The recent big-headed ant (Pheidole megacephala) invasion into the Laikipia Plateau has disrupted the mutualism between 4 arboreal ant species and a foundational east African myrmecophyte, (whistling-thorn acacia; Acacia [Vachellia] drepanolobium) that comprises virtually all woody cover in “black cotton” savannas. Initial surveys indicated that this ant causes wholesale extirpation of native ant mutualists, rendering trees vulnerable to destruction by megaherbivores (Riginos et al. 2015), and causing large declines in woody plant photosynthesis (Milligan et al. 2017, ESA). We conducted natural and manipulative experiments over two years to test the specific mechanisms whereby native and invasive ants alter the ecophysiology of this widespread savanna tree species. To attribute these changes to invasion, we measured leaf photochemistry and water relations in a before-after-control-impact design at multiple invasion and control sites in both wet and dry seasons. We also excluded herbivores and all ants from invaded and non-invaded acacia communities to measure their effects on tree physiology. Finally, we conducted a greenhouse experiment that excluded invasive ants from roots or roots and canopies of potted saplings, and recorded changes in leaf photochemistry, water relations, stem and root non-structural carbohydrate concentrations, and total biomass.
Results/Conclusions
In our natural experiment, big-headed ant invasion corresponded to a significant increase in net photosynthesis per-unit-leaf-area in the subsequent wet season (May 2018), and a significant decrease in leaf water stress. These differences disappeared during the subsequent dry season (September 2018). Our manipulative experiment revealed substantially lower canopy-level photosynthesis in invaded communities and large declines in photosynthesis per-unit-area in invaded tree communities that were exposed to vertebrate herbivores. The experimental removal of invasive ants did not impact adult host tree physiology. However, our greenhouse experiment showed that nesting by invasive ants in acacia sapling roots significantly increased water stress, decreased net canopy photosynthesis, and decreased root and leaf biomass. Our natural experiment results suggest that this invasive ant (which does not feed on nectar) induces a short-term increase in carbon assimilation in mature acacia trees, perhaps due to the displacement of costly nectar-demanding native ants. However, dramatic long-term reduction in carbon assimilation, in both greenhouse saplings and invaded tree stands exposed to ungulate herbivores, suggests that this biological invasion negatively affects sapling recruitment and carbon sequestration by acacias in this ecosystem.