In the last twenty years, big-headed ants (Pheidole megacephala) have progressively altered the ecology of black cotton savannas in Laikipia. This ant species, which has successfully invaded tropical and subtropical ecosystems around the world, likely spreads in untreated lumber, potted plants, and even industrial machinery and military equipment. Their invasions typically radiate from areas of human disturbance on the Laikipia landscape. In black cotton savannas, big-headed ants prey on native ants including Crematogaster acacia-ants. When native Crematogaster ants establish a colony on a host acacia (Acacia drepanolobium), they collect food rewards and nest in hollow cavities on the branches, and they deter large herbivores like elephants from inflicting substantial damage on the host plant. Big-headed ants exterminate these native ant defenders but do not provide any defense against herbivores; despite their high invasive potential, big-headed ants present as quite innocuous to humans. But over time, big-headed ants leave acacia trees vulnerable to heavy browsing by the large iconic herbivores found throughout black cotton savannas in Laikipia.
The effects of this invasion appear to cascade upward from the insect and plant community into higher trophic levels. Our research focuses on the ant-acacia A. drepanolobium because this tree dominates the overstory in black cotton savannas and serves various important ecosystem function roles that are unlikely to be replaced if its presence on the landscape is diminished through invasion. Invaded acacias no longer bear the substantial costs of housing and feeding their native ant defenders However, they suffer from heavy herbivory after invasion, reducing their photosynthesis by more than half. Without their biotic defense, these acacias pivot to a regrowth strategy that is insufficient for survival when herbivores are present. In just 10-20 years after big-headed ant invasion, entire groves of hundreds of acacia trees can be flattened into a grassland that no longer supports the browsing needs of black rhinoceros. PhD student Douglas Kamaru’s research at Ol Pejeta Conservancy (OPC) revealed further changes to predator-prey interactions: as acacia density declined and visibility increased in invaded areas, lions shifted from ambush-hunting smaller prey like plains zebra to targeting larger and more dangerous prey like buffalo.
This raises important questions: is this invasion causing irreversible changes? What strategies might manage or reverse the invasion? Our team has been investing in new tools and strategies to address these questions. We have built photographic monitoring systems in the roots of acacia study subjects at OPC (see attached video) that allow us to measure the decline in acacia roots as time since invasion increases, which may help predict acacia “starvation”. We are monitoring various acacia communities where big-headed ants died out in late 2023-early 2024 and native ants subsequently recolonized the surviving acacia trees. These die-offs appear to be driven by heavy rainfall, and we are now studying if an insecticide bait that specifically targets big-headed ants could be deployed following rains to effectively remove them from black cotton savannas and limit their future spread.
Universities: Iowa State University, University of Florida, University of Nevada-Reno (US National Science Foundation)
Principal Investigators: Dr. Jacob Goheen, Dr. Todd Palmer, Dr. Elizabeth Pringle