Abstract
Invasive ants shape assemblages and interactions of native species, but
their effect on fundamental ecological processes is poorly understood.
In East Africa, the big-headed ant (Pheidole megacephala) invades
monodominant stands of the ant-tree Acacia drepanolobium, displacing
native ant defenders and rendering trees vulnerable to elephants
(Loxodonta africana) and other browsing ungulates. We quantified A
drepanolobium photosynthesis and transpiration pre- and post-invasion by
P. megacephala. After ca. 5 years, ant invasion resulted in 69% lower
whole-tree carbon fixation during the growing season, despite
shorter-term (< 1 year) positive effects on photosynthetic
rates coinciding with the displacement of energetically costly native
ants. By experimentally excluding ants and large herbivores, we
demonstrate that reduced carbon fixation resulted largely from browsing
on trees by large herbivores in invaded areas. Our results from
individual trees likely scale up, highlighting the potential of invasive
species to alter carbon fixation and other biogeochemical cycles at
ecosystem scales.