Fifteen years ago, on night field trips to Lake Roland, Don Forester remembers hearing hundreds of upland chorus frogs celebrating spring with "almost deafening" song.
Now, their voices are silent. The frogs are gone, and nobody knows why.
"It's alarming. It's puzzling," said Dr. Forester, a biology professor at Towson State University and an authority on Maryland amphibians. "They may be telling us something we'd better listen to about our environment."
At a time when alarms are being raised worldwide about dramatic population declines, even extinctions, among certain amphibian species -- with evidence suggesting acid rain or depletion in the atmosphere's protective ozone layer as culprits -- Maryland is not an island.
Local friends of amphibians lament such disturbing signs as the disappearance of frogs and toads from a favorite little pond along Falls Road west of Ruxton, and a decline in salamanders beside a pristine spring near Sang Run in Garrett County.
They have sadly watched the march of development as it claimed woodland ponds and small spring wetlands, chasing the Eastern tiger salamander from the Western Shore and isolating some amphibian species in little pockets on the Eastern Shore.
"I've noticed that frogs in general seem to be a lot less abundant than 10, 15 years ago," said Edward Thompson, western regional ecologist for the state's Natural Heritage Program. "It's hard not to notice if you care about them."
Although none of the approximately 40 known species of amphibians in the state has disappeared, and local observers point to many like the bullfrog, American toad, spring peeper and green frog that are thriving, all are threatened by environmental change.
And several species -- including the upland chorus frog, Pseudacris triseriata feriarum -- may mirror the mysterious declines in the Western United States, Canada, Australia and the tropics reported during the past year at meetings of concerned herpetologists.
"The chronic problem for these animals is loss of habitat to development," said Jack Cover, curator of rain forest exhibits at the National Aquarium. "But even scarier is that some of these declines are in protected parks and wilderness. There's no obvious explanation."
Beginning last fall at the First World Congress of Herpetology in England, and at two later gatherings -- including an August "frog summit" in New Orleans -- herpetologists have painted a gloomy picture of population declines among dozens of species in at least 16 countries.
One of the most disturbing to scientists is the apparent disappearance of the golden toad, Bufo periglenes, a spectacular orange-skinned amphibian that lives in a protected half-square-mile of Costa Rica's pristine Monteverde Cloud Forest Preserve.
"It caught us by surprise. We probably should have had some captive populations of the golden toad. But they were so protective of it, they didn't allow any out," said Mr. Cover, who supervised the aquarium's highly successful breeding program for poison dart frogs.
The Western United States -- particularly California and the Pacific Northwest -- has experienced declines in so-called "montane" species that live in high-altitude mountainous regions.
Among those disappearing from areas where they once were abundant are leopard frogs in southern Colorado, mountain yellow-legged frogs from ponds in the High Sierras of California, the Yosemite toad, boreal toad, Cascade frog, western spotted frog and red-legged frog.
And the East Coast hasn't escaped the declines. Biologist Richard Wyman, director of the Huyck Preserve and Biological Research Station in New York state, said 20 of 30 species of amphibians surveyed recently in the Northeast were "uncommon, declining, rare or threatened" from causes other than habitat loss.
"Certainly, little bells should be going off. We feel that amphibians are early warning signs of trouble, like the canary in the coal mine," he said. "And the canary's coming up dead."
Amphibians have held their own since they emerged from the sea onto land about 400 million years ago, surviving the extinctions that killed off countless other animal species,
including the dinosaurs 65 million years ago.
But they remain tied to the water, where they are born and grow from tadpole or larvae to adulthood. Amphibians have evolved the ability to breathe through their moist skin, so they can absorb oxygen under or above water. On land, they can use human-like lungs.
"They're just a natural sponge," said Dr. Forester. "They're kind of the front-line vertebrate, right out there in contact with whatever is in the environment."
Acid rain is increasingly present in the environment, and there is evidence that it can affect amphibians in their vulnerable breeding pools. Lab research has shown that slightly higher acidity in their habitat can kill eggs or induce deformities.
Dr. Wyman said the red-backed salamander was being driven from the Catskills by acid rain, and Pennsylvania is noting similar effects. Tiger salamanders are disappearing in the Colorado Rockies, perhaps because of acid "pulses" from snow melting just as their eggs are developing.
In Maryland, Ed Thompson mentioned a "spot here in Garrett County that has one of the lowest pHs" in the United States. Acid-rain damage this year to the yellow birch, moss and lichens on Backbone Mountain in the southern part of the county was "the first time I've seen such devastation close to home."
"I guess it's almost an intuitive thing at this point," he said, "but animals so attached to the water are going to be affected sooner or later. There's only so much they can take."
A similar mix of intuition and laboratory evidence suggests that harmful ultraviolet radiation, normally screened out by Earth's atmospheric ozone layer, may kill or deform mountain-dwelling amphibians when ozone loss allows more radiation to reach the surface.
For species such as the golden toad and the high lake frogs of the American West -- mysteriously disappearing in pristine environments -- increased ultraviolet radiation is one of the few suspects, and research is under way to pin down its effects.
But herpetologists are a conservative group, and some are uncomfortable with talk of dead canaries and a "global" Armageddon for amphibians that is based largely on anecdotal evidence.
"We have an increasingly strong feeling there's a problem, but it's confusing," said David Wake, a herpetologist at the University of California at Berkeley who keeps a "frog log" of declines. "There's no identifiable pattern, and there are many areas where no declines have been noted."
Frogs, toads and salamanders are complex creatures, subject to natural boom-and-bust cycles from drought, flood and disease. Some species are so secretive or rare that scientists don't really know if populations are healthy or not.
And systematic field research on amphibians -- traditionally under-funded in favor of economically important animals such as
deer, game birds and fish -- often is the prov
ince of informed amateurs, such as members of the Maryland Herpetological Society.
The only statewide distribution survey in Maryland was published in 1975 by that nationally recognized group, led by Herbert Harris Jr., a professional photographer who has done respected fieldwork on amphibians for 30 years.
"We need to get data on a species-by-species basis, season after season," said Charles Stein, a Maryland ecologist for four decades and teacher at the Johns Hopkins School of Continuing Studies who is well-known through his environmental reports on National Public Radio.
Dr. Stein supports the prevailing view among herpetologists that such research will prove there is a real global problem, "probably from a combination of natural and man-made causes."
New surveys are being planned, internationally and in Maryland, a state that has good data on its endangered amphibians -- the Eastern tiger salamander, the green salamander, the Eastern narrow-mouth toad and the hellbender salamander -- but little on other species.
Local researchers want to know if rare amphibians like the barking tree frog, the carpenter frog and the spade-foot toad are holding their own or if the spotted salamander and ridge-and-valley salamander are declining as suspected in Maryland.
And they will listen this spring at their favorite ponds for the voices they no longer take for granted.
"They're telling us something that may be critical to our own survival on this planet," said Arnold Norden, a planner for the Maryland Department of Natural Resources.
"Whether we're smart enough to take notice of the warning in time, that's another question."