Bats in Acadia: Facing Challenges but Hanging On

A little brown bat resting on a tree in Acadia National Park.

Photo credit © Erickson Smith.

Whether you think they’re cute or creepy (we think they’re cute!), bats are fascinating and ecologically important members of our environment. They eat tremendous amounts of night-flying insects, including some considered human pests (think mosquitoes) and agricultural pests, and some bats pollinate plants, such as some of your favorite fruits and the iconic saguaro and organ pipe cactuses of the southwestern United States. Bats are found on all of Earth’s continents except Antarctica, and they comprise about 1,400 species worldwide, including 45 species in the United States and Canada. Bats, however, face many challenges. A recent threat in North America, including in Maine and Acadia National Park, is the disease known as white-nose syndrome (WNS).

Acadia National Park (NP), in coastal Maine, has a total of eight species of bats during at least part of the year. The park, almost 50,000 acres in size, is largely forested but also includes non-forested wetlands, freshwater lakes and ponds, and exposed rock features such as granite cliffs and outcroppings. What does Acadia not have that is often associated with bats? The answer is caves. So where do bats live in Acadia? How do park biologists keep track of their populations? And how are they doing since the arrival of WNS?

Acadia’s Bats and Where They’re Found

All bat species in Maine are also found in Acadia NP. Of these eight bat species, five can be found in the park all year (and hibernate here), and three are migratory (and not found in the park in the colder months). One of the year-round species, Northern long-eared bat (Myotis septentrionalis), is on the federal Endangered Species List. The other year-round species are little brown bat (Myotis lucifugus; state endangered), Eastern small-footed bat (Myotis leibii; state threatened), big brown bat (Eptesicus fuscus), and tri-colored bat (Perimyotis subflavus).

Bats in Maine use a variety of habitats, including forests, fields, wetlands, ponds, and mountain slopes. During their active period in May through mid-October (outside of winter hibernation), bats in Acadia use the following features for day roosts: rock crevices and outcrops, trees (small cavities or under loose bark), and, sometimes, buildings or other human structures. When hibernating in the park, bats primarily use rock crevices, including in talus slopes (see photo), and human structures.

Natural bat roosting and hibernating habitats in Acadia (left to right): a rocky cliff/outcrop, a rocky talus slope, and a tree with loose bark.

NPS photos.

The Threat and Impact of White-nose Syndrome

Some bat species in North America, including in Acadia NP, have been affected by the disease white-nose syndrome (WNS). White-nose syndrome, which can give a fuzzy white appearance to a bat’s muzzle and wings, is caused by a fungus and has resulted in dramatic population declines. Bats declined significantly in the park after the appearance of WNS in 2011–2012, with the three Myotis species and tri-colored bats the most affected. This disease has been a significant cause for concern and potentially makes it more difficult to monitor bats given their lowered numbers. Bats, however, persist in Acadia NP.

How do we Monitor Bats?

The National Park Service (NPS) protects bats and their habitats. Managers at Acadia NP have been monitoring bats since the mid-1990s. This long-term monitoring program is vital, because we need to know where bats are and how they’re doing in order to better protect them.

Finding bats?

Since bats are small animals that come out at night, their numbers dropped after the appearance of WNS, and the park is large—how do we find and monitor them?

Bats emerge from their daytime roost sites (two depicted here) to feed at night. An acoustic recorder (shown on right) will record their calls as they are catching insects or flying by.

Illustration by NPS/Jen Strona.

First, biologists have to be out when the bats are out—and that’s at night. No going to bed early for these folks. Second, there are multiple ways and tools to study and monitor bats. Acadia monitors bats mainly with mist nets and acoustic recorders. Biologists also use radio-tracking devices (placing small radios on individual bats), which can provide information on where bats roost and raise their young.

Biologists use mist nets to catch bats at specific locations in the park. By recording the number of bats caught each year, biologists can determine an index of abundance. This index can be compared over time to get an idea of how the population is doing. Mist netting involves catching bats in a fine, stationary net, collecting information on them (such as the type of bat and whether they show signs of WNS), and releasing them unharmed.

Another, more high-tech way of monitoring bats is based on their use of echolocation to navigate and catch insect prey. Since humans can’t hear bat calls, scientists developed acoustic call recorders to record the bats’ sounds. To analyze the recorded calls, special software is used that helps biologists identify the species that made the call, when the call was made, and whether the bat was feeding or navigating when it was recorded. This method of monitoring bats is efficient and less intrusive for the bats. Acadia NP biologists recently updated their acoustic monitoring program to best meet their monitoring goal of assessing long-term trends in the park’s bat community.

Want to hear an acoustic recording of a little brown bat?

