Bats are indifferent to whether we're playing soccer, baseball, or beach volleyball under our stadium lights. They only care about the game of catch they're playing with all the bugs attracted to the glow. As bats stuff themselves on swarms of sports-adjacent insects, though, our stadiums may be aiding certain bat species and wiping others out. Any bat that's willing to visit a lit-up sports stadium will find a bug bonanza there, says Corrie Schoeman, an ecologist at the University of KwaZulu-Natal in Durban, South Africa. But some bat species prefer to stay away from cities and lights. Could stadiums shift an area's ecological balance? To tackle the question, Schoeman visited seven sports stadiums around the city of Durban. Schoeman set up sound recording equipment near the bases of stadium lights. He gathered two-hour recordings on 12 nights at each stadium—half with the lights on, and half on nights when the stadium was dark. He also divided his recordings between the wet and dry seasons to see whether the time of year mattered. The first thing Schoeman discovered was much more bat activity at lit stadiums. When the lights were on—maybe for a home game or an evening practice—there was more bat activity in general. There was also more hunting in particular, which Schoeman could identify by the buzzing echolocation call a bat makes as it closes in on prey. The time of year didn't matter; neither did the type of land around the stadium. All that mattered was whether those big bulbs were switched on. Schoeman captured the sounds of nearly 4,000 bat flights over his equipment. Picking apart the squeaks, clicks and chirps, Schoeman could assign almost 90 percent of those calls to a species. He grouped the bats into three categories: "Urban avoiders" are species that stick to forests and caves. They usually don't fly very fast, and they like to hunt under the cover of dense vegetation. "Urban adapters" hunt along the edges of forests, and may take advantage of human spaces and structures. And "urban exploiters" are species that like to fly out in the open and roost in cities. Schoeman identified 12 bat species at the stadiums. Among these, four species stood out. They were present at every stadium and accounted for about 80 percent of the total bat activity he recorded, Schoeman says. All four dominant species—Chaerephon pumilus, Tadarida aegyptiaca, Otomops martiensseni and Scotophilus dinganii—are urban exploiters. It didn't surprise Schoeman to find these species taking advantage of stadium lights. "By definition...urban exploiters should utilize lit stadiums," he says. Urban avoiders, on the other hand, may stay away from these bright, open spaces because they're wary of predatory birds. And stadiums might not be the only human light sources providing food for urban exploiter bats, Schoeman says. Construction sites, for example, are another source of nighttime glow. These sites are obviously a home run for some bats. But they could be a problem for others, he says. If only a few urban bat species are benefiting from stadiums and similar light sources, the fitness of all other local bat species could go down. As stadium-loving bats thrive, they could outcompete shyer species. Certain bats could become locally extinct, Schoeman says. This would mean less biodiversity in the city overall. And that's an outcome with few fans.
Image: by swxxii (via Flickr)
Schoeman, M. (2015). Light pollution at stadiums favors urban exploiter bats Animal Conservation DOI: 10.1111/acv.12220