Mutualism as reciprocal exploitation: African plant-ants defend foliar but not reproductive structures.

The foundation of many plant-ant mutualisms is ant protection of plants from herbivores in exchange for food and/or shelter. While the role of symbiotic ants in protecting plants from stem- and leaf-feeding herbivores has been intensively studied, the relationship between ant defense and measures of plant fitness has seldom been quantified. We studied ant aggression, damage by herbivores and seed predators, and fruit production among Acacia drepanolobium trees occupied by four different acacia-ant species in an East African savanna. Levels of ant aggression in response to experimental disturbance differed strongly among the four species. All four ant species recruited more strongly to new leaf growth on host plants following disturbance, while recruitment to developing fruits was on average an order of magnitude lower. Host plants occupied by more aggressive ant species suffered significantly less vegetative damage from leaf-feeding insects, stem-boring beetles, and vertebrate browsers than host plants occupied by less aggressive ant species. However, there were no differences among fruiting host plants occupied by different ant species in levels of seed predation by bruchid seed predators. Fruit production on host trees was significantly correlated with tree stem diameter but not with the identity of resident ants. Our results demonstrate that defense of host plants may differ substantially among ant species and between vegetative and reproductive structures and that fruit production is not necessarily correlated with high levels of aggression by resident ants.