Divergent effects of different ant partners on host trees in a Kenyan ant-plant mutualism

Background/Question/Methods

Ant-plant mutualisms are often used as model systems for asking a broad range of ecological and evolutionary questions, yet we know little about how resource flow between these interacting partners affects nutrient and water budgets of host-plants, which may in turn strongly influence larger-scale ecosystem processes. We examined how different ant partners influence the foliar nutrient concentrations, water stress, and reproduction of an African ant-plant (Acacia drepanolobium), as well as the nutrient concentrations in soil surrounding the trees. We established a manipulative field experiment in central Kenya in which colonies of the four symbiotic ant species occupying A. drepanolobium were either left intact or experimentally removed from their host trees; this experiment has now been maintained for four years. In addition, a separate ~100-tree transect was established to assess differences in flowering and fruiting phenology for trees occupied by different ant species. 

Results/Conclusions

Our results demonstrate that different ant partners have strong and divergent effects on their host trees, changing foliar phosphorous concentrations by +17% to -23% and pre-dawn water potential by +30% to -16%. Significant effects of different ant partners were also detected in foliar nitrogen concentrations, leaf weight, and specific leaf area, as well as concentrations of available nitrogen in surrounding soils. In addition, the identity of ant occupants significantly influenced the timing of flowering (P<0.05) and level of fruit production (P<0.01) in host trees. Collectively, our results demonstrate strong and contrasting effects of different ant occupants on host-plant and soil resource availability, as well as reproductive parameters, raising the intriguing possibility that ant symbionts may differentially constrain or control the life history strategies of their plant hosts.