Trophic position and seasonal changes in the diet of the red wood ant F ormica aquilonia as indicated by stable isotope analysis

1. Red wood ants are among the most numerous generalist predators and strongly affect the composition of arthropod communities in forest ecosystems. However, their trophic position remains poorly understood. Stable isotope analysis was applied to study the trophic position of F ormica aquilonia and reveal seasonal changes in its trophic links with both myrmecophilous aphids and other invertebrates in a mixed forest of western S iberia. 2. The δ 15 N values of F . aquilonia exceeded those of herbivores and aphids by approximately 3.5‰. Despite obligate trophobiotic relationships with aphids, F . aquilonia occupied the trophic position of first‐order predator. The higher content of 13 C in the worker ants, compared with members of grazing food chains, was explained by their consumption of 13 C ‐enriched aphid honeydew. 3. Myrmecophilous tree‐dwelling aphids were enriched in 13 C and 15 N relative to grass‐inhabiting species, and the honeydew of tree‐dwelling aphids had higher δ 13 C values than those of the honeydew of grass‐inhabiting aphids. 4. The decrease in δ 13 C values of the worker ants from spring and summer to autumn apparently reflected the transition from the collection of tree sap and feeding on the aphid honeydew from trees with high 13 C content in the spring and early summer to a more diverse liquid diet in late summer, which included 13 C ‐depleted honeydew of aphids from herbs. 5. The prevalence of the 15 N ‐depleted aphid honeydew in the ants' diet in the second half of the summer is discussed as one possible explanation for the seasonal decline in δ 15 N values of the worker ants.