When the ants invade, lions stop killing zebras

Invasive ants are a big problem all over the world – and the big-headed ant is a particularly vexatious species.

Now research has shown how these little ants are upending a Kenyan ecosystem, causing lions to kill fewer zebras.

The ants are pushing out a native species that protects local trees, leading to reduced tree cover and making it harder for lions to hunt their preferred prey, according to a team of Kenyan and US researchers.

“These tiny invaders are cryptically pulling on the ties that bind an African ecosystem together, determining who is eaten and where,” says Professor Todd Palmer, an ecologist at the University of Florida, US.

The research is published in Science.

It revolves around the whistling-thorn tree (Vachellia Drepanolobium), a species of acacia that covers between tens to hundreds of thousands of square kilometres of East Africa.

This tree enjoys a mutual relationship with the native acacia ant (Crematogaster spp.). The ants use the trees’ food and shelter in exchange for dissuading herbivores, especially elephants, from eating the leaves with their painful bites.

“Much to our surprise, we found that these little ants serve as incredibly strong defenders and were essentially stabilizing the tree cover in these landscapes, making it possible for the acacia trees to persist in a place with so many big plant-eating mammals,” says Palmer.

In the last 20 years, another species called the big-headed ant (Pheidole megacephala), originally from an Indian Ocean island, has invaded the ecosystem around Laikipia, Kenya. These ants exterminate native acacia ant populations by killing adult ants and eating eggs and pupa, but they don’t protect the whistling thorn tree.

Two photos of landscape, photo a has more trees than photo b
An area not invaded by big headed ants (A) and an invaded area – note the difference in tree cover. Credit: Todd Palmer

Areas with invasive ant populations see 5-7 times as many trees broken and eaten by elephants.

“Oftentimes, we find it’s the little things that rule the world,” says Palmer. “These tiny invasive ants showed up maybe 15 years ago, and none of us noticed because they aren’t aggressive toward big critters, including people. We now see they are transforming landscapes in very subtle ways but with devastating effects.”

So what does this all have to do with lions? The current study, led by graduate student Douglas Kamaru, focusses on lion and zebra behaviour in Ol Pejeta Conservancy, Kenya.

Dry landscape with elephants
Elephants navigate a landscape invaded by big-headed ants at the Ol Pejeta Conservancy. Credit: Brandon Hays

The researchers investigated areas both with and without the big-headed ant and found that lions killed fewer zebras in invaded areas. This makes sense, since lions are ambush hunters: if there’s less tree cover, the zebras can see lions from further away.

Curiously, the lion population hasn’t declined in invaded areas. Instead, the researchers found that lions were switching their primary prey from zebras to the more dangerous buffalo.

“Nature is clever, and critters like lions tend to find solutions to the problems they face,” says Palmer.

“But we don’t yet know what could result from this profound switch in the lions’ hunting strategy. We are keenly interested in following up on this story.”

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