ST. PAUL — From a shrill chirp to a banjo-like strum, frog and toad calls are much more nuanced than the “ribbit” sound many children are taught.

For years, Maplewood volunteers have listened and documented the variety of calls made by Minnesota’s 14 species.

But organized listening to the amphibians is decreasing.

The state Department of Natural Resources discontinued its 23-year-old frog- and toad-calling survey in 2017. Maplewood volunteers refuse to quit, however, and are continuing to eavesdrop on the animals.

In 2006, naturalist Carole Gernes surveyed Maplewood’s wetlands, lakes and ponds. Gernes developed the four frog-monitor routes that volunteers use today. Locations on each route range from back yards to the edges of wetlands.

Volunteers continue to monitor Maplewood’s frogs and toads, as they have done for 12 years.

Claire and Mike Rohweder are among the volunteers this season. The Rohweders became Minnesota Master Naturalists in 2013. They then volunteered at the Maplewood Nature Center, where they were introduced to the monitoring program.

“We jumped on board with that,” Mike Rohweder said. “It sounded kind of fun.”

The center conducts an annual refresher course to go over species that volunteers will hear and quiz them on their knowledge. There’s also an online resource for newcomers to familiarize themselves with each call.

“I really like carefully observing the world around me, but I had no idea what a frog sounded like,” Claire Rohweder told the St. Paul Pioneer Press.

After finishing her fifth season of monitoring, she now tells her nephews which frog species they’ve found based on the recordings they play for her.

Volunteers must document the air temperature, wind speed, moon visibility, passing vehicles and distracting noises on the route.

At each of the 10 stops, volunteers must listen for five minutes, and record the frequency of the calls. If wind speeds reach 12 mph or rain is in the forecast, no surveys are conducted.

This year, there were three periods when volunteers could conduct listening sessions: April 15-30, May 15-June 5 and June 25-July 10.

Local volunteers submit data to the Nature Center, which is then studied by researchers.

This data aids efforts to research the city’s water quality.

For example, researchers noticed that frog calls in Knucklehead Lake were dropping. After that warning, the city installed a shoreline buffer to improve water quality in 2006.

In another instance, volunteer data detected an issue at a pond near John Glenn Middle School.

Mowing at the water’s edge caused grass clippings to wash into the water, decompose, deplete oxygen and boost algae growth.

Once the mowing ceased, native vegetation grew back and acted as a buffer to prevent soil, leaves and grass clippings from washing into the pond. It was a victory for the pond.

According to Maplewood naturalist Gernes, the water went from “pea soup without a peep” to having plenty of frogs again.