring naturally or are the result of pollution or other changes in the animals' habitat.
He said scientists analyzing short-term data might assume an amphibian population is fluctuating because of human-caused environmental problems, when, in fact, it is declining because of natural fluctuation.
To conduct their study, researchers at the ecology laboratory -- which is located on the U.S. Department of Energy's Savannah River Site, a nuclear materials production facility -- took a census of frogs and salamanders at a specific site and monitored their population.
The research began more than a decade ago at the Energy Department's request and is the longest ongoing census of an amphibian community in the world, Gibbons said.
"We are fortunate from a research standpoint that, during the development of the Defense Waste Processing Facility 14 years ago, the Energy Department recognized the need for a long-term ecological study to determine the effects of construction," he said. "Environmental welfare is an issue that deserves a long-term commitment of this nature."
The study has tracked changes at a pond called a "Carolina bay," a type of shallow depression that usually holds water either seasonally or year-round.
Five species of salamanders and 11 species of frogs and toads were known to breed at the bay.
Researchers erected an aluminum fence, known as a "drift fence," around the pond. Buried along each side of the drift fence are 44 pairs of buckets located 30 feet apart.
As the animals make their way toward the pond to breed, the drift fence stops them; they continue along the fence, trying to find a way around it, until they drop into a bucket.
Researchers check the buckets daily, collecting data, marking each animal and releasing it on the opposite side of the fence.
After the animals leave the pond, they again land in a bucket to be counted, marked and released. The newly metamorphosed young also are captured in the buckets as they leave the pond.
"By the end of a breeding season, we know how many adults bred, how many survived and how many young are produced," said David Scott, a research technician who contributed to the Science article. "The data allow us to assess how well or poorly a particular species is doing over time."
While this method doesn't work for all species of amphibians -- tree frogs, for example, climb over the drift fence -- it is effective for the four species studied for the latest research article: the marbled salamander, the mole salamander, the Eastern tiger salamander and the ornate chorus frog.