Credit: Flickr, Steve.

Not a lot of people know this, but bats provide excellent pest control that saves farmers billions each year. A new study, however, suggests that climate change is making migratory bats arrive in Texas (from Mexico) earlier than they used to two decades ago. This is a dangerous pattern: the bats risk not finding enough food since the insects and other creatures they prey on haven’t hatched or migrated themselves yet. This poor timing could significantly reduce bat populations, jeopardizing farming operations in the process.

Scientists at Rothamsted Research, an agricultural laboratory in England, initially analyzed radar data from some 160 U.S. weather stations to investigate how accurate radar is for gauging bat colony numbers and movements. When the bats emerge for their night-time foraging, they can cover the sky in massive clouds which show up on radar. However, over the course of their study, the researchers gained a more important insight — that bats were leaving their winter homes in Mexico earlier and reproduced sooner than they had two decades ago. The data that the researchers investigated tracked bat activity in Texas from 1995 through 2017.

According to Phillip Stepanian, Rothamsted meteorologist and co-author of the new study, this behavior coincides with warming temperatures experienced over the last decades.

“This was very surprising,” the researcher told Scientific American. “We weren’t out looking for climate change,” he says, “but then it suddenly became very obvious.”

What’s more, the researchers also found that more and more bats are overwintering at the Bracken Cave near San Antonio, Texas, rather than going back to their cold weather quarters in Mexico. This sort of behavior is unprecedented since the first bat survey in the area began in 1957. According to Stepanian, overwintering is another sign of altering behavior due to warming temperatures. Previously, a study on migratory bats in Indiana found that temperature variations also affected arrival and departure times. Early arrival at their summer nests can expose the bats to cold snaps that might freeze them to death — that’s besides not finding enough food.

“These bats spend every night hard at work for local farmers, consuming over half of their own weight in insects, many of which are harmful agricultural pests, such as the noctuid moths, corn earworm and fall armyworm,” said Charlotte Wainwright, co-author of the new study.

“We found that the bats are migrating to Texas roughly two weeks earlier than they were 22 years ago. They now arrive, on average, in mid March rather than late March,” the scientist added.

In such conditions, the bats might struggle to feed their pups or might even decide to skip reproduction entirely. Overall, this will lead to fewer and fewer bats, to the point that some Midwestern bats may be threatened with extinction.

Such declines could have grave consequences for human activity, particularly agricultural production. By one estimate, bats indirectly contribute around $23 billion to the U.S. economy by controlling pests such as plant-eating insects or by eating bugs that prey on pollinators. In the future, the researchers plan to further investigate the link between climate change and shifting bat migratory patterns. They also hope that weather radar networks around the world can be integrated to provide a continent-wide survey of bat populations.

Scientific reference: Phillip M. Stepanian, Charlotte E. Wainwright. Ongoing changes in migration phenology and winter residency at Bracken Bat Cave. Global Change Biology, 2018; DOI: 10.1111/gcb.14051