In this recording, the little brown bat makes numerous, quick echolocation calls, or chirps, to navigate or search for prey. The lower-pitched sounds that start half-way through the recording are not from a bat. The audio has been modified so the human ear can hear it, and the accompanying spectrogram (the moving image) is a graphic representation of the sounds. The red line at the top of the spectrogram indicates how loud the sound is (the amplitude), with larger waves representing louder calls. The main (center) part of the spectrogram shows the frequencies of the calls (scale in kilohertz on the right vertical axis). The varying intensity of colors, ranging from dark blue (low intensity or quiet) to red (high intensity or loud), indicates the loudness (amplitude) of each call.

Open Transcript 

Transcript

The little brown bat is making numerous, quick echolocation calls (or chirps) that occur at the same time as the thin, vertical, multicolored lines in the spectrogram. There are also lower-pitched sounds that start half-way through the recording, but they are not made by the bat.

Duration:

9.328 seconds

Acoustic Recording/Spectrogram Credit: Dr. Erik J. Blomberg, University of Maine

What does Recent Research Tell Us?

Recently, biologists at the park and partners at the University of Maine have been studying bat use of different areas in the park, the impact of WNS on their numbers, and how to better monitor the bats as population levels decrease.

Updated bat monitoring protocol

Under the revised bat monitoring protocol, biologists are sampling more (now one-half) of the park and revisiting the same survey sites every 2 years. Acoustic recorders are set out at a given site for about two weeks at a time in locations likely to be used by bats as travel corridors or foraging habitat.

Long-term bat declines

Based on data from the park’s mist net surveys, it appears that relative abundance of Northern long-eared bats and little brown bats declined by 90% or more after the arrival of WNS (Figure 1).

Fig. 1 Mist netting results showing capture rates of three Myotis bat species. Capture rates indicate how frequently bats of each species were caught. Given that species more abundant on the landscape are likely to be captured more often, capture rates can be used as an index of relative abundance.

In comparison, relative abundance of Eastern small-footed bats was fairly stable over the same period, and this species was the most frequently encountered over the second half of the study period. Note that while tricolored bats have also been affected by WNS it is rare to catch them with mist nets, so they don’t show up in this dataset. They do show up, however, in the park’s acoustic recordings. Also note that we captured particularly high numbers of little brown bats in 2009 because we surveyed near maternity colonies.

While the abundance of little brown bats and Northern long-eared bats declined significantly after the appearance of WNS, they stabilized at lower levels during 2015–2018 (based on both mist net data and acoustic data). These results provide some level of optimism for the persistence of bats in Acadia NP.

Bat presence in different areas of Acadia, 2019–2020

Based on acoustic monitoring in 2019–2020, little brown bats were the most common species in Acadia in those years, followed closely by Eastern small-footed bats. Each species was present in about one-half of the park sites surveyed. Northern long-eared bats were the least common species in 2019–2020, found in about one-fifth of the park sites. This information, and similar information on the other bat species recorded, will provide the baseline for bat monitoring in Acadia NP into the future.

What Can You Do to Help Protect Bats?

Given the many challenges bats face, monitoring programs like the one in Acadia NP are crucial for protecting bats now and in the future. If you’d like to do what you can to help bats, here are just a few ideas. See the links below the list for additional ways to help.  

• Learn about bats and share what you learn with others. There are many myths about bats. Learning the truth and sharing this knowledge will help you and others have a greater appreciation for bats.
• Turn off unnecessary lights and reduce outdoor lighting. Light pollution can disturb bat activity. Contributing to a dark environment can help improve conditions for bats.
• Foster natural habitat around your home. Plant a garden to attract insects that feed bats and pollinate plants. Where possible, leave dead and dying trees; they may be used by bats for roosting.
• Minimize the use of pesticides in your yard. This will leave more insects for hungry bats and will benefit insects that serve as pollinators. All bats in Acadia NP eat insects.
• Don’t disturb bats. Do not enter caves and mines where bats are known/suspected to hibernate. If disturbed, bats may become active and use up their fat reserves that are needed to sustain them through winter. Also learn more about WNS to help prevent its spread.

Sources for the above and additional ideas on How you Can Help:
U.S. Fish and Wildlife Service
National Park Service
White-nose Syndrome Response Team

Information in this article is based primarily on: Blomberg, E. J., C. Heilakka, B. Wheeler, L. Katz, M. Ingalls, and M. Donlan. 2023. The past, present, and future of bat population monitoring in Acadia National Park: Focused condition assessment. Natural Resource Report NPS/ACAD/NRR—2023/XXXX. National Park Service, Fort Collins, Colorado. [Publication Pending]