Bats help grow our crops, but climate change has them on the move
Rising temperatures are shifting bat migration patterns, with possible consequences for farmers.
First, let’s dispense with the myths. Bats aren’t blind. They won’t fly into your hair. The vast majority do not carry rabies. Bats are not flying mice. Only one bat — the vampire bat — drinks blood, and it’s not likely to be yours. Finally, if someone says you have bats in your belfry, it’s an insult to both you and to bats.
Bats deserve respect. We need them. They feast on insects that destroy crops and leave behind guano, an excellent fertilizer. Bats also pollinate plants like cocoa, banana, mango and agave. The vampire bat produces a powerful anti-coagulant in its saliva that’s used in a human drug to prevent strokes. Oh, and they just so happen to be the only mammals that can fly.
“Every time you eat corn-on-the-cob, you can thank bats for their role in managing populations of the corn earworm moth,” said Phillip Stepanian, a meteorologist with Rothamsted Research who studies the world’s largest colony of bats at Bracken Cave in southern Texas. “Even if we can’t convince you that bats are adorable, it’s hard to deny their important role in agriculture. Bats really are unsung heroes in our food production systems, and it’s our responsibility to understand the forces that threaten them.”
Today, climate change is one of those forces. It’s prompting bats — like many animal species in recent years — to change their behavior to adapt to a warming planet. Those small changes are sending ripples through ecosystems.
“Ecosystems are a complex web of connections, and it is difficult to change one component of the system without affecting the larger system,” Stepanian said. “When we observe a change in bat behavior, we have to wonder if it is an effect of some other change and whether it will result in additional changes elsewhere in the ecosystem.”
Data from a weather radar indicate that bats are migrating to Bracken Cave from Mexico roughly two weeks earlier than they did in 1995, arriving mid-March rather than late March. Some of them are also hanging around through winter instead of returning south, according to a new study in Global Change Biology authored by Stepanian and Charlotte Wainwright, also a Rothamsted meteorologist.
“When the radar detects bats flying out of the cave, it is because the bats are going out hunting for flying insects,” Stepanian explained. “During the winter, we wouldn’t expect to find many insects flying around, which is why most bats migrate back south where temperatures are warm and insects are abundant. When we see bats arriving earlier in the spring, or remaining over the winter, it suggests they have enough food to support them.”
It’s likely that more insects are surviving the winter —supporting more year-round bat residents in Texas— and that warmer overall temperatures are spurring those that do leave for Mexico to return to Texas earlier, according to Stepanian.
This insight into bat behavior was a surprising finding, since the researchers’ initial goal was to see whether they could monitor bat populations remotely without disturbing the colony. Stepanian said the subject is worthy of further study.
“When we observe a change in bat behavior, we have to wonder if it is an effect of some other change, and whether it will result in additional changes elsewhere,” he said. “For example, will changes in the timing of bat migration have an effect on their ability to regulate pest populations? Will farmers have to compensate by using more insecticides — and what further effects would that have on the ecosystem? At this point, we really don’t know.”
Bracken Cave is the summer home of more than 15 million Mexican free-tailed bats (Tadarida brasiliensis), and is the world’s largest bat colony, according to Bat Conservation International (BCI), which manages the cave near San Antonio. “The bats at Bracken Cave are an incredible natural phenomenon, and it is fascinating how we can monitor changes in their behavior over time using weather radar,” said Winifred Frick, BCI’s chief scientist.
In recent years, the cave has come under increasing stress from urbanization. BCI bought the land where the cave is located in 1992, and — with donations from its supporters — has continued to buy surrounding land to protect the bats and other native species from encroaching subdivisions.
“The nightly emergence of bats from Bracken Cave is a wondrous sight,” Frick said. “Millions of bats fly out of the cave and take to the night skies. Watching millions of bats fly out en masse is a spectacular event that is hard to describe, but one that you never forget once you’ve witnessed it.”
Stepanian agreed. “People don’t often encounter bats, certainly not as often as birds or butterflies, so they often get lumped in with other mysterious things that go bump in the night,” he said. “But public perception is changing. We often don’t get a chance to experience the epic scale of natural phenomena in our everyday life, and it’s easy to forget how spectacular our world is.
“These mega bat populations in the south-central United States give people access to a natural wonder,” he added. “After witnessing millions of bats taking flight into the sunset — just as they’ve done for thousands of years — it’s easy to appreciate how important they are.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.