Froggy went a missin'
by Joe Eaton

Even if the ominous headlines have become less frequent, the world's amphibians - frogs, toads and salamanders - are still in trouble. Fifteen North American species remain on the federal endangered species list, and more species may join them. The Southwest Center for Biological Diversity and a group called Christians Caring for Creation recently sued to have the arroyo toad of southern California federally listed as endangered. Some frog species endemic to Puerto Rico may already have become extinct.

The problem appears to be global, with reports of declines or apparent extinctions from Australia, Central and South America, Africa and Eurasia. Even species living in relatively pristine nature reserves have been affected. The golden toad has vanished from Costa Rica's Monteverde Cloud Forest Reserve, while the Yosemite toad has become hard to find in the high country of its national park.

Multiple causes have been suggested, and much research remains to be done. Unlike for birds, we lack good baseline data on amphibian population sizes, so the decreases are hard to quantify. Many species, especially among the salamanders, are small, cryptic, and hard to detect, even though they may outnumber and outweigh all other vertebrates in some environments.

Major suspects so far include these:

Ultraviolet radiation
The thinning of the ozone layor may be exposing amphibians to harmful doses of ultraviolet light in their vulnerable egg and larval stages. Studies in Oregon show a relationship between UV exposure and reproductive failure in some frog species.

Acid rain
Soil and water pH levels below 4-5 can be fatal to amphibian embryos and larvae. Acid rain may account for some of the reported declines in northern North America and Europe.

Habitat loss
The continuing destruction of freshwater wetlands has a direct impact on amphibians, most of which spend part of their life cycle in aquatic environments. This includes ephemeral habitats like vernal pools, where frogs gather to mate. Forest-dwelling salamanders are affected by clear-cutting and plantation silviculture.

Introduced predators
The voracious bullfrog, released in the wild in western North America, eats smaller frogs. Some artificially stocked game fish also prey on amphibians.

Disease
Frogs and toads in Australia, Panama and California have been found to be infested by a fungus, which coats their bellies and legs and appears to interfere with their respiration. High-altitude rainforest species seem to be most affected. UV radiation may make amphibians more vulnerable to other fungal conditions. Also, recent studies indicate that introduced fish may carry viral diseases that affect frogs.

Toxics
Herpetologists Robert C. Stebbins and Nathan W. Cohen, in their Natural History of Amphibians, (University of California Press) point to environmental contamination by man-made chemicals as "closest to being a 'single cause' behind widespread amphibian declines." Runoff carries a brew of contaminants to the wetlands where amphibians live or breed. Endocrine disruptors like PCBs and DDT may be playing havoc with amphibians' reproductive cycles. Mutagens may account for the highly publicized frog deformities found in the Great Lakes region (so far, only still-common species have been affected), although causation is controversial. Some chemicals, like the surfactant used in the herbicide Roundup, are directly toxic to amphibians. United States Geological Survey biologist Gary Fellers, who has studied the mountain yellow-legged frog in California, observes: "Those areas with the most contaminants are the areas with the fewest frogs."

A US government task force, with representatives from the Interior and Health and Human Services departments, the National Institutes of Health and the Environmental Protection Agency, has been convened to study the frog phenomenon and its implications for human health, and an eightfold increase in Interior's budget for frog research (to $8.1 million) is being proposed. The government has also created a web site (www.frogweb.gov) where US and Canadian residents can report sightings of amphibians, deformed or otherwise.

While the first reports of a decline were greeted with some skepticism, that is true no longer. "All amphibian biologists are now convinced that something unusual and catastrophic is happening to amphibians," says Ron Heyer of the Smithsonian. "We also think the amphibians are telling us something has happened to the habitat we share with the frogs. We need to act on the warnings the frogs are giving us."