Disruption of an ant-plant mutualism shapes interactions between lions and their primary prey

1. What they did

In Laikipia, Kenya, whistling-thorn trees cover huge areas, protected from elephants by native acacia ants that live in their swollen thorns. An invasive big-headed ant is killing off these acacia ants. The authors asked how this affects lions and their main prey, plains zebra, at Ol Pejeta Conservancy.

They measured visibility for three years in experimental plots with and without big-headed ants, fenced or open to elephants and other megaherbivores. They GPS-collared six lionesses from different prides, found 55 zebra kills by visiting clusters of GPS positions, mapped zebra density and lion activity, and used path analysis to test which links explained where zebra were killed. They also examined 18 years of kill records from anti-poaching patrols.

2. What they found

• Where big-headed ants had invaded and elephants could browse, visibility after three years was 2.67 times higher (48.2 m versus 18.1 m), because elephants broke and killed the undefended trees.
• Across the landscape, invaded locations had 13.45 m greater visibility than uninvaded ones.
• Zebra kills were less likely where visibility was higher: at typical visibility, the probability of a zebra kill was 2.87 times higher in uninvaded than in invaded savanna.
• Higher visibility did not change zebra density or lion activity; it reduced kills by removing the cover lions use to ambush zebra.
• Lion activity was similar in invaded and uninvaded areas, and lion numbers (about 53 adults and subadults) stayed stable for 13 years.
• From 2003 to 2020, zebra fell from 67% to 42% of lion kills, while buffalo rose from 0% to 42%, as whistling-thorn cover declined. Zebra and buffalo densities showed no directional change from 2014 to 2020.

3. What they concluded

By breaking the ant-tree partnership, a tiny invasive ant lets elephants open up the savanna, which makes zebra harder for lions to catch. Lions appear to have compensated so far by switching to buffalo, more dangerous prey. Disrupted mutualisms may be an overlooked driver of predator-prey relationships, and whether lions can keep adjusting as the ants spread remains an open question.

4. How tracking was or was not used

Kill sites were found by visiting clusters of GPS positions from the collared lions, and the prey species at each was confirmed in the field within 5 to 10 days. Visibility and the presence of big-headed ants were recorded at each kill site. Long-term kill records came from kills found by anti-poaching patrols. No trailing is described.

5. Tracker or observer certification

Not reported. The paper does not say who identified prey at kill sites or how their skill was checked.

6. Other technologies, skills or innovations

• Electrified megaherbivore exclusion plots.
• Path analysis with d-separation tests to compare competing explanations.
• Data and code are publicly available on Dryad.

7. Implications for conservation (from the paper)

• One of the world’s worst invasive species can reshape a savanna and its predator-prey relationships, with effects that spread across whole landscapes. Prey switching by lions to buffalo may hold numbers steady for now, but the future is uncertain as the ants advance.