Is That the Croak of the Pickerel? Frog Census Is a Tad Uncertain
David Dunn was standing utterly still near a dark pond, when a car pulled up. The driver asked what he was doing.
"I'm counting frogs for the government," Mr. Dunn said. The driver hit the gas. "Around here," says Mr. Dunn, a resident of rural Mississippi, "all you have to do is mention the government and people are gone."
Indeed, few people know about the late-night government project that Mr. Dunn and some 500 others have signed up for: America's annual frog-and-toad census, run by the U.S. Geological Survey.
Each spring, these volunteers stand quietly along stretches of the country's roads, listening to frogs. Their job is to estimate the abundance of the creatures by identifying each species' unique call. But now, scientists have devised a field experiment called "Ribbit Radio," which suggests the official frog-call count isn't all it's croaked up to be.
It can be harder counting frogs than people. Frogs are shy and nocturnal, with a preference for hiding in reeds or hovering under water. The best way to assess their populations isn't by seeing them, but by listening for their mating cries—from the chuckles and grunts of a wood frog to the cry of a pickerel frog, which sounds like a thumb riffling the teeth of a comb.
Volunteers of the North American Amphibian Monitoring Program, as the census is known, endure everything from the distracting sounds of cars and barking dogs to mosquitoes and chiggers. Sometimes, homeowners get anxious when they see a stranger in the dark, staring into the woods in search of frogs.
"I've had friends who were shot at," says Alvin Braswell, deputy director for operations at the North Carolina Museum of Natural Sciences and a frog counter.
A good frog census is important. Frogs have sensitive skins, so their changing population helps scientists track pollution, disease and other ecological maladies. Other research has indicated a sharp and somewhat mysterious decline in amphibians around the world, which helped spur the American census.
Yet the science of counting frogs can be rather unscientific. The census comes up with a general rating of abundance on a scale of one to three, rather than an exact number. Three represents a full chorus of constant frog calls, for instance. Scientists plan to use changes in ranking to track population shifts over time.
Sometimes frogs that are present may not get counted, because they're not in a vocal mood, or their calls are drowned out. Dr. Braswell once stood knee-deep in a North Carolina pond, tallying frogs at midnight. He was hoping to hear gopher frogs, which he knew were native to the area, but the din from the spring peepers was too much. "Shut up!" he bellowed. The peepers shut up. But no gophers croaked. "It was disappointing," he says.
And sometimes, census takers, in their eagerness to log rarer creatures, may record that they've heard frogs that simply aren't there.
"People misidentify species. Or in their enthusiasm, they simply make them up," says Ted Simons, a biologist at North Carolina State University. He devised the experiment called Ribbit Radio to test the skills of some frog-calling champions. His findings appeared in the August issue of the journal Ecology.
On a recent afternoon, Dr. Simons demonstrated Ribbit Radio. He sat under a tree and used a computer to relay recorded frog calls to 10 speakers in a field. His test subjects were two amphibian specialists, Dr. Braswell and Jeff Humphries of the North Carolina Wildlife Resources Commission. The two wrote down the frog species they thought they heard. Single frog calls were easy to identify. It got harder as calls from several species were played simultaneously and from varying distances.
The two herpetologists passed the demonstration easily enough, partly because they had done a similar test before, for the research in Ecology. But in that test, Dr. Simons included a trick to better mimic a real-world scenario, in which frog counters know all species that live in an area.
Hundreds of volunteers take part in the annual frog census. But are the counters getting it right? To test the accuracy of frog experts' skills in the field, Ted Simons developed Ribbit Radio. Gautam Naik reports.
He told the frog specialists to expect the calls of 10 species—but only played five. Several of the five experts wrote down names of "phantom" frogs whose calls weren't played.
The upshot, according to the Ecology study: Even when such phantom-call errors make up just 1% of detections, they can lead to a big overestimation of certain frog populations. Dr. Simons hopes the Ribbit Radio study will persuade the USGS to note such uncertainties in data, and offer more training.
Linda Weir, coordinator of the frog survey, was one of the participants in the Ecology study. She acknowledges the limitations of the census. "Ted's work has shown us that we need to look at" such errors more closely, she says.
Ribbit Radio was inspired by a similar experiment called Bird Radio, devised by Dr. Simons (who's also an ornithologist) and a friend, John Wettroth, who built the electronics for both projects. Bird Radio suggested identification errors also plague bird counts, which are also based on calls.
The USGS, which studies landscapes and natural resources, started the frog survey in 2001. The survey isn't a truly national sample. Twenty-two states in the eastern half of the country contribute data. Each volunteer is assigned a 15-mile route, with 10 sites, which have to be surveyed three times each year. At each site, the volunteer listens for five minutes.
Anyone can become a frog surveyor, provided they pass an online test to identify recorded frog calls. Two-thirds of testers succeed on the first go; the rest need two or more tries.
Last winter, Mr. Dunn, the Mississippi volunteer, bought a CD of frog sounds to practice. "I thought it would be a cakewalk," says Mr. Dunn, a retired history teacher, who scored only 35 out of 100 in his first attempt. He studied harder and passed the second time.
In 2006, another Mississippi frog counter, Nick Gault, heard an unusual sound on his first night out and feared he wasn't alone. "From my many zoo visits," he wrote in his account to the census, "I knew it was the low rumble of an African lion."
He crept back to his car. It took him a few minutes to realize the roar had erupted from a nearby animal sanctuary that housed large African cats.
In 2001, Ms. Weir was doing a survey in Maryland, when a police car rolled up. "I told the policeman I was listening to frogs, and he gave me a strange look," she recalls. "I was probably the talk of the station—the crazy lady who listens to frogs."
WSJ
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