Indirect interactions between trees, scale insects, and ants: The persistence of context-dependent mutualism in a coffee agroecosystem

The occurrence, intensity, and composition of mutualisms are dependent not only on the co-occurrence of mutualists, but also the broader biotic context in which they are embedded. We investigated the influence of the specific nest tree identity of the ant Azteca instabilis on the green coffee scale (Coccus viridis) density in a coffee agroecosystem in Southern Mexico. In particular, we tested the hypothesis that an indirect competitive interaction for ant-attendance occurs between a scale species (Octolecanium sp.) in the canopy of the shade tree Inga michelliana and C. viridis which inhabits coffee bushes (Coffea arabica) beneath the shade trees. Coffee bushes beneath a different shade tree species (Alchornea latifolia) were used as an indication of C. viridis density in a non-competitive environment. Results indicate that C. viridis occurs in significantly lower density adjacent to nests in Inga supporting the hypothesis of indirect competition. Additional experimentation suggests that there is a mutualism between Azteca and Octolecanium and that this interaction may be mediated by a hierarchy in ant-attendance of scale insects. Our results demonstrate the importance of considering the biotic context of mutualistic interactions. Implications of our results for the maintenance of a diverse community of scale insect predators (primarily Coccinellid beetles) and the spatial distribution, diversity, and abundance of other ant species nesting in the coffee layer will also be discussed.