NSF Award Search: Award # 1556905 (2024)

The invasive big-headed ant (Pheidole megacephala) is displacing native ant species in the savannas of East Africa. The treeAcacia drepanolobiumcovers large parts of the region and is a foundational species that provides critical habitat and forage for a diversity of large mammalian wildlife. The big-headed ant, however, is disrupting the mutualism betweenA. drepanolobiumand nativeCrematogasterants that typically live on the trees and protect them against mammalian herbivores, including elephants. In this study, conducted on Ol Pejeta Conservancy in central Kenya, we experimentally tested the separate and interacting effects of big-headed ant invasion and herbivory (using fence-out herbivory exclosures in invaded and uninvaded areas to simulate the prevailing land management scenarios of presence vs. absence of wildlife) onA. drepanolobiumtree growth and demography and assessed some of the cascading impacts of the invasion on wildlife.

While we predicted that the big-headed ant invasion and herbivory would have a strong interactive effect on tree demography, our demographic models based on 4 years of field data showed that herbivory is the overwhelming factor predicted to cause steep declines in tree numbers over the next 10 years. However, the presence of big-headed ants is predicted to cause additional decline in tree numbers relative to uninvaded areas. Trees in uninvaded areas had reproductive values2-3[TP1]times higher than trees in uninvaded areas. At these rates of decline in tree numbers, areas such as Ol Pejeta Conservancy are at serious risk of losing theirA. drepanolobiumtrees due to high herbivory and the exacerbating effects of the big-headed ant invasion. This would have strongly negative consequences for browsing wildlife such as black rhino, giraffes, elephants, and a variety of antelope species. Short-term protection of trees from herbivory, on the other hand, allowed even invaded trees to grow rapidly and may need to be incorporated into the management of wildlife-present properties and parks to maintain some trees.

We also found that trees in invaded areas, when exposed to herbivory, were photosynthesizing at a lower rate than trees in uninvaded areas. Invaded trees exposed to herbivory are visibly sickly, and our observations suggest that big-headed ants impact tree root health perhaps by nesting in the root system. When protected from herbivory, the trees appear to cope with this impact, but with the added stress of herbivory they are not doing well. We also found that invaded trees were fixing significantly less nitrogen from the atmosphere, likely because of the impacts of big-headed ants on tree roots and energy stores. Acacia trees play an important role in the savanna system?s nitrogen cycle, and the long-term effects of fewer trees fixing less nitrogen could be profound for grazing wildlife and livestock, since grasses benefit greatly from the nitrogen fixed by trees.

At present, however, the big-headed ant invasion is benefiting some grazing wildlife, especially plains zebra. Using survey counts and camera traps, we found that zebras prefer to be in invaded areas, likely because they feel safer in areas with higher visibility (fewer trees and less grass). We collaborated with Ol Pejeta Conservancy to fit GPS collars on X lions of different prides, and we used collar data to locate 147 kill sites. We found that zebras were less likely to be killed in areas invaded by big-headed ants, largely because of the effect of the invasive ants on habitat visibility. Thus, the ant invasion is having cascading effects through the ecosystem that alter even the behavior of top predators and their primary prey.

We did find one optimistic twist to the story of the big-headed ant invasion. While these ants extirpate the dominant native ants (Crematogaster mimosaeandC. nigriceps), a third species of native ant (Tetraponera penzigi) manages to persist and co-occupy trees with big-headed ants thanks to its non-aggressive behavior. This species is normally only present on a small fraction of trees, but in invaded areas we found it present on 12 times more trees. This ant species provides the tree with some protection from elephant herbivory, which may serve to reduce rates of tree decline within invaded landscapes.

Finally, with the help of NSF funding, we trained one postdoc, one Fulbright scholar, one Kenyan MSc student, one Kenyan research associate, and four Kenyan field technicians. All participants received training in insect and plant ID, and the design and implementation of large-scale ecological experiments.

Last Modified: 08/29/2022
Modified by: ToddMPalmer