Article

Amphibian Monitoring at Sleeping Bear Dunes, 2019

A green frog sits on a floating mat of grasses.
Green Frog is one of three species that is found in all the network parks.

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.

A green box is strapped to the trunk of a conifer tree. A hole in the trunk is visible above the box.
An automated recorder, or “frog logger.” Microphones come out of the top left and right of the box. A temperature logger (not shown) is hung nearby.

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 SLBE were recorded in 2019, with Eastern American Toad, Gray Treefrog, Green Frog, and Spring Peeper occurring at almost every site sampled. Wood Frog was found at one new site. The Northern Leopard Frog was not heard in 2019, though it was detected at five sites in 2018.

Phenology

Eastern American Toad was the first species to begin calling in early May. Gray Treefrog followed, with the first calls heard in mid-May, just days before the first Green Frogs were heard at SLBE09.

Call Index Values

The Eastern American Toad scored a CIV=3 at one site, while the rest scored CIV=3 at more than one site. Spring Peepers scored CIV=3 at all sites, with more than 50% of the recordings doing so at nine out of the 10 sites where it was heard.

A panel of 50 gray-scale bar graphs, one for each frog and toad species detected at each site. Site names are shown across the top, species names down the right side. The bottom axis shows Call Index Values ranging from 1 to 3.
Call Index Values (CIV) for each species at each site, Sleeping Bear Dunes 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 Sleeping Bear Dunes.

Tags

Sleeping Bear Dunes National Lakeshore

Last updated: January 10, 2024