COLLABORATIVE RESEARCH: Landscape-scale consequences of mutualism disruption: invasive ants threaten a widespread ant-plant mutualism in East Africa

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Throughout the natural world, one can find pairs of species living together and providing services for one another to their mutual benefit. These species interactions are called mutualisms, and they are thought to maintain much of the world's biodiversity. Mutualisms are also the basis for important ecosystem services like seed dispersal, pollination, and nutrient cycling. But what happens when an invasive species displaces one of the species partners in a mutualism? That is the question this project will address in the savannas of Kenya, where an invasive ant species is displacing the native ants that live on acacia trees. Native ants aggressively defend trees by attacking elephants and other herbivores that eat trees; in exchange, trees provide food (sugary nectar) and shelter (swollen thorns) for the ants. In contrast, an invasive species, big-headed ants, kill native ants but do not defend trees. Without their native bodyguards, trees are being eaten and killed by elephants more frequently. This project will assess the rate and extent of tree loss associated with the big-headed ant species invasion. Because many species of wild herbivores, including endangered black rhinos, rely on trees in one way or another, this project also aims to understand how tree loss affects habitat for wildlife. This research provides a unique opportunity to assess the domino effects of a critical species mutualism breakdown in a landscape with an abundance of elephants, giraffes, and other iconic species of large mammals. This project will test the hypothesis that the cascading effects of the invasive ant species P. megacephala on the foundational mutualism between Acacia drepanolobium and native ants are context-dependent. In savannas invaded by P. megacephala, elephants are expected to reduce tree cover, thereby lowering soil and plant nitrogen (N) and negatively affecting forage and habitat quality for wildlife (since A. drepanolobium is an N-fixer). In contrast, where elephants are absent, P. megacephala invasion should accelerate tree growth, as P. megacephala does not induce nectar production and thus may reduce metabolic costs to host plants. Over the long-term, this mutualism breakdown is predicted to lead to higher tree cover and soil N in invaded areas. This project will (1) quantify the effects of native versus invasive ants on growth, N-fixation, reproduction and biomass of trees in the presence and absence of wild herbivores, especially elephants; (2) use demographic models to project tree population growth rates; (3) use remote sensing to assess the rate of spread of the P. megacephala invasion in both wildlife-present and wildlife-extirpated areas; and (4) assess the effects of P. megacephala invasion on availability of soil and plant nutrients and resulting habitat use by wild herbivores.