The last 4 decades have not been good to frogs, toads, newts, and salamanders. According to the first worldwide study of amphibian populations, published in the 13 April Nature, their numbers have shrunk by about 4.5% per year since 1960.
Biologists have been sounding the alarm over declining amphibian populations for years. Species have disappeared even in protected or remote areas, such as Yosemite National Park and the Monte Verde cloud forest of Costa Rica. "No herpetologist disputes that a decline in population has occurred," says Ross Alford, a population biologist at James Cook University in Australia. But without a worldwide census, biologists could not assess the severity of the problem. To compound the difficulty, amphibian populations are notoriously variable; they routinely disappear from a pond only to reappear a few years later.
"What was missing was a large-scale, rigorous, quantitative look at the data," says Jeff Houlahan, a graduate student of wetland ecology at the University of Ottawa in Canada. "There was evidence all over the place, but no one had tried to pull it together." In 1997, Houlahan began to collect data (both published and unpublished) by e-mail from every researcher he could find who had ever counted amphibians.
With the help of collaborators in Switzerland and Russia, Houlahan eventually gathered figures on 936 populations of amphibians from every continent but Antarctica. The combined data left no doubt that the downward trend was both significant and worldwide. But Houlahan did find regional differences: In North America, amphibian populations decreased steadily for 40 years, while in Europe they stabilized in 1966. "It's possible that the damage caused by land development has already been done in Europe, and you're monitoring populations that aren't that vulnerable to it," Houlahan speculates.
Biologists hope that Houlahan's data bank will help tease out the most important causes of depopulation. "Sometimes the emphasis on unexplained declines might take attention away from obvious, garden-variety declines," such as those caused by land development, says conservation biologist Joseph Pechmann of the University of New Orleans.