Bats Along the St. Croix Riverway: Monitoring 2016–2018

The evening bat lives mainly in far southern Wisconsin but seems to be moving north. U.S. Army photo.

Bats across the country face climate change, habitat loss, wind turbines and a deadly fungal disease, white-nose syndrome (WNS). From 2016 to 2018, the Park Service's Great Lakes Network recorded bats at St. Croix National Scenic Riverway, on the Wisconsin–Minnesota border.

Eight species, two ways of wintering

• Four species are here only in summer, wintering in the southwestern U.S., Mexico and even Central America.
• Four stay and hibernate in caves or buildings. Three of them — little brown, northern long-eared and tricolored bats — are highly susceptible to WNS. The big brown bat hibernates too but has shown resistance.

What was known before

• 2009–2010: a graduate student caught little brown bats at maternity colonies in Saint Croix State Park and William O'Brien State Park, both on the Minnesota side.
• 2010, 2013 and 2019: the Wisconsin DNR drove acoustic survey routes in or next to the riverway, detecting seven species.
• 2016: evening bats were first documented as residents of Wisconsin — a maternity colony in the far south — and one was caught near the Twin Cities. None had been found at the riverway, but the species might be spreading north.

Table 1. Bat species at the riverway before monitoring began in 2016 and after three years of it (2018). * = hibernates here in winter.

Listening past human hearing

Bats call in flight to navigate and find food, and like birds, species can be told apart by their calls. But the calls are ultrasonic, so recording needs special microphones and software. Some species sound alike, and calls vary within a species, so the software isn't 100% accurate — a share of the recordings are checked by hand on a spectrogram.

Seven, plus one

• All seven known species were confirmed, plus the evening bat: 26 hand-verified recordings suggest it is at least occasionally in the park.
• All eight species were recorded in all three years; five (big brown, eastern red, hoary, little brown, silver-haired) at every site every year.
• Busiest sites: northwest of Sunrise Landing Campground — the most calls from big brown, eastern red, hoary and silver-haired bats in two of the three years — and Twin Springs Road, a quiet wetland backwater with good foraging, which had more calls per night from little brown, northern long-eared and tricolored bats than any other site in any year.
• Trends: big brown, eastern red, hoary, silver-haired, tricolored and evening bats look stable or slightly rising; little brown and northern long-eared bats look to be declining.

What comes next

The network is working with the NABat Midwest Bat Hub on statistical models of occupancy — the probability that a species is using an area, allowing for the fact that it is not always detected — especially for the species hit hardest by WNS.

When the project began, the Great Lakes were at the leading edge of WNS, and the monitoring gave parks a baseline to measure change against. In 2022 the network handed bat monitoring to the parks, so further work at St. Croix is up to park biologists.

Sources

Based on "Bat Monitoring Along the St. Croix Riverway, 2016–2018," Great Lakes Inventory and Monitoring Network, National Park Service; a work of the United States government in the public domain. The full report: Goodwin, K. R., and A. A. Kirschbaum, 2021, Acoustic monitoring for bats at Saint Croix National Scenic Riverway: Data summary report for 2016–2018, Natural Resource Data Series NPS/GLKN/NRDS—2021/1331 (download). The source's picture of eight bats is left out: it includes Wisconsin DNR photographs.