Mutualism disruption by an invasive ant reduces carbon fixation for a foundational East African ant‐plant

Invasive ants shape assemblages and interactions of native species, but their effect on fundamental ecological processes is poorly understood. In East Africa, Pheidole megacephala ants have invaded monodominant stands of the ant‐tree Acacia drepanolobium, extirpating native ant defenders and rendering trees vulnerable to canopy damage by vertebrate herbivores. We used experiments and observations to quantify direct and interactive effects of invasive ants and large herbivores on A. drepanolobium photosynthesis over a 2‐year period. Trees that had been invaded for ≥ 5 years exhibited 69% lower whole‐tree photosynthesis during key growing seasons, resulting from interaction between invasive ants and vertebrate herbivores that caused leaf‐ and canopy‐level photosynthesis declines. We also surveyed trees shortly before and after invasion, finding that recent invasion induced only minor changes in leaf physiology. Our results from individual trees likely scale up, highlighting the potential of invasive species to alter ecosystem‐level carbon fixation and other biogeochemical cycles. In our study, we investigated how a foundational ant‐plant mutualism between Acacia drepanolobium and native ant partners [1] can be disrupted by the introduction of a non‐native ant that extirpates native ants but does not engage in mutualism with the host plant [2]. A combination of natural experiments and surveys of sites that been invaded for >5 years showed that invasion caused large declines (ca. 69%) in whole‐canopy photosynthesis during the wet season, but that decline may lag >5 years after the initial introduction of the non‐native ant [3]. Further, a field experiment manipulating the occurrence of ants and elephants showed that large herbivores were the proximate driver of this photosynthesis decline [4], likely because invaded trees were vulnerable to chronic and severe herbivory because of the loss of their native mutualist ant partners. This large reduction in carbon fixation is broadly relevant in black cotton savannas, in which A. drepanolobium is a foundation tree species that serves many biogeochemical and ecological roles, and our findings also demonstrate how biological invasions can drive changes in carbon cycling by altering native species interactions. Photo credit: Macro photography [1,2] by Ben Cherry.