The Effects of Termite Mound Induced Spatial Heterogeneity on an Acacia-Ant Mutualism

Abstract

In the Laikipia district of Kenya, black cotton soils are dominated by Acacia drepanolobium, an ant-plant with four ant symbionts. Beneath this monospecific overstory, termite mounds produce local-scale resource heterogeneity. The effects of this spatial heterogeneity on the mutualism between A. drepanolobium and its four ant symbionts are studied through observational work, experimentation, and development of a mathematical model. Observational data demonstrate that trees closer to mounds are more likely to be occupied by the dominant and most positively mutualistic ant species Crematogaster mimosae, but they also experience more elephant damage, which is associated with lower fruit production. Further, C. mimosae was found to defend its host from herbivory less aggressively close to termite mounds. An experiment was conducted to test the hypothesis that lowered C. mimosae aggression close to mounds is the result of increased protein access for the ant colonies on termite mounds. Provision of protein to ant colonies in resource-poor, off-mound locations resulted in diminished ant aggression, supporting the hypothesis. A mathematical model developed for this system suggests that C. mimosae should be more competitively dominant as it gets closer to mounds, and that termite mounds promote coexistence of two ant species, C. nigriceps and C. mimosae.