1 Stable isotopes signatures (δ13C and δ15N) of the most important tree-dwelling ants in an olive orchard were examined, together with the signatures of the most common herbivores, predators and sap-sucking insects. The olive orchard consists of separate subunits (trees) surrounded by a matrix of grasses or bare ground, and the role of ants in such a system is not fully understood.
2 None of the selected ant species was exclusive to the olive trees because they were also observed foraging on vegetation (mainly thistle) under the tree crowns. Hence, the relative contributions of these two sources of energy (olive trees versus herbs/grasses) were assessed by comparing the δ13C of ants with the signatures of plants and those of other arthropods collected on the trees and on nearby thistles. Next, the trophic level occupied by the ants and their ecological role within the olive food web were determined by examining the δ15N values and their relationship with indices of ecological performance measuring the potential pressure exerted by each species on the ecosystem.
3 The analysis of 13C signatures revealed a different contribution of the two energy sources, olive trees versus herbs and grasses, with the former being more important for ants. The analysis of 15N signatures suggested separate roles for different ant species: some (Crematogaster scutellaris, Lasius lasioides) occupied a higher trophic level, mostly involved in predation, whereas others (Camponotus piceus, Camponotus lateralis) occupied a lower level, probably involved more in homopteran tending. A fifth species (Camponotus aethiops) was in an intermediate position. Finally, the δ15N levels of the species were significantly correlated with indices of ecological performance.