Abstract
1. Studying the feeding ecology of ants can reveal their trophic position and allow inferences on interactions with other organisms. We investigated the nutritional ecology of ants at the Alpine tree line (from subalpine forests to alpine grasslands), testing the hypothesis that changing food availability reflects upon ant feeding preference and trophic position. Five slopes with five sites each were sampled using a combined experimental (baiting) and chemical (stable isotope) approach.
2. Sugar resources were most preferred by the whole ant community in the alpine environment and on the tree line and were therefore likely limiting in these habitats. This shift was not detected in the two dominant ant taxa occurring over the investigated gradient, viz. the slave ant Formica lemani and mound-building wood ants (Formica s. str.).
3. Yet, stable carbon (C) and nitrogen (N) isotope signatures of both these ant species likewise revealed a shift in resource use over the ecotone, from a lower trophic position in subalpine forests to a more carnivore-dominated lifestyle in alpine grassland. Moreover, wood ants were found to occupy a higher trophic position compared to F. lemani.
4. In conclusion, energy resources turned out to be more limiting for ants in the alpine environment, but less so in subalpine forests. The low trophic position of these ant species in the forest is considered to be a result of abundant trophobiotic associations with honeydew-producing homopterans that occur in far larger numbers on conifer trees in the forest.