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USGS Research Finds Human Activity Leading Reported Cause Of Bat Deaths

Paul Cryan
U.S. Geological Survey
Bats flying in Texas

How do bats die? Over time, the answer to that question has changed.

They used to die by accident. Or by getting eaten. Perhaps they got caught in a natural disaster like a fire or flood. Many were intentionally killed by humans, who feared them for a variety of reasons. Nowadays, the ways bats die has changed. 

They still die from those old causes. But new reasons for death, some related to human activity, have come to dominate the reports of bat mortality.

Paul Cryan, a research biologist with the U.S. Geological Survey in Fort Collins, investigated the causes of bat death today and historically.

When Cryan looked at reports of bat deaths worldwide, what he found was a striking change in what was killing bats - at least in the deaths that are being reported. The change started around 2000, when wind turbines came on the scene. Bats, for reasons that are still somewhat unclear, sometimes fly into turbines and die.

Now that there are more wind farms, they do this even more often. Since 2000, wind turbines have become the cause for more than 35 percent of reported bat mortality events worldwide, Cryan and his co-authors report in their paper, published in the journal Mammal Review.

The other big killer in recent years is white nose syndrome, a fungus that has devastated bat populations in North America since the early 2000s. (It does not have the same effect on bats in Europe.) Cryan researches white nose as well, and he and others believe the fungus will continue to spread west across North America , continuing to devastate the bats it infects.

Since the study looked at nine different ways bats could die, Cryan was struck by the lack of one type of death: viral and bacterial disease. Bats are known for carrying and spreading disease, but it seems those diseases rarely kill them.

"I think the one that really jumped out at us was the lack of evidence for these previous disease-related die-offs… we just didn't really find that," Cryan said.

This might hold an interesting clue for scientists, Cryan speculated. Understanding why bats are less susceptible to diseases that often plague mammals that congregate in high numbers in small spaces (think humans in cities) could help researchers better understand diseases that affect humans.

"There's probably a lot these bats still have to teach us about how they deal with these infectious diseases, how they stay healthy."

The other big takeaway, said Cryan, is that humans have the power to change the deaths caused by them. If wind turbines have become a big bat killer - policy and science can work to change that.

"We have seen an uptick in human caused mortality in bats and those are the places where we can focus our efforts and make a difference," Cryan said.

Stephanie Paige Ogburn has been reporting from Colorado for more than five years, primarily from the Western Slope.
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