The Chorus of Frogs

Frogs Are Excellent Indicators of Environmental Quality

My field season begins with the chorus of the frogs.

While male frogs have been calling for a few weeks in the lower Hudson Valley, up here in the St. Lawrence Valley they have just begun. Wood frogs, boreal chorus frogs, spring peepers, and northern leopard frogs are the first to breed. As the season progresses, male pickerel frogs, American toads, grey tree frogs, green frogs, and bullfrogs in turn will chorus, until lastly in late June the mink frogs begin to broadcast their distinctive staccato chatter.

My students and I visit a set of twenty wetlands each summer to record data on which frog species are calling, and approximately how many of each species are in the chorus. We use a standard protocol designed by the North American Amphibian Monitoring Program, or NAAMP ( ). Volunteers in the eastern and central United States have been visiting assigned wetlands for a decade or more at least three times a year, using the NAAMP protocol to record the variety and abundance of calling frogs and toads.

Why are my students, and so many NAAMP volunteers, willing to endure the inconvenience of standing near a dank, mosquito-infested wetland for five minutes, listening to frogs? Certainly the sound of a full frog chorus is a thrilling, unforgettable experience. However, the scientific reason is more practical: frogs are excellent indicators of environmental quality. North American frogs all breed in water, and their eggs and larvae develop in an aquatic environment. Any contaminants or pathogens in water readily pass through the sensitive amphibian skin. Thus, the diversity and abundance of frogs at a water-body can indicate something about its water quality. Frogs are affected by salt and other contaminants from roads, and by fertilizer and pesticide runoff from lawns and farms. These amphibians, because they migrate among wetlands or between wetlands and upland habitat, are also affected by roads and road traffic, and by changes in land use in the surrounding landscape that affect habitat quality and accessibility.

Thus, because frogs are easy to detect and identify by their calls, and because frogs are sensitive to many different kinds of environmental stresses, and because frog species differ in how sensitive they are to these stressors, quick and inexpensive frog call surveys provide a valid way to assess environmental health, and to detect changes in health over time. I use frog call surveys as one tool for evaluating whether restored wetlands function as well as natural wetlands, and whether wetlands within a Great Lakes Area of Concern (the Massena AOC ) are comparable to wetlands outside of the AOC.

I am part of a team that has been analyzing the entire NAAMP data to ask what kinds of human stressors affect frog species diversity and abundance. Each team-member analyzes a set of NAAMP data with his or her university class, then we meet annually at the National Center for Ecological Assessment and Synthesis (NCEAS ) in Santa Barbara, California to put all of the data together and search for patterns. So far, we have found good evidence that roads and road traffic have a negative impact on frogs throughout the eastern US.