As acacia ants have dwindled, elephants have been able to knock down and eat more whistling thorn trees. With fewer trees, lions have lost the cover they rely on to stealthily attack zebras, their primary prey.
The study, which was published Jan. 25 in the journal Science, found that lions are less likely to kill zebras in uncovered areas. They killed 25 percent fewer zebras between 2003 and 2020, the study said.
“When you have invasive species, they can affect other species that wouldn’t seem to have anything to do with the species invasion in kind of weird, unpredictable ways — ways that are hidden, but very serious,” study co-author Jacob Goheen, a University of Wyoming zoology professor, told The Washington Post.
The findings were a result of years of research on animals and insects in a nature conservancy in Kenya. A study from March 2015 found that whistling thorn trees, which are native to East Africa and can grow about 18 feet tall, were suffering after big-headed ants had invaded the Ol Pejeta Conservancy in Kenya’s Laikipia County.
Acacia ants live in whistling thorn trees, which provide them nectar. In return, the ants protect the trees from predators — a symbiotic relationship. But after big-headed ants kill acacia ants and eat their eggs, larvae and pupae, elephants are no longer warded off and they can break down more trees in those areas. Roughly 75 percent of the conservancy’s trees have vanished, Goheen said.
Another study published in May 2019 found that lions killed more zebras in areas of the Ol Pejeta Conservancy that had more tree cover. Lions typically hunt zebras by hiding behind trees and chasing them when they approach.
Researchers from both studies worked together to try to connect their findings to determine whether the invasive ants were affecting lions’ behavior.
In May 2018, researchers attached GPS collars to six lions in the Ol Pejeta Conservancy. Using computers, researchers then tracked the lions’ locations in the roughly 140-square-mile protected savannah.
Lions usually moved fast, and sometimes in groups, when they were chasing prey. So when researchers saw a cluster of red tracking points on their computers — indicating that multiple lions were moving quickly around the same area — they predicted that lions had hunted prey there. Within a few days, researchers drove to the area to record which animal the lions had killed and how much tree cover was nearby. They drove with an armed security guard in case an animal attacked them, researcher Douglas Kamaru said.
Researchers continued that routine for about two years, determining that the six lions they were tracking had killed about 140 animals. Roughly half of those kills were zebras, which lions were more than two times more likely to kill in areas with tree protection than in uncovered areas, the study said.
Despite lions eating fewer zebras, researchers don’t expect the vulnerable species’ population to dwindle in the conservatory. That’s because lions found a new food source: African buffalo.
While zebras usually run away when they spot lions, buffalo often fight back, making cover less important before the hunt. Still, buffalo aren’t lions’ first choice for food, Goheen said. Killing a zebra takes one or two lions; killing a buffalo takes between five and 10, Goheen said.
As big-headed ants continue to expand in the area, researchers said other problems could emerge. For example, endangered black rhinos eat whistling thorn trees but could lose their food source, study co-author Corinna Riginos said.
“The big-headed ant invasion is kind of changing everything, and it takes time for all these dynamics to play out,” Riginos said. “We’ve already seen evidence of big changes. What happens next?”
