Context-dependent benefits from an indirect ant-plant mutualism: Competition for ant attendance limits ant benefits to coffee

Background/Question/Methods

Biotic and abiotic contexts can alter the outcome of defense mutualisms between plants and their partners. This conditionality may be specifically true for indirect ant-plant mutualisms mediated by honeydew-producing hemiptera (hereafter hemiptera) because plants may compete for ant attendance with other plants that also host high densities of hemiptera. In this study we tested the hypothesis that ant (Azteca instabilis) limitation of coffee berry borer (seed predator) would be conditional upon the species of tree ants nest in. We predicted coffee below Inga spp. trees (many hemiptera) would have weaker benefits from A. instabilis than coffee below non-Inga spp. (few hemiptera). In six coffee plantations we compared the borer infestation of coffee plants (on 3-5 plants) and the number of coffee plants foraged upon by A. instabilis below Inga spp. and non-Inga spp. trees with active A. instabilis nests as well as paired control trees without A. instabilis. We also estimated the total benefit of A. instabilis nests to coffee (mean proportion infested in control tree – A. instabilis tree × number of coffee plants foraged upon by A. instabilis × mean number of berries per coffee plant).

Results/Conclusions

There was twice as many hemipterans in the A. instabilis nest trees of Inga spp. relative to non-Inga spp. Azteca instabilis activity was 50% higher and there were 14 times more hemipterans on coffee below non-Inga spp. relative to Inga spp. nest trees. Across all coffee plants there was a 6 times higher proportion of coffee fruits infested with the berry borer below control trees (without A. instabilis) relative to paired A. instabilis trees. The difference between each tree pair (control – A. instabilis) was 66% greater for non-Inga spp. relative to Inga spp. trees, although this differences was not significant (pest control per coffee plant). Azteca instabilis foraged on 86% more coffee plants under non-Inga spp. relative to under Inga spp. trees. Estimation of nest level reduction of berry borer was 6 times greater for non-Inga spp. (254 berries rescued) relative to Inga spp. (40 berries rescued). Results suggest that at nest level scales, A. instabilis suppression of the berry borer and indirect benefits to coffee are limited by abundance of hemiptera in nest trees. Thus, it appears competition for ant attendance between coffee and trees may alter the strength of the ant-hemiptera-coffee association.