In 2006, a deadly fungus started killing bat colonies across the United States. Now, an environmental economist has linked their loss to the deaths of more than 1,300 children. The study, published in Science on Thursday, found that farmers dramatically increased pesticide use after the bat die-offs, which was in turn linked to an average infant mortality increase of nearly 8%. Unusually, the research suggests a causative link between human and bat wellbeing. “That’s just quite rare – to get good, empirical, grounded estimates of how much value the species is providing,” said environmental economist Charles Taylor from the Harvard Kennedy School, who was not involved in the study. “Putting actual numbers to it in a credible way is tough.” The crisis for bat colonies began in 2006, when a fungus called Pseudogymnoascus destructans hitchhiked from Europe to the US. P destructans grows on hibernating bats in winter, sprouting as white fuzz on their noses. It can extinguish a bat colony in as little as five years. When Eyal Frank, an assistant professor at the University of Chicago, learned about the disease, called white-nose syndrome, he realised it provided a perfect natural experiment to demonstrate the value of a bat. Bats eat 40% or more of their bodyweight in insects every night, including many crop pests. What would their disappearance mean? In infected areas, he found, farmers compensated for the loss of bats by significantly increasing their use of insecticides – by 31.1% on average. Next, Frank looked at infant mortality – a metric commonly used to judge the impact of environmental toxins. Infected counties had an infant death rate 7.9% higher, on average, than counties with healthy bats, despite pesticide use being within regulatory limits. That equates to 1,334 extra infant deaths. Frank tested other factors that might plausibly explain the rise in deaths: unemployment, the opioid epidemic, the weather, differences among mothers, or the introduction of genetically modified crops, but none explained the increase in pesticide use or the rise in infant deaths. He spent a year “kicking the tyres on the study”, and the results held. It provided “compelling evidence”, he said, “that farmers did respond to the decline in insect-eating bats, and that response had an adverse health impact on human infants”. It is unusual for a study of this type to suggest causation, not just correlation, said Taylor. “A lot of papers that try to link pesticides to outcomes are correlational in nature,” said Taylor. “People who are exposed to more pesticides, for example, might have other risk factors – like, farm workers are exposed to a whole host of other socioeconomic risks that could explain why there might be different health outcomes.” White-nose syndrome, however, essentially creates a randomised controlled trial: because the spread of white-nose syndrome was closely monitored, Frank could compare counties that had lost their bats with those the disease had not yet reached. “The bat disease wasn’t expected, and it shouldn’t have preferentially targeted certain groups over others,” Taylor said. A number of recent studies have shown how collapsing populations of wildlife can have unexpected knock-on effects for people. In June, Frank and another researcher estimated that the collapse of India’s vulture population may have resulted in 500,000 human deaths – because without the scavenging birds to eat rotting meat, rabies and other infections proliferated. The findings on pesticide use also echo previous research, including a study of Taylor’s. In the US, cicadas emerge en masse at intervals of 13 to 17 years. Taylor found that pesticide use increased in cicada seasons, as did infant mortality. People born in cicada years had lower test scores and were more likely to drop out of school. Columbia history professor David Rosner, who has spent his career investigating environmental toxins, said the study joins a body of evidence going back to the 1960s that pesticides adversely affect human health. “We’re dumping these synthetic materials into our environment, not knowing anything about what their impacts are going to be,” he said. “It’s not surprising – it’s just kind of shocking that we discover it every year.” Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on Twitter for all the latest news and features