Cooperation among species - or mutualism - plays a critical role in structuring ecological communities, often underlying the persistence of foundation species. While foundation species can stabilize ecosystems at landscape scales, their ability to persist is often underlain by keystone interactions at smaller scales. Acacia drepanolobium is a foundation tree, comprising > 95% of woody cover in East African black-cotton savanna ecosystems. Its dominance is underlain by a keystone interaction with obligate, symbiotic ants in which it provides housing (swollen thorns) and carbohydrate-rich nectar from extra-floral nectaries (EFN). In return, it gains protection from catastrophic damage from mega-herbivores. While the mechanisms by which symbiotic ants facilitate the dominance of A. drepanolobium have been described, we know less about how other putative key players, such as non-ant mutualistic partners influence the functioning of this interaction. The dominant ant, Crematogaster mimosae, tends scale insects for carbohydrate-rich honeydew. We coupled observational surveys with manipulative field experiments to examine the role of scale insects in this specialized ant-plant interaction. Specifically, we ask if this third-partner strengthens the ant-plant mutualism by making the ant a better protector of the tree, or weakens the mutualism by being costly to the tree while providing little additional benefit.
Results/Conclusions
Our results suggest that a third-partner strengthens this ant-plant mutualism. Surveys of 130 trees revealed a positive relationship between scale density and ant activity and aggression. Experimentally reducing scale insects, but not other resources for ants (domatia and EFN), resulted in a 2.3X increase in elephant damage. This weakened protection was driven by a reduction in ant colony size and defensive behavior. Reducing scales may have a larger effect on ants than reducing EFN because ants likely have more control over regulating carbohydrates from scales than from EFN. When we reduced EFN, we found an increase in scale density and the number of ants tending scales, but reducing scale insects had little effect on the proportion of active EFN or on ants tending EFN. Acacia drepanolobium occurs in a semi-arid environment and produces little new growth and EFN during the dry season. Thus, one way in which scale insects likely strengthen this mutualism is by acting as a buffer, allowing ants to continue to gain carbohydrate resources when EFN production is low. Our results suggest that scale insects play an important role in this specialized ant-plant mutualism that underlies the dominance of a foundation tree.