Researchers say a pressure drop created by turbines can cause bats' lungs to burstThe mystery was alarming to wildlife experts: large numbers of dead bats appearing at wind farms, often with no visible signs of injury.
Researchers now think they know one reason: Wind turbines cause bats' lungs to explode. More specifically, a sudden drop in air pressure created by the blades can cause fatal internal hemorrhaging, researchers at the University of Calgary said in a study.
The toll taken on bats highlights a delicate balance facing the wind industry—how to be "green" without causing other unintended environmental consequences.
Some of the best sources of wind—coastlines and mountaintops—also happen to be in the path of migratory birds. Wind farms installed on mountain ridges also have triggered fears over soil erosion, and some environmental groups—citing land use laws designed to keep Mother Nature unspoiled—have fought proposed wind farms.
With the deaths causing a stir among wildlife advocates, an unusual partnership called the Bats and Wind Energy Cooperative is seeking ways to strike a delicate balance between protecting the bat population and meeting the nation's growing demand for renewable energy.
"We support the development of clean energy, but to make it 'green' we have to do everything we can to minimize the environmental impacts," said Ed Arnett, project coordinator for the cooperative.
Some wind experts dismiss fears over turbines' impact on wildlife. They point to a 2007 study by the National Academy of Sciences that concluded far more birds and bats have been killed in collisions with vehicles and buildings than in collisions with turbines.
But wildlife experts are particularly protective of bats because the mammals have low reproductive rates, meaning even small numbers of fatalities can affect their populations.
"Once you start taking a small number of bats out of the general population, the risk of endangerment or extinction vastly increases," said Joseph Kath, the endangered species project manager for the Illinois Department of Natural Resources.
Thus far, there have been no reports of endangered or threatened bat species being killed at wind farms in North America. Most bats felled by turbines have been migrating species like hoary bats, eastern red bats and silver-haired bats.
The concern over bats is fairly recent. Since the 1980s, when wind farms were in their infancy, wildlife biologists have been more worried about protecting birds from spinning turbines. Bat deaths at wind farms largely went unnoticed.
Then in 2003, researchers stumbled upon an estimated 1,400 to 4,000 bat carcasses at the Mountaineer Wind Energy Center in West Virginia and recorded extensive bat fatalities at wind farms in Pennsylvania and Tennessee.
Wildlife experts were taken by surprise.
"These are unforeseen circumstances," Arnett said. "Most of us didn't anticipate this being a problem."
Since then, the chorus of voices calling for greater protection for bats at wind farms has grown louder. Last summer, the American Society of Mammalogists called for wind farms to avoid "bat hibernation, breeding and maternity colonies."
Still, the explanation for why bats with no external signs of injury were being found dead at wind farms was largely a mystery until August.
That's when researchers at the University of Calgary reported that 90 percent of bats felled near one wind farm showed signs of barotrauma, or fatal internal hemorrhaging, of the lungs that occurred because of drops in air pressure near the spinning blades.
The condition affects bats more than birds because bird lungs are more rigid and can withstand sudden changes in air pressure, according to the study, which was published in the journal Current Biology.
The study may explain why bat fatalities often outnumber bird fatalities at wind farms. In Illinois, it is estimated three times as many bats (93) as birds (31) died during a year at the 33-turbine Crescent Ridge Wind Farm in Bureau County, a consulting firm reported last year.
The firm, Curry & Kerlinger, deemed the findings "small and not likely to be biologically significant."
But given a decline in several bat species in the eastern United States, "the possibility of population effects, especially with increased numbers of turbines, is significant," the National Academy of Sciences study stated.
Illinois is expected to increase the number of wind farms dramatically in coming years. The state has mandated that 25 percent of its electricity be generated by renewable resources by 2025, with about 75 percent of that renewable energy coming from wind. Illinois has 915 megawatts of capacity installed with the capacity to build 9,000 megawatts.
Arnett said the cooperative doesn't discount the Calgary study but has conducted studies of its own, using night-vision cameras, that found bats also have been killed by collisions with turbine blades.
There are several theories as to why bats might be flying close to turbines. Some think bats might confuse turbines with large, dead trees because many species found dead use such trees to roost. Others hypothesize that turbines may attract insects, which attract hungry bats.
The cooperative has been looking for ways to bring down the death toll, including studies of the effectiveness of ultrasonic sounds that would deter bats and curtailing the spinning of turbines until it's too windy for bats to fly.
Arnett said the latter step may have some economic consequences. But he expressed confidence that the wind industry can continue to grow without harming bat populations.
"It's not choosing one or the other," Arnett said. "It's finding a balance, and I'm convinced we can solve this problem."