Listening for Frogs and Toads in the Apostle Islands, 2014–2019

Eight kinds of frogs and toads live in Apostle Islands National Lakeshore:

Two more — the boreal chorus frog (Pseudacris maculata) and the American bullfrog (Lithobates catesbeianus) — live outside the park but appear to be absent inside it.

Why listen to frogs

Among amphibians, frogs and toads stand out for their calls, each distinct and identifiable by species, so their populations can be monitored with remote recorders. It matters because amphibian declines are among the world's most prominent conservation problems. Most amphibians need both land and water in their lives, which makes them sensitive to temperature, rain and humidity — and so good indicators of climate and habitat change.

Frog loggers

A "frog logger". Its microphones come out of the top left and right of the box; a temperature logger hangs under the board's roof. NPS photo.

Ten permanent monitoring sites were set up in 2014. From April, the recorder at each site takes a 5-minute sample every hour from 5:00 p.m. to 3:00 a.m., plus four extra samples on the half hour between 7:30 and 10:30 p.m. A sensor logs the temperature each hour.

The recordings show:

• Occupancy — whether a species is at a site at all.
• Abundance — tracked through how the strongest calling at each site, and how often a species is heard, change from year to year.
• Phenology — added in 2018: the date each species first calls every year. It is tracked only for three groups common to all the network's parks: Eastern American toads, green frogs and treefrogs. Gray and Cope's gray treefrogs sound so alike, even on recordings, that they are counted together.

From a sub-sample of the recordings, each species gets a Call Index Value (CIV) for its strongest calling: 1 means calls that do not overlap — few males singing — and 3 means calls too numerous to count individuals. Followed over time, CIVs show whether a species is increasing or decreasing.

What the recorders heard in 2019

• Six of the eight species were recorded. Green frogs and spring peepers were at every site.
• Mink frogs were heard only at the Stockton and Michigan Island lagoons, though they are known on Long Island and at the Sand River estuary.
• No Cope's gray treefrogs were heard, and no northern leopard frogs were detected — though the leopard frog is known on Long Island, with unconfirmed reports from Bear, Devils, Stockton and York Islands.
• American bullfrog, boreal chorus frog and pickerel frog were not detected; none is known in the park, despite several unconfirmed reports.

Calling season. Frogs start calling earlier at the network's southern parks and later at the northern ones; the Apostle Islands fall in the middle. Eastern American toads were first, trilling on 15 May at the two mainland sites. Green frogs followed in the last week of May on Stockton, Sand (west) and Basswood Islands, and the first treefrogs joined in the second week of June at the mainland sites, Presque Isle lagoon and Sand Island (west).

Chorus strength. Several sites and species never passed a CIV of 1 — few calling males, as with Eastern American toads and gray treefrogs at both Stockton Island sites. By contrast, green frogs on Outer Island and spring peepers at most sites reached 3, a sign of large populations.

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). Image from the National Park Service's page.

What's next

The recorders switch on every spring. Data since 2019 are being analysed, and the park plans regular updates on its amphibians.

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network, "Amphibian Monitoring in the Apostle Islands, 2014–2019." https://www.nps.gov/articles/000/amphibian-monitoring-in-the-apostle-islands-2014-2019.htm
• Amphibian monitoring in Great Lakes Network parks: https://www.nps.gov/im/glkn/amphibians.htm · the article as a PDF: https://irma.nps.gov/DataStore/Reference/Profile/2301973
• The original's photo of an Eastern American toad on Hermit Island is credited to a named photographer, not the NPS, and is not reproduced here.
• Rewritten in hubnx's own words.