Bats, Wind Energy and Agriculture
A recent Science report puts dollar figures on the pest-control value of bats to North American agriculture: between $3.7 and $53 billion a year. The study follows concern that bat populations are under severe pressure from a pair of threats: wind turbines and an infectious disease called white-nose syndrome.
From the USGS press release, "Bats Worth Billions to Agriculture":
The loss of bats to white-nose syndrome has largely occurred during the past 4 years, after the disease first appeared in upstate New York. Since then, the fungus thought to cause white-nose syndrome has spread southward and westward and has now been found in 16 states and 3 Canadian provinces. Bat declines in the Northeast, the most severely affected region in the U.S. thus far, have exceeded 70 percent.
From the Science article, "Economic Importance of Bats in Agriculture" (sub. req.):
At the same time, bats of several migratory tree-dwelling species are being killed in unprecedented numbers at wind turbines across the continent. Why these species are particularly susceptible to wind turbines remains a mystery, and several types of attraction have been hypothesized. There are no continental-scale monitoring programs for assessing wildlife fatalities at wind turbines, so the number of bats killed across the entire United States is difficult to assess. However, by 2020 an estimated 33,000 to 111,000 bats will be killed annually by wind turbines in the Mid-Atlantic Highlands alone.
One explanation for bats' vulnerably to wind turbines appears in a 2008 Science Daily report, "Why Wind Turbines Can Mean Death For Bats":
Power-generating wind turbines have long been recognized as a potentially life-threatening hazard for birds. But at most wind facilities, bats actually die in much greater numbers. Now, researchers reporting in Current Biology ... think they know why.
Ninety percent of the bats they examined after death showed signs of internal hemorrhaging consistent with trauma from the sudden drop in air pressure (a condition known as barotrauma) at turbine blades. Only about half of the bats showed any evidence of direct contact with the blades.
"Because bats can detect objects with echolocation, they seldom collide with man-made structures," said Erin Baerwald of the University of Calgary in Canada. "An atmospheric-pressure drop at wind-turbine blades is an undetectable—and potentially unforeseeable—hazard for bats, thus partially explaining the large number of bat fatalities at these specific structures.