Listening for Frogs on the Upper Mississippi River, 2019

Why listen to frogs

Frogs and toads are unusual among amphibians: each species has its own identifiable call. That makes it possible to monitor them with remote recorders, or "frog loggers."

Monitoring matters because amphibian declines are among the most prominent conservation issues in the world. Most amphibians need both land and water in their lives, which makes them sensitive to changes in temperature, precipitation and humidity — and so indicators of climate and habitat change.

Nine frog and toad species are known on the upper Mississippi River:

A "frog logger." Its microphones come out of the top left and right of the box; a temperature logger is attached underneath. NPS.

How the recorders work

The National Park Service's Great Lakes Network set up ten permanent monitoring sites in 2015 in Mississippi National River and Recreation Area (MISS). From April, a recorder at each site takes 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 every hour.

The recordings show:

• Occupancy — whether a species is present at a site.
• Abundance — how the loudest calling at a site changes from year to year, and how often a species is heard.
• Phenology — the dates each species first calls each year, tracked since 2018 for three species common to all the network's parks: Eastern American Toad, Green Frog and the treefrogs. Gray and Cope's gray treefrogs sound so alike, even on recordings, that they are combined.

A sub-sample of recordings is scored with a Call Index Value (CIV), the maximum calling intensity: 1 when calls do not overlap and individual males can be counted, up to 3 when calls are too many to count. Followed over time, CIVs show whether a species is increasing or decreasing.

What they heard in 2019

Eight of the nine species were found. The most widespread were the Eastern American Toad, Gray Treefrog, Green Frog and Northern Leopard Frog; rarer were the Wood Frog (two sites), Cope's Gray Treefrog (three) and Boreal Chorus Frog (three).

Blanchard's cricket frog was heard at just one site — but among the network's parks, the Mississippi River is the only place it is found. That has drawn others, including the Minnesota Department of Natural Resources, to look for it, and surveys beyond the monitoring program have found it at several more sites along the river.

The spring peeper was absent again, as in every earlier year. The American bullfrog has not been detected at a monitoring site but has been confirmed at Pigs Eye Lake; the Mississippi River and Indiana Dunes are the only network parks where bullfrogs are known.

Calling starts earlier here than in most of the network's parks. In 2019 the Eastern American Toad called first, on April 23. Treefrogs (both species) were heard in mid-May, the earliest on May 4, and Green Frogs from late May.

Every species reached CIV 3 at one site or more, except Blanchard's cricket frog (at most 2) and the wood frog (at most 1).

Call Index Values for each species at each site, Mississippi National River and Recreation Area, 2019 — from 1 (individuals can be counted, with silence between calls) to 3 (a full, continuous, overlapping chorus). NPS.

What's next

The recorders switch on each spring to capture frogs and toads emerging from winter and calling for mates. Data since 2019 are being analyzed, with regular updates to come. More: the network's amphibian monitoring and a PDF of this report.

Sources

Based on "Amphibian Monitoring on the Upper Mississippi River," Great Lakes Network, National Park Service; a work of the United States government in the public domain. A photograph of a Blanchard's cricket frog credited to an individual photographer is not reproduced.