Listening for Frogs and Toads at Indiana Dunes, 2019

Frogs and toads are the amphibians that call, and each species' call can be told apart from the others. That lets the National Park Service's Great Lakes Network follow their populations with recorders left in the field, known as "frog loggers." The work matters because amphibian declines are among the world's most prominent conservation problems. Most amphibians depend on both water and land over their lives, which leaves them sensitive to changes in temperature, rainfall and humidity and makes them indicators of shifts in climate and habitat.

The nine species

Indiana Dunes National Park has nine known species of frogs and toads:

Indiana Dunes is one of just two parks in the network where the American Bullfrog lives.

How the recorders work

A frog logger. Its microphones stick out from the top left and right of the box, and a temperature logger sits in the shaded cup below. National Park Service photo.

The park's ten permanent monitoring sites were set up in 2013. Each April, the recorder at every site starts taking a 5-minute sample once an hour from 5:00 p.m. to 3:00 a.m. It also takes four extra samples in the evening, on the half hour between 7:30 and 10:30 p.m. A sensor logs the temperature every hour.

The recordings answer two questions:

• Occupancy — whether a species is present at a site at all.
• Abundance — followed through changes in a species' peak calling intensity at each site from year to year, and through how often it is heard.

In 2018 the network added phenology, the date each species first calls each year and how that date shifts over time. So far it follows only the species common to every park in the network: Eastern American Toads, Green Frogs and treefrogs. Gray Treefrogs and Cope's Gray Treefrogs sound so alike that even recordings struggle to separate them, so where both occur their data are pooled for this purpose.

A subset of recordings is scored with a Call Index Value (CIV), the highest calling intensity heard for each species. It runs from 1, when calls do not overlap and there are few singing males, to 3, when calls are too many to count individuals. Following CIVs over the years can show whether a species is becoming more or less abundant.

What 2019 showed

The recorders picked up eight of the park's nine species in 2019. Eastern American Toads, Gray Treefrogs, Green Frogs and Spring Peepers turned up at every site sampled, and Boreal Chorus Frogs at all but one.

Timing. Frogs start calling earlier at Indiana Dunes than at any other park in the network. Eastern American Toads began at three sites in mid-April and at the others a few days later, when the first treefrogs started up. Treefrog calling arrived in three waves: around 20 April at three sites, around 4–6 May at three others, and on 16 and 21 May at the remaining two. Green Frogs were first heard on 15 May, and every site had recorded them by 22 May.

Intensity. Across all sites, over half of the Spring Peeper recordings scored CIV 3. For American Toads, every recording at four sites scored CIV 1, as did more than 50% of recordings at the other four sites where they were heard.

Call Index Values for each species at each site, Indiana Dunes National Park, 2019. A value of 1 means individuals can be counted, with silence between calls; 3 means a full chorus, with constant, overlapping calls. Image from the National Park Service's page.

Continuing

The recorders still switch on every spring to catch frogs and toads emerging from winter and calling for mates. Data gathered since 2019 are being analyzed, and the network plans regular updates on the park's amphibians. More on the network's amphibian monitoring is at nps.gov/im/glkn/amphibians.htm.

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network: "Amphibian Monitoring at Indiana Dunes, 2019." The source spells the Spring Peeper's genus Psudacris; it is corrected to Pseudacris here. Its photo of an American Bullfrog, credited to a named photographer, is not reproduced.