Amphibian Monitoring at Pictured Rocks National Lakeshore, 2019

Eastern American Toad soon after transforming from a tadpole and coming onto land.

Gary Casper

Toads and frogs are unique among amphibians in that they produce calls that are unique and identifiable. This makes it possible to monitor their populations using remote recorders, or “frog loggers.” Monitoring is important 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 indicators of climate and habitat change.

Six frog and toad species are known to occur at Pictured Rocks: 

Common Name	Scientific Name
Eastern American Toad	Anaxyrus americanus americanus
Gray Treefrog	Hyla versicolor
Green Frog	Lithobates clamitans
Mink Frog	Lithobates septentrionalis
Spring Peeper	Psudacris crucifer
Wood Frog	Lithobates sylvaticus

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

NPS

Frog Loggers

Ten permanent amphibian monitoring sites were established in 2013. In April, an audio recorder at each site begins collecting 5-minute samples once an hour between 5:00 p.m. and 3:00 a.m. each day. Four extra samples are collected on the half-hour mark in the evenings between 7:30 and 10:30 p.m. A temperature sensor records temperature once an hour.

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.

A sub-sample of the recordings are used to assign a Call Index Value (CIV) to each species. CIV is the maximum call intensity, ranging from CIV=1 (non-overlapping calls indicating few singing males) to CIV= 3 (calls too numerous to count individuals). CIVs can be analyzed over time to determine if a species is increasing or decreasing in abundance.

The Latest (as of 2019)

Five of the six species known to occur at PIRO were recorded in 2019, with Eastern American Toad, Gray Treefrog, Green Frog, and Spring Peeper occurring at almost every site sampled. The Mink Frog is known to occur at Sand Point but has never been heard at a monitoring site.

Phenology

Eastern American Toads were the first species heard in 2019, with calls beginning in mid-May. Gray Treefrogs were next, the first of which were heard in late May. This was followed by Green Frogs, which began calling on 1 June

Call Index Values (CIV)

All species scored a CIV=3 at one or more sites except for the Wood Frog, which scored a maximum of CIV=2 at two of the five sites where it was heard. Spring Peepers scored CIV=3 at all sites where it was heard.

Call Index Values (CIV) for each species at each site, Pictured Rocks National Lakeshore, 2019. CIVs range from 1 (individuals of the species can be counted, there is silence between calls) to 3 (full chorus for the species, calls are constant, continuous, and overlapping).

What's Next?

The automated recorders continue to turn on each spring and capture the sounds of frogs and toads coming out of their winter slumber and looking for mates. Data collected since 2019 are being analyzed and we will publish regular updates on the status of amphibians at Pictured Rocks.

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/2301978