Their numbers dwindling, two at-risk bat species coexist

Indiana bats and northern long-eared bats coexist in central Indiana, but the populations of these two imperiled species are falling. To better protect them, researchers wanted to understand how they differed in their prey and their use of the landscape.

The two species share a lot in common, including the threats they face. Both are threatened by white-nose syndrome, a deadly fungal disease that has decimated bat populations in much of North America. And like many other wildlife species, both are declining due to habitat loss.

“But there are a lot of things we didn’t know about their basic biology, like which specific food items they are eating,” said researcher Timothy Divoll, a data scientist at Brown University’s Center for Computation and Visualization. As part of his doctoral research, Divoll conducted a study published in Environmental DNA looking at both species’ habitat and food preferences in hopes of informing forest management to ensure that both species are better conserved. Indiana bats (Myotis sodalis) are federally endangered. Northern long-eared bats (M. septentrionalis) are federally threatened.

“We really wanted to understand how these species coexist to better help land managers, particularly forest managers, understand what roles bats play in the forest and the insects they’re eating,” said coauthor Joy O’Keefe, assistant professor and extension wildlife specialist at the University of Illinois. “We also wanted to see how forest management ties into bat habitat.”

To conduct the study, the researchers looked at two sites in very different landscapes. One was in an urban-rural transition area, close to a major airport, with riparian patches of forest. The other was in a contiguous forest surrounded by farms. At both sites, they caught bats and took fecal samples over four seasons. DNA barcoding of fecal samples back at the lab allowed them to get a better idea of what specific prey species bats were eating. This was a new approach. While previous researchers examined fecal samples for the remains of prey, it could only show prey that survived digestion and left hard parts in fecal samples, like crunchy bits of beetles, crickets and wasps. Using DNA barcoding, Divoll and his co-authors could capture a much broader picture of what the bats ate, including hundreds of species of moths, flies and other soft-bodied prey in addition to numerous hard-bodied insects.

One thing the researchers specifically paid attention to was prey size. They found that northern long-eared bats tended to eat slightly larger insects than Indiana bats—probably because the species gleans its prey from the forest floor or tree trunks, where it can find bigger insects. Indiana bats, on the other hand, are more likely to catch smaller insects in mid-air while flying. The researchers believe these different ways of catching prey may be one reason the two species can coexist. They also knew from a previous study that northern long-eared bats use a much smaller area to forage at night compared to Indiana bats, suggesting the bats use different hunting strategies as well.

When they looked at which plants the insects were associated with, the team realized the importance of plant diversity. “Managing forests to maintain a diversity of plants and also different heights of the forest with different ages of trees and shrubs will increase the forest’s capacity to host a more diverse array of insects for bats to eat,” Divoll said.

That makes controlling invasive plant species an important strategy. Invasive autumn olive or bush honeysuckle, for example, can crowd out native plants from the understory. “It homogenizes the forest,” O’Keefe said.

Maintaining healthy habitats for the bats is particularly important as white-nose syndrome continues to decimate bat populations, the team found, so that the bats have the resources they need to rear their pups and prepare for winter hibernation.

The researchers saw the threats to both bat species firsthand. During the course of their work, they watched northern long-eared bat populations sink below Indiana bat numbers, even though that species was once the rarer one. “It was an ambitious effort on our part, working against time, to get these samples before the bats disappeared,” O’Keefe said.