Non-native ant species unsettles delicate savanna food ecosystem

Saturday February 10 2024

Ants milking aphids on a thistle. BIOSPHOTO | DENIS BRINGARD


In Kenya, an unassuming yet invasive ant species is disrupting the intricate balance of an ecosystem, ultimately influencing the hunting patterns of the continent’s most iconic predator, the lion.

A study reveals that a tiny and seemingly innocuous invasive ant species, identified as big-headed ants (Pheidole megacephala), is impacting tree cover in an East African wildlife area, making it harder for lions to hunt its preferred prey, zebra.

Led by University of Wyoming doctoral candidate and Kenyan scientist, Douglas Kamaru, the research reveals that the species of invasive ants has been eating up another ant species that has historically protected the acacia dominant tree species.

“The spread of the big-headed ant, one of the globe’s most widespread and ecologically impactful invaders, has sparked an ecological chain reaction that reduces the success by which lions can hunt their primary prey,” the researchers wrote in a study published last week in the journal Science.

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“We show that a tiny invader reconfigured predator-prey dynamics among iconic species,” wrote the researchers, led by UW PhD student Douglas Kamaru, who’s part of Prof Jacob Goheen’s research group in the UW Department of Zoology and Physiology.


Other members of the team were drawn from the University of Nairobi, Karatina University, The Nature Conservancy, the University of British Columbia, the University of Florida, Duke University, the University of Glasgow, the University of Nevada-Reno and the US Geological Survey.

In the middle of it all is the Whistling-Thorn Tree, which is historically protected from leaf-eating animals by a species of native ants that nests in the trees’ bulbous thorns and feeds on its nectar. In return, the native ants ferociously defend the trees from gigantic plant eaters, like elephants, giraffes and other herbivores by biting them and emitting formic acid — an arrangement called mutualism.

Then arrives the big-headed ant, which effectively kills these native acacia ants that protect the whistling-thorn tree. Big-headed ants overrun the place over the past two decades, do not protect the trees from elephants, allowing them to browse and break trees at five to seven times the rate of uninvaded areas.

Having lost their bodyguards, the acacia trees are being obliterated. Lions, which are ambush predators, rely on the tree cover to stalk and hide before pouncing on zebras. Less tree cover means lions are not as successful at ambushing prey.

The native ants are particularly effective at defending the trees against elephants, “thereby stabilising savanna tree cover across entire landscapes,” wrote the researchers.

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In published studies from the early 2000s, Todd Palmer, an ecologist and professor at the University of Florida, and co-principal investigator in this current study, began to unravel the complexities of this congenial relationship in East Africa between plant and animal species.

“Much to our surprise, we found that these little ants serve as incredibly strong defenders and were essentially stabilising 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,” Palmer said.

“Oftentimes, we find it’s the little things that rule the world,” Palmer said.

“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.”

The scientists found that the increasing openness across the landscape had led to a significant decrease in zebra kills across the landscape. Making the best out of a bad situation, lions have adapted by shifting their diets from zebras to African buffalo, which though equally vulnerable to lion predation in the invaded areas, are larger, hang out in groups, and more difficult to kill.

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“Although the invasion of big-headed ants has shaped the spatial distribution of zebra kills, and the frequency of zebra kills has declined over time, prey switching by lions to more formidable prey seems to have thus far prevented any cascading effects on lion numbers,” the researchers concluded.

Although the overall lion population has remained stable, the scientists say as big-headed ants advance across the landscape, it is yet not clear if this may further jeopardise populations of lions — a species already on the brink of endangerment.

Palmer notes the importance of understanding the cascading effects of the ant invasion on the ecosystem. The disruption caused by these seemingly insignificant ants underscores the delicate balance of nature, demonstrating that even minor changes can have far-reaching consequences.

“These ants are everywhere, especially in the tropics and subtropics. You can find them in your backyard, and it is people who are moving them around,” Palmer said.

“We are working with land managers to investigate interventions, including temporarily fencing out large herbivores, to minimise the impact of ant invaders on tree populations.”

The scientists say as they grapple with the intricate relationships within ecosystems, finding solutions to mitigate the impact of invasive species becomes crucial for the preservation of iconic landscapes and their inhabitants.