Where have all the amphibians gone?
Census-taking by ear: listening for frogs on a warm spring night.
On a warm, late spring night, Joe Martinez stands on the side of a back road in the Blue Hills, 25 miles south of Boston. He waits patiently, clipboard in hand, as he listens closely for a frog call to break the wooded darkness.
Other all-too-familiar sounds engulf the air instead: Cars roar by with music spilling out, and airplanes fly overhead, stifling the subtler sounds of nature.
Dr. Martinez, a herpetologist involved in science education, and monitoring partner David Deitz, director of a medical software company, are out surveying frog and toad calls for the volunteer-based North American Amphibian Monitoring Program (NAAMP).
Martinez steps back into the car with Dr. Deitz. "Believe it or not, in the past we've heard frogs at that site," says Deitz as he drives half a mile to the next monitoring site, the second of 10.
The situation there and in all the sites that followed is the same: not one humble croak in the nocturnal air.
According to Martinez, they are surveying under some of the worst conditions. The lack of rain over the past month is probably one of the main reasons they have not heard any calls. Frogs are more likely to be out calling when it is warm and wet.
This was the first time in three years that they have been "completely shut out," Deitz says.
Over the past 50 years, many populations of the world's 5,000 species of amphibians (frogs, toads, salamanders, and newts) have been declining or completely disappearing.
In 1991, more than 3,000 scientists and conservationists in more than 90 countries joined the Declining Amphibian Population Task Force (DAPTF) in an effort to understand why amphibians are disappearing.
The monitoring program, which is sponsored by the United States Geological Survey (USGS) and is part of the task force, relies on volunteers to gather long-term amphibian data across Canada, the US, and soon, Mexico.
Scientists have long considered amphibians as early indicators of environmental problems because they live on land and in water. Most amphibians have permeable skin, which allows substances to be absorbed and concentrate in their bodies.
Declines have been attributed to a variety of causes, including habitat destruction, chemical contamination, changes in the ozone layer, acid rain, parasites, and viruses.
Yet not all amphibians are in decline, and within a single location, some species may be in decline while others are stable.
"A real hard thing with amphibian populations is trying to differentiate between natural population fluctuations and real trends," says Scott Jackson, regional coordinator of the Massachusetts Amphibian Calling Survey. "Amphibian populations tend to fluctuate a lot. So the best thing to do is collect data over a wide area and over a long period of time."
Most scientists agree that although declines are evident across the globe, there is no single factor responsible.
"The thing we have to be careful about with amphibians is making blanket statements," says Michael Lannoo, professor of biology at Ball State University in Muncie, Ind., and coordinator for the US task force group. "We lack the long-term research data to make the type of conclusions we want to. It takes a lot of people and a lot of time."
According to Sam Droege, biologist and head of monitoring development for the USGS at the Patuxent Wildlife Research Center in Maryland, the scientific design of the frog-calling survey is quite rigorous. Researchers at the USGS at Patuxent help set up monitoring programs in each state, acting as advisers and consultants.
Across the US, volunteers receive training in how to identify each calling frog species in the area, how to conduct a frog-call survey, and the protocol behind the survey technique. Volunteers are then assigned a calling route in their area. Volunteers collect data on temperature, wind, cloud cover, and rainfall. They listen for frog calls and then write down which species they hear and the intensity. The volunteers then forward the data to the USGS at Patuxent, where they are compiled and analyzed.
"It will take at least 10 years to extract some idea of long-term changes in order for it to be statistically valid," Dr. Droege says.
In February, the USGS launched Frogwatch USA, a less scientifically rigorous program than NAAMP, geared toward monitoring frogs and toads at specific ponds or wetland sites. "Even a person's backyard can become an important site for information," says Droege. "The primary reason why we do Frogwatch is that we get data for one particular pond."
"With monitoring, you can't go back in time. It's not reproducible," he says. "But with Frogwatch, you can always go back and see how things change over time."
Some scientists believe deformities could also be contributing to the decline. Over the past five years, frogs in Wisconsin, Minnesota, and Vermont have been found with missing or extra limbs and eyes.
Dr. Lannoo points out, however, that deformities have been known to occur in pre-industrial times, suggesting that there is a natural fluctuation. At the same time, Lannoo questions the reliability of volunteer data and the scientific validity of the calling survey project, arguing that scientists need something more tangible than "I heard a call." "Amphibian research has to be a combination of education and hard science," he says.
Still, Droege says, "It's a tremendous way to get people away from their television sets and of breaking the myth that 'scientists are the only ones that can do this and I can't.' "