Bats at a Loss

Worldwide, almost a quarter of all bat species are threatened with extinction, with even common species starting to disappear. As with other wildlife, declines in bat populations are tied to habitat loss due to human development.

“In addition to loss of forest and wetland habitat, other factors related to urbanization impact bats,” says Susan Loeb, SRS research wildlife biologist based in Clemson, SC. “Urban areas have less snags and hollow trees for bats to roost in and fewer of the insects they eat.”

Parks and preserves are increasingly recognized as important for conserving biodiversity as pressures from development, introduced species, and climate change increase. Though large national parks and forests are assumed to provide the greatest conservation value, smaller parks often contain a large number of unique wildlife species, and serve as refuges for creatures that don’t move very far during their lives as well as rest stops for migrating species.

“Around 2000, bat researchers started to realize that parks in urban and suburban areas might play a particularly important part in bat conservation, but we didn’t really have the studies to back that up,” says Loeb.

Though bat inventories had been taken for some American cities, there hadn’t been any surveys of bats specific to urban and suburban parks. In 2004, the National Park Service contacted Loeb about surveying bats in some of the Coastal Plain parks in South Carolina and Georgia as part of a larger national resource inventory. What started out as a coastal inventory turned into a broader study of parks in a number of different types of regions.

From 2004 through 2007, Loeb and technicians, some led by nowretired wildlife biological technician Chuck Dachelet, (who was well seasoned in mist-netting bats that ranged from the tremulous Indiana bat to the combative big brown bat) surveyed 10 National Park units in the Piedmont and Coastal Plains of North Carolina, South Carolina, and Georgia. Parks ranged from the highly urban Chattahoochee River National Recreation Area in Atlanta, GA, to the relatively rural—and mostly wetland— Congaree National Park in the Upper Coastal Plain of South Carolina.

Loeb and her team used mist nets and acoustic detectors to identify and count bats, recording both species richness (total number of different species present) and diversity (numbers of individuals of each species present). Loeb and her fellow researchers were used to working through the night in dark forests but the park survey offered a different experience. “We actually caught a couple of bikers in our mist nets,” says Loeb. “There were a lot of people out there, running and walking. It was a good opportunity to tell people about bats and our research.”

The researchers had predicted that species richness would increase with park size and that both species richness and diversity would decrease with development. What they found was more complicated.

“Contrary to our prediction, species richness was not related to park size and did not decrease with development,” says Loeb. “Species richness in urban parks was as high, sometimes higher than in rural parks, with some of the urban and suburban parks containing rare and sensitive species such as the Rafinesque’s bigeared bat and the small-footed bat.” (Loeb cautions that since most of the parks in the study were over 250 acres in size, the lower threshold at which the size of the area studied becomes a factor might not have been reached.)

Big Browns Take Over

Though species richness was relatively high in urban parks, species diversity was low. As the researchers expected, the higher the human population density and percent developed land in an area, the lower the species diversity for bats. But these results were not only due to reduced habitat, but also to the increased dominance of big brown bats.

“Declining species diversity in relation to urbanization can be traced to the dominance of big brown bats in the urban parks we studied,” says Loeb. “We found that numbers of big brown captures were positively correlated to human population density and developed land.”

Big brown bats have been shown to dominate all the urban bat communities studied in the United States to date. Loeb speculates that the urban environment might actually be favorable for species such as big brown bats because they readily roost in buildings and bridges, and, with their large body size and long wing span, can fly longer and farther to forage than smaller species.

But smaller and more sensitive species are still present, even in highly urbanized settings. Loeb and fellow researchers found Rafinesque’s big-eared bats, a species of concern, in the highly urban Ocmulgee National Monument in Macon, GA.

“Though our results show that urban parks may serve as important refuges for some bat species, the low numbers of individuals suggest that some species in these areas may be vulnerable to local population extinction,” cautions Loeb. “Management of urban as well as rural parks should strive to conserve as much bat roosting and foraging habitat as possible.”

For more information:

Susan Loeb at 864–656–4865 or sloeb@fs.fed.us

Recommended reading:

Loeb, S.C.; Post, C.J.; Hall, S.T. 2009. Relationship between urbanization and bat community structure in national parks of the Southeastern U.S. Urban Ecosystems. 12: 197–214.