The spread of the deadly white-nose syndrome, which has decimated U.S. bat populations in the Northeast and Midwest, appears to be tied to an increase in infant mortality that claimed more than 1,000 lives, according to new research published in Science.

The study provides an unusually clear link between biodiversity and human health. Bats “add value to society in their role as natural pesticides, and this study shows that their decline can be harmful to humans,” says Eyal Frank, an environmental economist at the University of Chicago and the study’s author.

Frank has established himself as an economist with a knack for quantifying the ways that a species’ decline can become a matter of life and death for people. Ecologists often talk about the ways human health is tied to environmental conditions, such as the emergence of diseases that originate from wildlife as deforestation eats away at their habitat. But it can be hard to pinpoint a clear cause-and-effect relationship when ecological systems are so complex. Frank looks for cases that enable him to compare what happens to places that suffer some environmental loss with similar places that don’t. This is referred to as a “natural experiment” that approximates the kind of control scientists can get in the lab.

He was the co-author of a recent study that tied a crash in vulture populations to more than 500,000 deaths in India. In that case, the birds provided a valuable health service by feasting on rotting livestock carcasses. In places where the winged cleanup crews vanished, death rates rose. The likely reasons: water pollution worsened, and rabies cases increased as feral dogs filled the niche left by vultures.

A somewhat similar dynamic appears to be at play with the bats. People have extolled the benefits of bats that feed on insects, because they help control crop-damaging pests. That, in turn, could mean that farmers have to spray their fields with less synthetic pesticides that can be poisonous to people.

While it makes sense in theory, it’s another thing to demonstrate clear causal connections. Maybe bats don’t eat enough bugs to make a big difference. Maybe farmers respond in other ways than chemical warfare if bats disappear. Or maybe the additional pesticides don’t have a clear effect on people’s health.

The spread of white-nose syndrome provided Frank with the kind of natural experiment he seeks. The syndrome is caused by a fungus, Pseudogymnoascus destructans. Native to Europe, it showed up in the U.S. in the early 2000s. Scientists in upstate New York discovered caves where bats hibernate were littered with the corpses of the little mammals. Some survivors had a fuzzy white coating around their noses, hence the name.

Since that initial discovery, researchers have concluded that the fungus kills when it irritates the hibernating bats enough that they rouse in midwinter. European bats appear to have adapted to the fungus, but many in the U.S. aren’t. The awakened bats burn crucial energy stores and starve. The fungus can decimate local bat populations, killing as many as 99% of a species. On average, nearly 75% of bats in a colony are killed off.

Because this disaster unfolded gradually, hitting some places but not others, Frank was able to compare what happened when white-nose syndrome came to town. The effect was both clear and grim.

In counties where bat infections were confirmed, insecticide use rose by 31% on average. There was no similar change in nearby counties with no known cases of white-nose syndrome. There was also no comparable increase in the use of other chemical tools such as herbicides that target plants, further evidence that the bat disappearance led to a surge in insect problems.

At the same time, counties hit by bat die-offs also witnessed an 8% increase in infant mortality – not counting deaths from homicides or accidents such as car crashes. Counties without white-nose saw no similar increase. Other social factors, such as unemployment, couldn’t explain the pattern. Frank calculated that added up to an additional 1334 infant deaths between 2006 and 2017 in 245 U.S. counties.In addition to the deaths, the loss of the bats translated into financial losses for farmers totaling nearly $27 billion due to a loss of crop sales, Frank found.  “When bats are no longer there to do their job in controlling insects, the costs to society are very large,” he noted. “The cost of conserving bat populations is likely smaller.”

Much of the news about white-nose syndrome is grim. The fungus has now appeared across the U.S. Earlier this year, the National Park Service announced the fungus turned up in New Mexico’s famous Carlsbad Caverns.

But there are some glimmers of hope. Among other things, scientists are testing a possible vaccine for bats; using UV light to clear the fungus from caves; spraying caves with antifungal chemicals; treating bats with probiotics; and even building artificial caves where cooler conditions appear to help bats endure the fungus.

If scientists are questioned about whether all this effort is worth it to save animals that often provoke fear or disgust, now they can point to other reasons for the work: More than $27 billion and 1,300 children.

Frank, E. “The economic impacts of ecosystem disruptions: Costs from substituting biological pest control.” Science. Sept. 6, 2024.

Image: Bureau of Land Management via Flickr