Listening for Frogs on Isle Royale: 2019 Amphibian Monitoring Results

Seven frog and toad species are known on Isle Royale, with an eighth, the northern leopard frog (Lithobates pipiens), possibly present:

Why listen to frogs

Frogs and toads have distinctive calls, so their populations can be tracked with remote recorders — "frog loggers." That matters because amphibian declines are among the world's most prominent conservation issues. Most amphibians need both land and water, so they respond to changes in temperature, rainfall and humidity, making them good indicators of climate and habitat change.

A frog logger, with microphones on top of the box and a temperature logger beneath the roof. NPS

How it works

Since 2015, Isle Royale has had ten permanent monitoring sites. From April, each recorder takes a 5-minute sample every hour from 5 p.m. to 3 a.m., plus four extra half-hour samples between 7:30 and 10:30 p.m.; a sensor logs the temperature hourly. The recordings show:

Each species also gets a Call Index Value (CIV): 1 when individual calls can be counted, with silence between; 3 for a full, continuous chorus.

2019 results

• All seven known species were recorded. American toads, green frogs and spring peepers were at almost every site.
• Wood frogs turned up at six sites — more than before, probably because recorders started earlier, when wood frogs call most.
• Gray treefrogs, mink frogs and boreal chorus frogs were at only one or two sites each. Mink frogs call rarely and are hard to confirm without a sighting, but the single one heard at site ISRO04 appears genuine.
• The northern leopard frog is still unconfirmed on the island.

First calls come later on Isle Royale than at most of the network's parks: American toads on May 26, green frogs on May 29, and the rare treefrogs as early as June 20.

Call Index Values for each species at each site, Isle Royale, 2019. Image from the National Park Service's page

Most species scored 1 or 2. Spring peepers reached a full chorus (3) at least half the time at every site; the only other species to score a 3 was the green frog, at a single site.

The recorders keep switching on each spring as frogs and toads emerge to find mates, and later data are being analyzed.

Sources

Based on "Amphibian Monitoring on Isle Royale, 2019," National Park Service, Great Lakes Inventory and Monitoring Network; a work of the United States government in the public domain.