Bats Adapt to Disturbed Habitat

Land use history affects where Rafinesque’s big-eared bats roost

Rafinesque’s big-eared bat (Corynorhinus rafinesquii) is considered a rare and sensitive species. The bats are small, with a body length of three to four inches, ears just over one inch, a wingspan just shy of a foot, and they weigh around half an ounce — less than a slice of bread.

Though their range includes much of the Southeast, they’re not found in great numbers, with bottomland hardwood habitat loss and disturbance limiting suitable roosts for these little fliers. U.S. Forest Service research ecologist Susan Loeb examined a range of habitat conditions in South Carolina and published new study results in the Journal of Mammology.

rafinesques-big-eared-bat
The broad, short wings of Rafinesque’s big-eared bats limit their ability to travel long distances but help them to reach and extract insects. Photo by USFS.

“We know that the bats prefer to roost in bottomland hardwood areas, usually in large, hollow trees,” said Loeb. “In general, they prefer larger trees with large cavities, but that varies across sites — and they will also roost in old houses, barns, or bridges.”

Loeb sought to understand how land use or disturbance history affected big-eared bats’ roosting habits. She examined bat habitat at three bottomland hardwood sites along the Savannah River floodplain in South Carolina with timber harvest histories ranging from less to more disturbed:

  • The James W. Webb Wildlife Center and Management Area (Webb) has been managed by the South Carolina Department of Natural Resources since 1941, and its bottomland areas haven’t seen any logging disturbance since that time.
  • Extensive cypress logging took place at the Department of Energy Savannah River Site in the 1900s, with additional widespread harvest activity in the 1960s to 1980s and again in some areas in the 1990s.
  • The Groton Plantation bottomlands were logged multiple times in the 1940s, 1970s, and 1980s.

From 2009 to 2011, Loeb established 49 transects across the three sites, located 361 trees with cavity openings at their base, and surveyed those trees anywhere from two to nine times. She found Rafinesque’s big-eared bats roosting in 67 of the basal cavity trees.

Loeb recorded tree attributes like species, height, and diameter, along with cavity sizes and locations — basal, chimney, or both. Other site characteristics were also noted, including temperature, rainfall, and how often the sites experienced flood conditions.

rafinesque-big-eared-bat-in-roost
Rafinesque’s big-eared bats switch roosts every few days but return regularly to previously used roosts. Photo by Charles Dachelet, USFS.

When she analyzed her data, Loeb learned that although the bats at the least-disturbed Webb site used a large range of cavity volumes or sizes, they selected the largest trees available.

At Webb, the average diameter of trees bats roosted in was five feet, whereas trees used at the more-disturbed sites were about three to four and a half feet in diameter.

Bats at the Webb site primarily used trees in bald cypress and swamp tupelo stands and rarely roosted in upland areas.

Bats at the two more-disturbed sites, Groton Plantation and Savannah River Site, used the broadest roost niches, meaning they used a greater number of different tree species–oaks, hickories, sycamore, blackgum, and sweetgum in addition to cypress and tupelo.

They also roosted in more habitat types, including upland and less frequently flooded areas, than bats at the less-disturbed Webb site.

Overall, bats were more likely to use larger cavities, and cavity characteristics were the more important factors influencing their roost selection.

“These results suggest that Rafinesque’s big-eared bats can, in fact, adapt to a wide range of habitat conditions–as long as trees with large cavities are available for their roosts,” adds Loeb. “More research is needed to determine the long-term viability of these bats in disturbed habitats, and that knowledge will help us design the most appropriate management strategies for their survival.”

Read the full text of the article.

For more information, email Susan Loeb at sloeb@fs.fed.us.

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