For anyone who still doubts that amphibian populations are declining worldwide, Jeff E. Houlahan has a message: Give up.
The ecologist at the University of Ottawa worked with an international team to crunch the biggest data set yet assembled for amphibian populations. The team collected information from 200 scientists who’ve examined 936 populations in 37 countries—most in Europe and North America.
Individual species and places vary, but “at a global scale, amphibians have declined over the past several decades,” Houlahan and four coauthors announce in the April 13 Nature.
“We wanted to just pile the data as high as we could and see if we have declines,” Houlahan says.
Although he cautions that records before the 1980s are scarce, Houlahan notes that 1960 to 1966 saw the sharpest plunge, perhaps 15 percent overall. Since then, declines have stayed around 2 percent per year. Scientists, however, didn’t raise the alarm until the late 1980s. “We were catching on decades late,” he laments.
Previous evidence came mostly from anecdotes or short-term studies, the team contends. Houlahan started a grander project while a stay-at-home dad caring for toddling twins. In free moments, he sent E-mail asking for data from every amphibian researcher he could find.
His team calculated yearly population changes for all the studies. In an approach that’s stirring debate, the researchers also tracked ratios of the numbers of declining populations and rising ones.
“Many frog populations should decrease more often than they increase,” argues Ross A. Alford of James Cook University in Townsville, Australia. One population boom gets whittled away by several seasons of dwindling. Houlahan’s second analysis therefore “could be misleading,” Alford says.
Despite his questions, Alford agrees with the study’s conclusion. So does Joe Pechmann of the University of New Orleans, who in a 1991 article cautioned against labeling every population dip a decline.
Few if any herpetologists doubt the decline, notes Ron Heyer of the Smithsonian Institution in Washington, D.C., chair of the Declining Amphibian Populations Task Force. Such a massive analysis “is still significant,” he says.
The international director for the task force, Tim Halliday of the Open University in Milton Keynes, England, comments that Houlahan’s results could come “largely from the impact of habitat destruction alone.” Against that backdrop, he sees local assaults from climate change, disease, pollution, and ultraviolet radiation.
The director of the longest-running daily frog census, according to the Guinness Book of World Records, makes no special claims about amphibian declines. Whit Gibbons of the Savannah River Ecology Lab in Aiken, S.C., has an article coming out in BioScience on global reptile declines. The trouble “extends far beyond amphibians,” he sighs.