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Gathering Evidence

A concerted effort by the enlisted scientists has provided us with far greater documentation of amphibian decline than we had in 1990 when the task force was formed. One suspicion that researchers confirmed is that most amphibian declines and disappearances are directly related to habitat modification. Furthermore, when the habitat change is dramatic, so are the effects. For example, in the United Kingdom, where many-in some areas 80 percent-of breeding ponds have been filled in over the past 50 years, all six native amphibian species have suffered dramatic population declines. Elsewhere, along a well-studied area on Volcan Tajumulco, the highest mountain in Guatemala, only 1 of 8 species of salamanders was able to survive after cattle ranchers converted the upper cloud forest zone into grazing pastures. Herpetologists also discovered that seemingly modest changes in habitats can also have profound effects. For instance, to the casual observer, it would appear that the arroyo toad (Bufo microscaphus californicus), whose habitat now exists entirely within uninhabited parks in California, is well protected. But the major streams that fed the best breeding sites have been dammed, and what remains of the stream bed plains is now being overrun by all-terrain sport vehicles. Because the larvae cannot live in the silty conditions that result from these modifications, toad populations have decreased alarmingly.

Perhaps the most disturbing finding, however, is that amphibian declines are occurring in diverse locations in relatively undisturbed habitats. Consider the following cases:

In Australia, herpetologists have known since the late 1970s that populations of R. silus, the gastric brooding frog, were declining in pristine sites. After learning at the First World Congress of Herpetology that the decline might be symptomatic of a worldwide problem, the Australians launched a campaign to inventory all known amphibian localities throughout their rainforests, and to initiate long-term monitoring programs in some key areas. The researchers had since counted 14 frog species from remote habitats whose once-abundant populations had either completely vanished or had been reduced to only a few frogs.

In California, biologists Charles Drost and Gary Fellers, both of whom are now with the U.S.Geological Survey, devised a clever approach to evaluate the status of amphibian populations in Yosemite National Park. Using extensive field notes of biologists Joseph Grinnell and Tracy Storer-who recorded detailed descriptions of the area’s amphibian breeding sites between 1915 and 1919-Drost and Fellers were able to reassess the amphibian populations at the same sites. The fact that the researchers were able to relocate every site proved that no obvious change had occurred in the habitat during the intervening 75 years. Sadly, they also found that most of the amphibians were gone: whereas Grinnell and Storer counted 7 different amphibian species at 70 locations, Drost and Fellers could now find only 4 at 26 sites.

The elfin forests on the ridge crest at Monteverde, Costa Rica, have witnessed perhaps the most notorious disappearance of an amphibian population from an undisturbed habitat-that of Bufo periglenes, the golden toad. Among the world’s most colorful amphibians, the brilliant golden males differ dramatically from the equally flamboyant black, red, and yellow females. Largely because of their spectacular beauty, golden toads-known to science only since the 1960s (although the Quakers who colonized the Monteverde area were aware of their existence before then)-served as the focus of concerted efforts to conserve the local habitat. In fact, a golden toad is depicted on the same sign with a panda to mark the entrance to a 328-hectare preserve established in 1972 by the Tropical Science Center of Costa Rica and the World Wildlife Fund for Nature. Later endeavors by other conservation groups tripled the size of the preserve to 10,500 hectares and finally more than doubled it again by adjoining it to the 16,000-hectare Children’s International Rainforest.

Despite these conservation efforts, the golden-toad population crashed in 1988. During April and May of 1987, “more than 1,500 toads gathered to mate in temporary pools at Brillante, the principal known breeding site,” report biologists Martha Crump and Alan Pounds, in the March 1994 issue of Conservation Biology. “But in 1988 and again in 1989, only a single toad appeared at Brillante, and a few others gathered 4 to 5 kilometers [to the southeast]. During 1990 to 1992,” the researchers note, “despite our intense surveys, no golden toads were found.” Nor have any been seen since.

In Puerto Rico, researchers have discovered that two species, including Eleutherodactylus jasperi-one of the world’s few viviparous frog species (which, like mammals, produce live young instead of eggs)-have apparently become extinct though their habitat still appears suitable.

In Ecuador and Venezuela, eight species have been reported absent from the cloud forests of the Andes mountains. One genus in particular, the Atelopus, was once incredibly abundant (researchers could collect hundreds in an hour). But in 1990, Enrique LaMarca, a biologist at the University of the Andes in Venezuela-having spent more than 300 hours during 34 separate field trips searching for the frogs-reported finding only one specimen of A. mucabajiensis and two A. soriani. Another species in the genus, A. oxyxrhynchus, which LaMarca reported observing walking by the dozens on the forest floor, has not been seen since 1978.

In the Atlantic Forests of southeast Brazil, specifically at a well-studied site in Boraceia, So Paulo, seven common amphibian species disappeared in 1979. The site has since been revisited numerous times by several herpetologists including Jaime Bertolucci, a doctoral student at the University of So Paulo, who conducted an intensive year-long study of the ecology of tadpoles. But none of the species that disappeared in 1979 have ever been found.

Similarly well-documented studies have found amphibian disappearances or declines from relatively undisturbed habitats elsewhere in these and other regions, including the U.S. Rocky Mountains and the Cascade Mountain Range in Washington, Oregon, and California.

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