Ecology of ant communities on the alpine tree line ecotone

Alpine ant communities are poorly studied from an ecological perspective, even in central Europe. At five mountains situated in the Central Alps, elevational transects from the upper montane forest into the alpine environment, crossing the tree line ecotone, were investigated focusing on the change of community composition and the trophic position of dominant species. Fourteen ant species were recorded. Species richness was found to be elevated directly at the ecotone, along with higher functional diversity of the ant community. Both these results were mostly driven by a higher abundance of Myrmicinae species directly at the ecotone. Mound-building red wood ants dominated numerically in the forest, while the subordinate alpine slave-ant Formica lemani dominated in alpine grassland. A constrained ordination of the ant community data revealed that shrub cover, soil humus content and the density of wood ants influenced significantly the presence and abundance of all other ant species. Feeding preferences were investigated experimentally (offering baits) and analytically (using stable isotopes). Baiting revealed a high preference for sugar resources, therefore likely the most limited resources for ants in subalpine forest as well as alpine grassland. Yet, feeding responses of the ant community shifted over the ecotone towards an even higher sugar preference in the alpine zone. The same shift was also found to occur intraspecifically in the two dominant ant taxa (wood ants and alpine slave ant). Stable C and N isotope signatures of these dominant ants likewise revealed a shift in the used resources, from a more herbivore-like trophic position in subalpine forest to a more carnivore-dominated life style in alpine grassland. Further, wood ants were found to occupy a higher trophic level compared to slave ants, although the contribution of trophobiosis was initially expected to be higher for this latter subgenus. The lower trophic position in the forest is most likely facilitated by strong trophobiotic associations with honeydew-producing homopterans that occur in far larger numbers on conifer trees in this habitat. The isotopic signature of Formica ants changed over the gradient. To our knowledge, this is the first evidence that also under natural conditions ants undergo shifts in their trophic niche. This finding contradicts a recent assumption which forecasted the opposite.