Restoration projects have triggered more than one metamorphosis along the Stony Run stream in Baltimore. The latest one—wriggling, hopping and croaking—has been a special source of delight. After a long absence, frogs are making a comeback.

Stony Run flows into the Jones Falls and ultimately the Chesapeake Bay. Until recently, it had little habitat to attract or sustain a frog population. Like many urban streams, its once meandering path had been gouged by stormwater runoff into a straight chute with high banks.

“It was a big, incised channel, with no pools or riffles,” said Eric Schott, a professional biologist and member of the Jones Falls Watershed Association.

That changed earlier this year when the city of Baltimore led an aggressive stream makeover that restored the channel to a more natural shape and flow.

“Throughout the summer, people told us they heard frogs, and that was encouraging,” Schott said. “Then we started to see them jump in as we walked past. I found some extremely large bullfrog tadpoles, too.”

Reports of frogs in Stony Run got attention because few, if any, had appeared in the urban stream for at least five years. The frogs, biologists say, serve as a living indictor of restoration success.

“I think we’re seeing frogs in the stream now because the physical habitat is more suitable,” Schott said.

According to a 2006 report by the U.S. Geological Survey, habitat loss, caused by the removal and degradation of wetlands and forests, is the leading threat to frog populations in the United States. But frogs, and amphibians in general, have declined worldwide because of a host of pressures that also include water pollution, climate change and disease.

Scientists are concerned about the decline because amphibians are one of the first species to feel the direct impact of environmental problems.

“Their skin is very porous,” said naturalist David Mizejewski of the National Wildlife Federation. “An exchange of liquids and gases occurs right through their skin.”

This makes amphibians very susceptible to absorbing toxins in the environment or reacting to changes in their surroundings. When amphibians are deformed or are dying in large numbers, they provide an early warning of problems that may later effect the broader ecosystem.

The global decline of amphibians became a concern approximately 15 years ago. Since then, scientists have worked across national boundaries to document the change and research its causes. The Global Amphibian Assessment unites the efforts of 600 scientists from 60 countries, coordinated by the World Conservation Union, Conservation International and NatureServe. In the United States, many universities and federal agencies have also increased their research efforts.

There’s a role for citizens, too. Frogwatch USA, managed by the National Wildlife Federation, relies on “citizen scientists,” volunteers who collect and submit information about local frog populations to a central database.

“We have a limited number of scientists, but lots of interested people who can help make observations that are useful to scientists,” Mizejewski said.

The program was launched in 1998 in partnership with the U.S. Geological Survey. Since then, more than 1,395 volunteers have monitored 1,942 sites in 49 states.

Frog watchers monitor local sites during the frog breeding season. In the Chesapeake watershed, the breeding season may last from February through August, depending on the location. Volunteers choose the site and decide how often to visit. Twice a week is recommended.

As was the case along Stony Run, the most important piece of equipment for detecting frogs is often the ear. About 30 minutes after sundown, Frog watchers eavesdrop for exactly three minutes on the breeding songs of their local frogs.

“They have pretty distinct calls,” Mizejewski said. “The bullfrog has a deep, throaty call. The green frog sounds like the plucking of a banjo string, and spring peepers have a really high-pitched chorus that’s almost deafening. Wood frogs are one of my favorites. They sound like a bunch of ducks calling.”

The Frogwatch USA web site provides online recordings that teach volunteers how to recognize the breeding calls of different species. Volunteers in the Chesapeake region are most likely to hear green frogs, wood frogs, bullfrogs, spring peepers and American toads, as well as leopard frogs, pickerel frogs and cricket frogs.

Volunteers make note of the species they hear and use a simple scale to estimate their numbers. They also record information about temperature, wind and rainfall. Then they submit the information online or send it to the program coordinator by mail.

The data helps to track the ranges and locations of individual species of frogs and toads and will eventually help to assess population changes. “It’s also a great way to get a ‘green hour’ into your day and encourage a moment of stillness,” Mizejewski said. “That doesn’t happen too often anymore.”

Susan Muller of the Howard County, MD, Parks & Recreation Department agrees. Muller coordinates a team of 100 volunteers who monitor 35 sites on county-owned land. Muller finds that Frogwatch attracts retired people, families and students earning community service credits. The volunteers not only collect valuable data but also build stronger ties to their local environment.

“They get very possessive about their sites,” Muller said. “They’ll say, ‘I’ve seen people fishing for turtles in the pond. Should they be doing that?’ or ‘There’s lots of trash out here. Do you know anyone who could help with a cleanup?’”

Muller trains her volunteers by using Frogwatch materials and leading field trips to practice identifying the calls. They soon become active stewards with sharp ears and smart questions.

“People should be concerned if they have a pond or wetland behind their house and aren’t hearing something,” Muller said. “They should want to know why.”

To hear the breeding calls of frogs in your state, or to learn more about Frogwatch USA, visit www.nwf.org/frogwatchusa/.