Wildlife & Nature

Little Brown Bats Slowly Recover from Deadly Fungus

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Once considered the most common bat in the region, little brown bats were significantly impacted by white-nose syndrome. (Ann Froschauer/USFWS)

Surveys of bat maternity roosts in barns and attics around Rhode Island suggests that populations of little brown bats, which had declined by 95-98 percent across the Northeast as a result of a deadly fungus, are beginning to recover.

“It’s speculative at this point, but from what I see anecdotally and in the data, we’re seeing evidence of recovery,” said Charles Brown, a wildlife biologist with the Rhode Island Department of Environmental Management (DEM) who monitors bat populations in the state. “It seems like they hit bottom — at least I hope they did — and we’re seeing evidence of the bats rebounding and recovering to some degree.”

Once considered the most common bat in the region, little brown bats are one of the species most significantly impacted by white-nose syndrome, a fungus found in the caves where the bats hibernate. First discovered in 2006 in a cave in upstate New York, the fungus soon spread to bat hibernation caves throughout the Northeast. It has now been detected in 35 states and seven Canadian provinces, and it has been confirmed as a cause of death for 12 North American bat species.

Brown began monitoring maternity roosts in Rhode Island in 2010, after the fungus had already had a significant impact on local bats, most of which are believed to hibernate in caves in Vermont, New Hampshire and New York.

“Female little brown bats are here in the summer in their maternity roosts, where they’re not exposed to white-nose syndrome,” Brown said. “But they’ve come from a cave somewhere like Vermont where they’ve been exposed to it all winter, and then they go back there the following winter and may be re-exposed. We’ve learned that some bats have developed a resistance and are persevering, but every year bats are dying and not coming back.”

Nonetheless, little brown bat numbers seem to be growing in the Ocean State.

“We’ve seen some maternity colonies blink out through the years, whether because they all died or because they got down to such low numbers that the bats ended up going to another colony,” Brown said. “My guess is that there are probably fewer colonies on the landscape now than there used to be, but in general, all of our sites showed good recruitment this year.”

Brown and a team of DEM staff and volunteers visit each of 15 bat maternity roosts twice each summer to count how many of the animals emerge to feed at dusk. Some of the sites contain little brown bats, others have big brown bats, and some have both species. June visits tally breeding females, while July visits count both adult females and their pups, which are able to fly and feed themselves by then. Males do not spend time in maternity roosts.

“We had good recruitment of pups this year,” he said. “Some years we don’t see that. But, in general, this year we saw bats that did well as far as pups making it to flight stage. Whether they survive their first winter of life may be a different story, however.”

Rebuilding bat populations is a long, slow process, according to Brown. Female bats give birth to just one pup each year, and the pup survival rate during their first year is only about 50 percent.

“We’re not going to see a significant recovery in our lifetime,” Brown said. “We’re not going to see numbers we used to see for a long time. We need to think long-term about protecting maternity roost sites. If they’re going to recover, they need habitat, they need places to feed, they need places to raise their young. We’re going to have to maintain our forests and open spaces. If we don’t protect habitat, there won’t be a place for bats to come back to.”

Brown is also worried about the long-term stability of the buildings containing bat maternity roosts. An old barn in Coventry that has long been a maternity roost is collapsing and may no longer be maintaining the environmental conditions that the bats require to reproduce.

Not every bat species in Rhode Island was as impacted by white-nose syndrome as the little brown bat, however. Brown said big brown bats, some of which also migrate to caves in northern New England, are less susceptible to the fungus.

Bats that do not hibernate in caves, such as red bats and hoary bats, were not affected by the fungus at all.

Brown monitors these other bat species by conducting mobile acoustic surveys — driving five 20-mile transects twice each summer with a bat detector mounted on his vehicle to listen for the calls of bats. He also traps and bands bats at various locations each year, though the coronavirus pandemic put all bat handling on hold to ensure that humans do not transmit the virus to bats.

The data he collects provides insights about where various species are found and how populations are trending from year to year. He shares this information with the North American Bat Monitoring Program, which assesses regional population trends.

“We know more about bats than we did 10 years ago, but we still don’t know a lot,” Brown said. “There are still a lot of unanswered questions, especially about bat migration.”

Brown is particularly concerned about the impact of the growing number of wind turbines on migrating bats.

“It’s well documented that bats suffer mortality at wind turbines, and I don’t think that’s being adequately addressed by the regulatory process in place now,” he said. “We know bats move in the offshore environment in great numbers. We know bats are attracted to turbines, though we don’t know why. And we know they’re killed by wind turbines.”

Rhode Island resident and author Todd McLeish runs a wildlife blog.

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  1. Is there anything that the public can do to help? Where do bats roost during the day? Are bat houses useful?

    • Donna, in years past, my father and several neighbors tried building bat houses, but with no success attracting them, so I guess they’re finding suitable sites, at least in my area.

  2. Thank you, Todd, for this report. I’m sure the relatively low mosquito population in my wetlands-adjacent neighborhood is due to these wonderful creatures. We do see them at dusk, and I see no ‘Mosquito Squad" signs anywhere in the vicinity.

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