The more we think we understand amphibian decline, the more mysterious it becomes-and the biggest mystery of all is whether we will act to stop it.
Why did it die out? The golden toad (Bufo periglenes) is by now perhaps the world's most famous amphibian, but it probably no longer exists it lived only in the Monteverde Cloud Forest Reserve in Costa Rica--a foggy that of dense upland forest wracked by heavy winds that blow off the Caribbean. The toad's main babitat was on one cold, wet ridge called Brillante where it emerged en masse in spring for five to ten days at a time, to mate in rainwater that pooled against the roots of the trees.
On the somber forest floor, these mating congregations were a spectacle of intense, almost hallucinatory color. The males were an improbable flaming orange and had eyes like black beads. They were only about 5 centimeters long they looked like Mayan treasure come to life. The females were slightly larger and colored so differently you wouldn't have thought they belonged to the same species; they were greenish black with bright red blotches edged in yellow. Photos of the male were used in the publicity campaign to establish the reserve. Its photos still adorn tourist posters. Eventually, it became the "poster road" for amphibian decline.
The last time the roads appeared en masse was in 1987. In 1988 only 10 toads were seen. A year later scientists found just one solitary male. In 1990 they found no toads at all. Initially, it was thought that perhaps the toads were just "hiding out"-- skipping a bad breeding season or two. The springs at Monteverde had been slightly warmer and drier than usual, and unfavorable spring weather is known to reduce the breeding populations of many other amphibians. But that generally does them no long term harm the creatures are usually back in force with the next good spring.
And over the very long term amphibians have indeed prospered. At some 350 million years of age, Amphibia is the world's oldest terrestrial vertebrate class. (A class is a taxonome group at the level of for example mammals or birds). Scientists have thus far identified nearly 5,000 species of frogs, toads, salamanders, newts and caecilians (legless largely subterranean creatures). There are more species of amphibians than there are of mammals. Amphibians' collective domain includes every continent except Antarctica, and probably most of the world's major islands. They achieve their greatest variety in tropical and warm temperate forests, but they also live in deserts, grasslands, northern bogs--even tundra, in the case of the wood frog (Rana sylvatica), one of four North American frogs that can "freeze solid" and survive--the only vertebrates known to have this ability.
Measured against the full breadth of this ancient and widespread class, the golden toad's plight didn't initially look very portentous. But it wasn't just a matter of the golden toad. By 1990, 19 other amphibian species had gone into serious decline or disappeared entirely from Monteverde. And it wasn't just a matter of Monteverde: as the 1990s wore on, reports of declines and disappearances emerged from most of the regions where amphibians were reasonably well monitored--in North America and parts of South America, in Europe and Australia.
Yet Monteverde cast a long shadow: many of these other declines unfolded in a way that bore an uncanny resemblance to events in Costa Rica. They were, in the first place, very rapid. They sometimes involved whole assemblages of species, rather than just one or two. And they were occurring not just in areas that were obviously disturbed, but in some of the world's most carefully protected parks. These were not the kinds of losses that could be readily predicted--or explained. The "Monteverde syndrome" suggested that something peculiar was happening to Amphibia--something bad enough to distinguish it from the broader tragedy we have come to know as the biodiversity crisis.
What Makes Them So Vulnerable?
"Amphibian" is a Greek construction meaning "double life"--a reference to the fact that the typical amphibian lifecycle is partly aquatic and partly terrestrial. That can make amphibians doubly vulnerable: disturbance of either water or land can affect them. In water, for example, some species have fairly narrow temperature requirements. Some do best in still water, others need flowing water. And many are particular about where they will breed. In southwestern California, the endangered arroyo toad (Bufo microscaphus californicus) does not reproduce well unless it lays its eggs on the sandy bottom of a slow moving stream. Some frogs and salamanders will lay eggs only in the shallow "vernal pools" that appear with the spring rains and disappear with the summer heat. This is a kind of evolutionary gamble with the weather: the young are safe from predatory fish in a vernal pool, but they must reach their terrestrial phase before the pool dries.
Given such preferences, it's not surprising that a primary ingredient of amphibian decline should be that standard form of environmental corrosion: habitat degradation. Many amphibians, for example, are forest animals and the world is currently losing about 14 million hectares of natural forest each year-that's an area larger than Greece. (It's true that tree plantation cover is expanding, but plantations do not generally provide an ecological substitute for natural forest; see "Paper Forests," WORLD WATCH, March/April 1998.) Even when the result is not outright deforestation, logging can devastate amphibian populations.
Consider the logging boom in the U.S. southeast, where the forests shelter the world's richest assemblage of salamanders. About 60 percent of all salamanders belong to a lineage called the Plethodontidae, which lack lungs. These creatures breathe through their skin, which must remain moist at all times to facilitate gas exchange, or they'll suffocate. Plethodontids are consequently extremely sensitive to changes in temperature and humidity. Even selective logging is likely to reduce a population, because it opens up the canopy and dries out the floor. Clearcutting a population's habitat is a death sentence. In the U.S. southeast, the logging of mature hardwood forest, the primary salamander habitat, is expected to overtake the hardwood growth rate by 2010. More and more of the region's rich salamander diversity is likely to end up sharing the plight of the red hills salamander (see page 18).
Deforestation- induced losses are almost certainly far greater in the tropics, although we generally know far less about them. The extreme case appears to be Sri Lanka. As recently as 1993, the island's amphibian fauna was thought to comprise only 38 species, but a recent five-year survey of the remaining rainforest turned up more than 200 additional amphibian species, which are apparently endemic to Sri Lanka (that is, they occur nowhere else). Today Sri Lanka is believed to have the world's highest amphibian diversity, in terms of the number of species per unit area. And yet that diversity is probably just a shadow of what it once was. Over the past 150 years or so, the island has lost 96 percent of its original rainforest cover. When survey researchers checked the records of naturalists who were exploring Sri Lanka before 1900, they found that more than half of the amphibians mentioned by their predecessors were no longer present. Most of Sri Lanka's surviving natural forests are legally protected, but th ey continue to dwindle in the face of illegal logging, primarily for fuelwood.
Habitat loss is thought to be the leading cause of amphibian decline, but it obviously cannot account for the "Monteverde syndrome." Places like Monteverde would seem to be about as close as it's possible to get to pristine--their habitats are intact. And yet the amphibians in these places are apparently reacting to dramatic changes. But these are changes that most of us either don't see, or that we just don't read as "unnatural."
Toxics are the usual suspects in cases of invisible damage, and there's no question that amphibians are highly vulnerable to them. Amphibians have thin, permeable skin that readily absorbs contaminants; their eggs lack protective shells and are highly permeable as well. So it's hardly surprising that in heavily industrialized areas, pollution is frequently invoked as a cause for local declines. In some centers of heavy industry, the pollution is so intense and pervasive that it's a wonder there are any amphibians left to study. Here, for example, are the types of pollution that appear to be injuring amphibians in Ukraine: heavy metals, pesticides, aromatic hydrocarbons, acid rain, and radioactive waste.
But pollution is taking a toll in healthy-looking landscapes as well. In Britain, the acidification of ponds is a major factor in the endangerment of the Natterjack toad (Bufo calamita). The...