Listening for Frogs at Sleeping Bear Dunes, 2019

Frogs and toads are unusual among amphibians in making calls that are distinctive and identifiable, so their populations can be monitored with remote recorders, known as "frog loggers." Monitoring matters because declining amphibian populations are one of the most prominent conservation problems in the world. Most amphibians need both land and water during their lives, which makes them sensitive to changes in temperature, precipitation and humidity, and good indicators of climate and habitat change.

Six frog and toad species are known at Sleeping Bear Dunes National Lakeshore:

How the frog loggers work

An automated recorder, or "frog logger." Microphones come out of the top left and right of the box; a temperature logger hangs nearby. National Park Service.

The Park Service's Great Lakes Inventory and Monitoring Network set up ten permanent amphibian monitoring sites in the park in 2013. Starting each April, a recorder at every site captures a 5-minute sample once an hour from 5 p.m. to 3 a.m., plus four extra samples on the half hour between 7:30 and 10:30 p.m. A sensor logs the temperature hourly.

The recordings show:

• Occupancy, whether a species is present at a site;
• Abundance, tracked by how the loudest calling at a site changes from year to year and how often a species is heard;
• Phenology, the date each species first calls each year, tracked since 2018 for three species found in every park in the network: the Eastern American toad, the green frog and the treefrogs. The gray treefrog and Cope's gray treefrog sound so alike, even on recordings, that their data are combined.

A sample of the recordings is used to give each species a Call Index Value (CIV), a measure of the most intense calling heard: 1 when individual calls do not overlap and the few calling males can be counted, up to 3 when there are too many calls to count individuals. Tracking CIVs over time shows whether a species is increasing or decreasing.

What the recorders heard in 2019

• Species: five of the six species were recorded. The Eastern American toad, gray treefrog, green frog and spring peeper turned up at almost every site, and the wood frog was found at one new site. The northern leopard frog was not heard at all, though it had been detected at five sites in 2018.
• First calls: the Eastern American toad started calling first, in early May, followed by the gray treefrog in mid-May and, days later, the first green frogs.
• Call intensity: the Eastern American toad reached a full chorus (CIV 3) at one site, and every other species did so at more than one. Spring peepers reached a full chorus at every site where they were heard, and in more than half the recordings at nine of those ten sites.

Call Index Values for each species at each site, 2019, from 1 (individuals can be counted, with silence between calls) to 3 (a full, continuous, overlapping chorus). National Park Service.

The recorders switch on every spring to capture frogs and toads emerging from winter and calling for mates. Data collected since 2019 are being analyzed, and the network plans regular updates on the park's amphibians.

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network, "Amphibian Monitoring at Sleeping Bear Dunes, 2019." https://www.nps.gov/articles/000/amphibian-monitoring-at-sleeping-bear-dunes.htm
• PDF of the article: https://irma.nps.gov/DataStore/Reference/Profile/2301979; network amphibian monitoring: https://www.nps.gov/im/glkn/amphibians.htm
• The Park Service's page spells the spring peeper's genus "Psudacris"; the correct spelling is Pseudacris. A green frog photograph by Gary Casper on that page is not reproduced here.