What the frogs said: listening to amphibians in the Great Lakes national parks

Spring's peeping frogs are loud enough to cut through a car window, and in seven national park units around the Great Lakes, the National Park Service listens to them with recorders. This update on the Great Lakes Network's amphibian monitoring explains why toads and frogs make good monitors, how the recorders work, and what they heard in 2019, the data year the update reports.

Boreal chorus frogs can be found in splash pools on the rocky shoreline of Isle Royale National Park. NPS/T. Gostomski

Why listen to frogs

Toads and frogs have unique calls, so remote recorders, or "frog loggers", can monitor their populations. The top reason the network parks did so is that amphibian declines are one of the most prominent conservation issues in the world. Most amphibians need both land and water in their lives, so they react to changes in temperature, precipitation and humidity and signal climate and habitat change.

The recordings answer two questions. Occupancy is whether a site holds a species at all. Abundance is tracked by how the maximum call intensity changes across years and by how often a species is heard. From 2018 the network also began tracking phenology, the trend in each species' first-calling date. For now it does that for three species found in every park: the Eastern American Toad, the Green Frog and the treefrogs. Gray Treefrog and Cope's Gray Treefrog sound so alike that even recordings struggle to separate them, so their data are combined for phenology.

How the recorders work

Recorders are in seven of the network's nine parks; St. Croix Riverway and Voyageurs take part in a separate program with a different protocol. They switch on in spring, typically mid-April, and record for five minutes at the top of every hour from 5:00 p.m. to 3:00 a.m. each day, plus three more samples on the half hour in the prime evening calling period. A sub-sample of recordings from each park is scored with a Call Index Value, a measure of calling intensity for each species. Over time those values show whether a species is increasing or decreasing.

An automated recorder, or frog logger: microphones come out of the top corners and a temperature logger is attached underneath. NPS

Who lives where

The table shows each species' status in each park: A absent, E extirpated, P present, U unconfirmed. Parks: APIS Apostle Islands, GRPO Grand Portage, INDU Indiana Dunes, ISRO Isle Royale, MISS Mississippi River, PIRO Pictured Rocks, SLBE Sleeping Bear Dunes.

What was heard in 2019

Chart from the National Park Service: number of frog and toad species heard in audio recordings at each park, 2019 (St. Croix Riverway and Voyageurs data not shown).

• Apostle Islands: six of eight species recorded; Green Frog and Spring Peeper at every site.
• Grand Portage: five of eight; Green Frog and Spring Peeper at every site.
• Indiana Dunes: eight of nine; Eastern American Toad, Gray Treefrog, Green Frog and Spring Peeper at every site.
• Isle Royale: all seven; Eastern American Toad, Green Frog and Spring Peeper at almost every site.
• Mississippi River: eight of nine; most widespread were Eastern American Toad, Gray Treefrog, Green Frog and Northern Leopard Frog.
• Pictured Rocks and Sleeping Bear Dunes: five of six at each; Eastern American Toad, Gray Treefrog, Green Frog and Spring Peeper at almost every site.

Species that are hard to confirm

The methods are sufficient for every species in the parks, but four are hard to confirm: American Bullfrog, Cope's Gray Treefrog, Mink Frog and Northern Leopard Frog. Ambient noise can mask the bullfrog, Mink Frog and leopard frog, and Cope's Gray Treefrog sounds like the Gray Treefrog. Mink Frogs tend to call in the morning, so early-morning samples improved detection. Leopard frog calls are low, quiet and sporadic, and come from a few small areas within larger wetlands, so the recorders can miss them by chance. Data collected since 2019 is being analyzed, and updates will be posted.

Sources

• National Park Service, Great Lakes Inventory and Monitoring Network, "Amphibian Monitoring in the Great Lakes Network Parks: 2023 Update" (https://www.nps.gov/articles/000/amphibian-monitoring-in-the-great-lakes-network-parks-2023-update.htm); rewritten in hubnx's own words. A gray treefrog photo credited to a person alone is left out; one species name (Pseudacris crucifer) is spelled as in the scientific name, correcting a typo in the source's table.