Amphibian Monitoring in the Great Lakes Network Parks: 2023 Update

Gray treefrogs can be gray or green or both. They are able to change colors to match their surroundings. Gray treefrogs are found in all of the Great Lakes Network parks.

T. Gostomski

The high-pitched peeping of Spring Peepers is one of the earliest signs of spring. And it can be loud. Drive by a roadside pond and the calls can penetrate the closed windows of your vehicle. Stand next to a pond full of calling peepers, and you will hear nothing else.

Toads and frogs produce calls that are uniquely identifiable, making it possible to monitor their populations using remote recorders, or “frog loggers.” Such monitoring was important to the network parks for many reasons, the top one being that declines of amphibian populations are one of the most prominent conservation issues in the world. Most amphibians need both land and water during their lifetimes, so they are sensitive to environmental changes including temperature, precipitation, and humidity, making them an indicator of climate and habitat change.

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

NPS/T. Gostomski

Occupancy vs. Abundance

Recording frog and toad calls allows us to determine occupancy and abundance. Occupancy is a measure of whether or not a site is occupied by a species. Abundance is tracked by assessing how the maximum call intensity changes on sites across years, and by how many times we hear that species across years. In 2018, we also began tracking phenology, or the trends in annual first-calling dates for each species over time. For now, we are only tracking phenology for three species common to all parks in the network: Eastern American Toads, Green Frogs, and treefrogs. The Gray Treefrog and Cope’s Gray Treefrog have very similar calls that are difficult to distinguish, even with recordings, so for phenology purposes, we combine data for both species.

An automated recorder, or “frog logger.” Microphones come out of the top left and right of the box. A temperature logger is attached to the bottom of the box.

NPS

The Where and the How

Recorders are located in seven of the nine network parks. (St. Croix Riverway and Voyageurs participate in a separate monitoring program that uses a different protocol.) Recorders are programmed to turn on in the spring (typically in mid-April) and recordings are collected for five minutes at the top of each hour between 5:00 p.m. and 3:00 a.m. each day. Three additional samples are collected on the half-hour during the prime evening calling period.

A sub-sample of recordings from each park are used to assign a Call Index Value (CIV)—a measure of calling intensity—for each species. Over time, we can use CIV values to determine if a species is increasing or decreasing in abundance.

Occupancy of frog and toad species at Great Lakes Network parks.

A = Absent, E = Extirpated, P = Present, U = Unconfirmed. APIS=Apostle Islands, GRPO=Grand Portage, INDU=Indiana Dunes, ISRO=Isle Royale, MISS=Mississippi River, PIRO=Pictured Rocks, SLBE=Sleeping Bear Dunes

St. Croix Riverway and Voyageurs participate in a different monitoring program, so their data are not shown.

Common Name	Scientific Name	APIS	GRPO	INDU	ISRO	MISS	PIRO	SLBE
American Bullfrog	Lithobates catesbeianus	U	A	P	A	P	U	U
Blanchard’s Cricket Frog	Acris blanchardi	A	A	E	A	P	A	A
Boreal Chorus Frog	Pseudacris maculata	P	P	P	P	P	U	A
Cope’s Gray Treefrog	Hyla chrysoscelis	P	A	P	A	P	U	U
Eastern American Toad	Anaxyrus americanus americanus	P	P	P	P	P	P	P
Fowler’s Toad	Anaxyrus fowleri	A	A	E	A	A	A	U
Gray Treefrog	Hyla versicolor	P	P	P	P	P	P	P
Green Frog	Lithobates clamitans	P	P	P	P	P	P	P
Mink Frog	Lithobates septentrionalis	P	P	A	P	A	P	A
Northern Leopard Frog	Lithobates pipiens	P	P	P	U	P	U	P
Pickerel Frog	Lithobates palustris	A	A	A	A	A	A	U
Spring Peeper	Psudacris crucifer	P	P	P	P	E	P	P
Wood Frog	Lithobates sylvaticus	P	P	P	P	P	P	P
Western Chorus Frog	Pseudacris triseriata	A	A	A	A	A	A	U

Number of frog and toad species heard in audio recordings at each park, 2019. APIS=Apostle Islands, GRPO=Grand Portage, INDU=Indiana Dunes, ISRO=Isle Royale, MISS=Mississippi River, PIRO=Pictured Rocks, SLBE=Sleeping Bear Dunes. St. Croix Riverway and Voyageurs data not shown.

2019 Results

APIS

Six of eight species recorded, with Green Frog and Spring Peeper occurring at every site.

GRPO

Five of eight species recorded, with Green Frog and Spring Peeper occurring at every site.

INDU

Eight of nine species recorded, with Eastern American Toad, Gray Treefrog, Green Frog, and Spring Peeper occurring at every site.

ISRO

All seven species recorded, with Eastern American Toad, Green Frog, and Spring Peeper occurring at almost every site.

MISS

Eight of nine species recorded, the most well distributed being Eastern American Toad, Gray Treefrog, Green Frog, and Northern Leopard Frog.

PIRO

Five of six species recorded, with Eastern American Toad, Gray Treefrog, Green Frog, and Spring Peeper occurring at almost every site.

SLBE

Five of six species recorded, with Eastern American Toad, Gray Treefrog, Green Frog, and Spring Peeper occurring at almost every site.

See individual park briefs for more detailed information.

Challenges

Our methods are sufficient for monitoring all species found in the parks, but there are some challenges. The most difficult species to confirm are American Bullfrog, Cope’s Gray Treefrog, Mink Frog, and Northern Leopard Frog. The bullfrog, Mink Frog, and leopard frog calls can be obscured by ambient noise, while Cope’s Gray Treefrog sounds similar to the Gray Treefrog. Mink Frogs tend to call in the morning, so collecting early morning samples has improved their detection rates, but, leopard frog calls are low frequency, low volume, and sporadic. They also tend to call from a few small areas within larger wetlands, making it more likely to miss them by random chance.

We continue to adjust and improve our methods to capture the best representation of amphibian populations in the network parks. Data collected since 2019 is being analyzed, and we will continue to post updates on what we find.

For more information on amphibian monitoring in Great Lakes Network parks:

Amphibians (U.S. National Park Service) (nps.gov)

Download a PDF of this article: 
https://irma.nps.gov/DataStore/Reference/Profile/2301972