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Biological invasions can lead to the reassembly of communities and understanding and predicting the impacts of exotic species on community structure and functioning are a key challenge in ecology. We investigated the impact of a predatory species of invasive ant,Pheidole megacephala, on the structure and function of a foundational mutualism betweenAcacia drepanolobiumand its associated acacia‐ant community in an East African savanna. Invasion byP. megacephalawas associated with the extirpation of three extrafloral nectar‐dependentCrematogasteracacia ant species and strong increases in the abundance of a competitively subordinate and locally rare acacia ant species,Tetraponera penzigi, which does not depend on host plant nectar. Using a combination of long‐term monitoring of invasion dynamics, observations and experiments, we demonstrate thatP. megacephaladirectly and indirectly facilitatesT. penzigiby reducing the abundance ofT. penzigi’s competitors (Crematogasterspp.), imposing recruitment limitation on these competitors, and generating a landscape of low‐reward host plants that favor colonization and establishment by the strongly dispersingT. penzigi. Seasonal variation in use of host plants byP. megacephalamay further increase the persistence ofT. penzigicolonies in invaded habitat. The persistence of theT. penzigi–A. drepanolobiumsymbiosis in invaded areas afforded host plants some protection against herbivory by elephants (Loxodonta africana), a key browser that reduces tree cover. However, elephant damage onT. penzigi‐occupied trees was higher in invaded than in uninvaded areas, likely owing to reducedT. penzigicolony size in invaded habitats. Our results reveal the mechanisms underlying the disruption of this mutualism and suggest thatP. megacephalainvasion may drive long‐term declines in tree cover, despite the partial persistence of the ant–acacia symbiosis in invaded areas.