“Sometimes we wonder just what the heck people think we’re doing,” said Pat Hartman, as he strained to hear frogs croaking in the distance while cars zoomed by on the road behind him. Chris Brueske, his survey partner, determined their location with a Global Positioning System.
After a minute, Hartman’s watch beeped, signaling the end of their listening period. The men jumped into action, filling out a log sheet noting the types of frogs they heard and their estimates of the relative density of the frog populations.
|Volunteer Pat Hartman logs frog calls.|
Hartman and Brueske were in Lebanon Hills Regional Park in Eagan in mid-April as volunteer monitors for the Minnesota Frog and Toad Calling Survey.
Along with hundreds of other volunteers across the state, they were helping the Department of Natural Resources track the health of frog populations and the ecosystems the frogs inhabit.
Worldwide, amphibian populations are in trouble. According to the International Union for Conservation of Nature’s “red list,” nearly half of the world’s species of frogs and toads are threatened. Habitat loss, disease and climate change are factors in the decline.
“Frogs are great indicators of the health of wetlands, and of water quality,” said Krista Larson, who coordinates the survey for the DNR Nongame Wildlife Program. So the DNR came to believe that monitoring amphibian populations could prove a valuable gauge of the health, not only of frogs, but also of the state’s wetlands.
The survey hasn’t yielded any conclusive results on either frogs or wetlands – yet.
Survey data collected since 1997 indicate that spring peepers, as well as tree frogs, seem to have declined as much as 10% statewide, according to Larson. The American bullfrog, on the other hand, has become more common. Once found mainly in southeastern Minnesota, bullfrogs now live in much of the state.
Populations of the other 11 species of frogs and toads found in Minnesota appear to be holding steady.
Prior to 1994, the DNR did not monitor frog and toad populations.
“It was a big gaping hole,” said Larson. An explosion in the number of deformed frogs sighted in Minnesota prompted the DNR to copy a population survey model already in use in Wisconsin.
The Frog and Toad Survey makes no attempt to put a number on frog populations, a near impossibility given the elusiveness of the creatures. Brueske and Hartman didn’t sight a single frog during their April stops in Lebanon Hills, and they have rarely done so over the years. They were, however, barraged with frog calls at each of the 10 listening stops on their route.
At each stop, they ranked the presence of each species they heard — based on the volume of those calls — on a 0-3 scale. Zero being none, 1 – individuals can be counted with space between calls, 2 – individuals can be distinguished but there is some overlapping of calls, and 3 – calls are constant, continuous and overlapping.
For example, chorus frogs were a 3 at every stop on Brueske’s and Hartman’s run, and wood frogs were an intermittent 1, audible only at some stops. They identified no other species.
Recording the volume of amphibian voices at pre-determined spots repeatedly “is a way of measuring change over time,” said Larson. Volunteers are asked to travel their assigned route three times each year. The first is done in the spring when water temps rise above 50 degrees, the second in early to mid-summer when waters are generally above 60, and the final run later in summer when lakes and wetlands reach 70 degrees.
Frog calls are mainly produced by males attempting to attract females. Most species do so by inflating their vocal sacs, forcing air over their vocal chords.
Friends since they attended St. John’s University a decade ago, Hartman and Brueske have been listening to frog and toad calls for four years. They circle Lebanon Hills on a pre-designated two-hour route that takes them from a lakeshore to deep in the midst of a housing development, to a forested dirt road and finally to a highway overpass.
Hartman, a biology teacher at Mahtomedi Learning Center, and Brueske, who works for the Minnesota Department of Health and holds a degree in chemistry, do their best to assure the validity of their results.
Before volunteers can have their data logged in the official DNR record, they must pass an online test in which they listen to audio recordings of multiple frog species and attempt to identify their calls.
“Ninety-two percent this year,” Brueske said, half bragging, half joking.
Volunteers are only required to know the frog calls relevant to their part of the state. This process makes it fairly easy to flag outliers. On that night in Lebanon Hills, Brueske and Hartman detected only two relatively common species, chorus frogs and wood frogs.
“We know they know their calls,” Larson said of the volunteers who make the survey possible. “And they really want to help, so they do study – every once in a while, I get .mp3 clips that people have recorded, asking me what frog this could be.”
True to form, Brueske and Hartman carried the official DNR frog call CD in the car with them. To make sure they were hearing wood frogs, which have become rarer on the early spring run, they listened intently to track after track of frogs recorded in the wild.
Of all the calls, Hartman said, the American bullfrog (an invasive species in most of Minnesota) is the most distinctive. “It sounds like a slow lightsaber,” he said. Wood frogs sound a bit like a duck quacking, and spring peepers sound almost alien, high pitched and far-away.
Brueske’s and Hartman’s Lebanon Hills route is one of the original Twin Cities Metro area routes designated by volunteers in the early 1990s. Most of the routes statewide were randomly assigned via a computer model in 1998.
The placement model, a result of a partnership with the National Amphibian Monitoring Program and the United States Geological Survey, scattered approximately 250 dots on a map of the state, according to potential volunteer density. The participants then fleshed out the routes, and the routes have been run consistently, with as little change as possible, for the last 11 years.
The DNR publishes a year-end report based on survey data, available at the survey’s home page: www.dnr.state.mn.us/volunteering/frogtoad_survey/index.html. But due to the relative infancy of the survey, data analysis has been fairly minimal.
The annual report lists species trends as rising, falling, or stable, but it is done as a single, statewide analysis. Larson hopes to do more with the data in coming years, including analyzing populations by region.
She believes, for example, that spring peepers, once common in the Twin Cities, seldom are heard by volunteers now. The most certain trend in the data, the increase and spread of American bullfrogs, comes from bullfrogs being sold as pets and bullfrog tadpoles being sold as fishing bait, according to Larson.
But certainty in the apparent decline in spring peepers and tree frogs is proving to be more difficult. “It looks like a 10 percent drop, but it’s completely dependent on weather, and we’ve had some odd springs,” Larson said.
A more average spring this year, in terms of both temperature and precipitation, could show both species returning to previous levels Larson said. Or this year’s data, she said, could shed more light on whether Minnesota is “seeing a real, biological decline” in amphibian species.