A study published in a German scientific journal this summer indicates that wind turbines in one area may pose a serious risk to populations of bats over the better part of a continent. The study, performed by researchers from the Leibniz Institute for Zoo and Wildlife Research (IZW), showed that pipistrelle bats killed at German wind turbines likely originated from countries as distant as Scandinavia, Poland, the Baltic countries, and Russia. An estimated 200,000 bats are killed each year at German wind turbines, raising the possibility that Germany will become a sink for European bat populations -- and raising troubling implications about the effect of wind turbines on California bats.
The IZW researchers analyzed the fur of killed bats and recorded the ratio of stable hydrogen isotopes in the fur's keratin. Hydrogen has two stable isotopes -- a lighter isotope, H1, with no neutrons, and H2 or deuterium, which has one neutron in its nucleus. The two are chemically identical, but their ratio in the environment varies, with deuterium more common in the north of Europe. As an organism grows, it takes in deuterium in whatever ratio it is available in the environment, providing a geographical marker in the organism's chemical composition. By matching the ratio of deuterium to H1 in fur samples then matching that ratio to a map, the researchers were able to determine where killed bats were raised.
As bats killed at German wind facilities may have come from places 1,000 miles or more away, and as bats have very slow reproduction rates of only 1 or 2 offspring a year, the study suggests that wind turbines in Germany may well be depressing bat populations across the entire northeastern portion of Europe, in an area perhaps a million square miles in extent.
The bats most vulnerable to wind turbine injuries -- which seem to stem mainly from lung trauma from steep gradients in air pressure rather than from collisions -- are migratory species that live in trees. In California, one of the most common tree bats is the hoary bat, which migrates in large groups across a wide swath of North America from Canada to Mexico. Though the hoary bat is unusual among bats in that it can have litters of three or four pups, the German study nonetheless suggests that poorly sited wind turbines in bat migration areas might depress hoary bat populations across the western part of North America.
A 2011 paper by Paul Cryan of the USGS Fort Collins Science Center in Colorado reports that bat deaths have skyrocketed with the advent of wind turbine installations:
Since 2004, unprecedented rates of bat fatalities have been documented at multiple wind energy sites across the United States and Canada, as well as in several European countries. In the United States, bat fatality rates at turbines are variable across sites and regions. Despite standardized and well validated methods for measuring and comparing fatality rates across sites rarely being employed, estimates to date for individual wind energy sites range from just below one bat per installed megawatt per year (bats/MW/yr) to as high as 70 bats/MW/yr. These fatality rates for bats generally exceed the fatality rates of migratory songbirds at wind turbines, and far exceed any documented natural or human-caused sources of mortality in the affected species of bats. Some large wind energy facilities (e.g., 100-300 MW) are estimated to have fatality rates of 10-20 bats/MW/yr, which means that single wind energy facilities are causing the deaths of thousands of bats per year. With approximately 40,000 MW of turbines currently installed in the United States and Canada, and an average published bat fatality rate of 11.6 bats/MW/yr, more than 450,000 bats may already perish at turbines each year in North America. This number might even be an underestimate due to problems with earlier fatality estimation equations and because bat fatality rates appear to be increasing with deployment of larger turbines. [Emphasis added.]
If U.S. wind turbines truly do pose more of a threat to some American bats than the dreaded White Nose Syndrome now devastating eastern Bat populations, then the German study is sobering indeed. It may be that even a few isolated wind installations may harm bat populations across a broad landscape.
As Cryan says, distressingly,
In some parts of the country, bat researchers who only rarely catch hoary bats in the wild can now walk beneath turbines at certain wind energy facilities during autumn and find more dead hoary bats on the ground in a few weeks than they have caught during their entire careers.