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

Abstract

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.

Automated detection of an insect‐induced keystone vegetation phenotype using airborne LiDAR

“…In practical ecological and conservation applications, ecologists and managers are typically less concerned with a model's reported accuracy than its ability to recapitulate ecosystem dynamics. Ground-truthing 16 ha UAV flights would be immensely convenient for tracking the ecosystem health of this savanna, as changes in ant occupancy are closely linked to savanna productivity, herbivore activity and carbon cycling (Milligan et al, 2021;Palmer et al, 2008Palmer et al, , 2010.…”

“…Such an application of our protocol is timely and relevant: an invasive ant species, the big-headed ant (Pheidole megacephala), established an invasion front less than 2 km from our plot. Pheidole megacephala out-competes all four mutualist ant species and does not defend acacias from browsers, leading to increased herbivore and pathogen damage and loss in above-ground carbon sequestration (Milligan et al, 2021(Milligan et al, , 2022. Such shifting invasion fronts are difficult to detect on the ground but could be delineated from monitoring the retreat of C. nigriceps-occupied trees with future LiDAR surveys.…”

“…Changes in community composition could be indicative of drought-induced vegetation die-off, fire-induced succession events or invasive species incursion (e.g. invasive Pheidole megacephala ants out-competes symbiotic ants, leading to increased herbivore and pathogen damage and loss in above-ground carbon sequestration in the region; see Milligan et al, 2021Milligan et al, , 2022. Therefore, monitoring the ant occupancy of each tree can be used as a means of assessing ecosystem stability (Câmara et al, 2019;Houadria & Menzel, 2017;Vandermeer et al, 2008).…”

Epiphytic ant‐plant obtains nitrogen from both native and invasive ant inhabitants

“…The invasive electric or little fire ant, Wasmannia auropunctata , from South America displaces the native Tetraponera aethiops from the domatia of Barteria fistulosa in Gabon, resulting in an increased incidence of lianas encroaching on host trees (Mikissa et al, 2013). Similarly, the African big‐headed ant Pheidole megacephala displaces the native ant inhabitants of Vachellia drepanolobium (Visitacao, 2011) and causes a marked increase in the level of elephant herbivory to the plants by failing to defend plants (Riginos et al, 2015) even resulting in a decrease of tree‐level photosynthesis (Milligan et al, 2021). To the best of our knowledge, there is no published research examining how an invasive ant affects a nutrient‐provisioning ant‐plant mutualism.…”

“…ant inhabitants of Vachellia drepanolobium (Visitacao, 2011) and causes a marked increase in the level of elephant herbivory to the plants by failing to defend plants (Riginos et al, 2015) even resulting in a decrease of tree-level photosynthesis (Milligan et al, 2021). To the best of our knowledge, there is no published research examining how an invasive ant affects a nutrient-provisioning ant-plant mutualism.…”