Ants are making lions on the Kenyan savanna eat fewer zebras : Short Wave At the Ol Pejeta Conservancy, a wildlife preserve in central Kenya, lions and cheetahs mingle with zebras and elephants across many miles of savannah – grasslands with "whistling thorn" acacia trees dotting the landscape here and there. Twenty years ago, the savanna was littered with them. Then came invasive big-headed ants that killed native ants — and left the acacia trees vulnerable. Over time, elephants have knocked down many of the trees. That has altered the landscape — and the diets of other animals in the local food web.

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When tiny, invasive ants go marching in ... and alter an ecosystem

When tiny, invasive ants go marching in ... and alter an ecosystem

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An invasion of big-headed ants has changed the landscape at the Ol Pejeta Conservancy in Laikipia, Kenya. Elephants wander a landscape that has fewer trees and more open grasslands. Brandon Hays hide caption

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Brandon Hays

An invasion of big-headed ants has changed the landscape at the Ol Pejeta Conservancy in Laikipia, Kenya. Elephants wander a landscape that has fewer trees and more open grasslands.

Brandon Hays

At the Ol Pejeta Conservancy, a wildlife preserve in central Kenya, lions and cheetahs mingle with zebras and elephants across many miles of savannah – grasslands with "whistling thorn" acacia trees dotting the landscape here and there.

There used to be many more acacia trees.

Twenty years ago, the savannah was covered with them. These trees provided food and shelter for native acacia ants and in turn, the ants defended the trees against animals, like elephants, that would eat them. When elephants grabbed tree leaves, these native ants would swarm up the inside their trunks and bite them.

The whistling thorn trees and the acacia ants had a steady, mutualistic relationship and supported each other for many years. Then – an invasive ant disturbed their relationship, setting off a cascade of events that changed how elephants act and what lions eat, the effects of which are still playing out today. The chain of events is documented in a paper published Thursday in the journal Science.

Around twenty years ago, the invasive, big-headed ant appeared in the region, says Jacob Goheen, a professor in the Department of Zoology and Physiology at the University of Wyoming and co-author of the paper. "It showed up in people's houses and other centers of human activity. To the best of our knowledge, it was introduced in bushels of produce from somewhere in the Indian Ocean," he says.

Invasive, big-headed ants wipe out native acacia ant populations, killing adults and eating their eggs and young. Patrick D. Milligan hide caption

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Patrick D. Milligan

Invasive, big-headed ants wipe out native acacia ant populations, killing adults and eating their eggs and young.

Patrick D. Milligan

These big-headed ants are small – about a third of the size of the native acacia ants – but vicious. They form supercolonies with hundreds of thousands of ants, and kill entire populations of the native ants when they encounter them.

That leaves the acacia trees undefended — and susceptible to elephants.

Beyond eating the leaves, the elephants have been pulling down branches and knocking the trees over. "We are seeing these areas open up, from the dense acacia [cover] to open landscape, grasslands," says Douglas Kamaru, a PhD student in Goheen's research group at the University of Wyoming and co-author on the paper. He says 70-80% of the trees in the park have been cleared in the past twenty years.

This landscape transformation has made it harder for the lions in the park to catch zebras, their most common prey. Typically, the lion's hunting strategy relies on the element of surprise. They hide behind trees, stalk their prey and pounce. In more open landscapes, the zebras can see the lions coming and have time to escape.

Lions catch zebras more successfully in areas covered with acacia trees, where they can better hide and stalk their prey. Victoria Zero hide caption

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Victoria Zero

Lions catch zebras more successfully in areas covered with acacia trees, where they can better hide and stalk their prey.

Victoria Zero

Kamaru, Goheen and their team found the lions were almost 3 times more likely to catch zebras in wooded, tree-covered parts of the park – like more of the park used to be – than on the grasslands that have opened up recently.

These findings took years of field work and observations and experiments tease out.

"Ecology is really messy. It's often hard to study interactions among multiple species," says Kaitlyn Gaynor, an assistant professor in the Department of Zoology at the University of British Columbia. Gaynor was not involved in the study, though she wrote a commentary, published alongside the research, in Science. "What this study did, really elegantly, was follow a disturbance, step-by-step, through this complex web of interactions," they said, essentially providing scientific documentation for the butterfly effect, in this time and place.

As for the lions at the reserve, their populations are stable for now. There are still areas in the park with tree cover, where they're having decent luck catching zebras. There's also some evidence they're supplementing their declining zebra diet with more buffalo.

But the invasive, big-headed ants are still encroaching at a rate of about 160 feet each year, and it's not clear if they can be stopped.

The ants' impact extends beyond lions, for instance, to endangered black rhinos, which have relied on the now-dwindling acacia trees for food, Goheen, at the University of Wyoming, says. "One of the surprises about this [study] was that these ecological chain reactions, that are triggered by an invasive species, affect a bunch of other species that have seemingly very little to do with that ant-tree mutualism," says Goheen.

The researchers say they'll continue watching and measuring the changes in the area to see if efforts to control the ant invasion work, whether the lions continue adapting their diets successfully and how other animals on the reserve are affected by this cascade of ecological changes. Another wild card, they say, is the warming and drying climate, which could cause huge shifts to the ecosystem.

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Today's episode was produced by Rachel Carlson, Gustavo Contreras and Elena Burnett. It was edited by Rebecca Ramirez, Christopher Intagliata and Viet Le. Rebecca also fact-checked alongside Rachel. Stu Rushfield was the audio engineer.