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.