Short-term effects of wood ants (Formica aquilonia Yarr.) on soil animal community structure

Wood ants are known to affect the structure of invertebrate communities in tree canopies but it is not clear whether wood ants can affect the structure of soil animal communities. To isolate the direct effects of wood ants on soil fauna, I manipulated wood ant density in a 6-week mesocosm experiment by placing mesocosms (O 14 cm, height 13.5 cm) permeable to ants either on or outside the ant trails. The mesocosms contained litter, humus, and a diverse soil fauna typical of coniferous forest. I controlled earthworm species composition and biomasses by inoculating 470 mg fresh mass of epigeic lumbricid Dendrobaena octaedra (Sav.) or Dendrodrilus rubidus (Sav.) to the mesocosms. A 15-fold difference in ant density affected both the taxonomic and the trophic structure of the soil invertebrate community. The biomass of epigeic arthropod predators, Arachnida and Opiliones, was reduced in the high ant density treatment, whereas the biomass of mesofauna predators increased in the presence of ants. Although total earthworm biomass was not affected by the wood ants. reproductive output of epigeic earthworm Dendrodrilus rubidus increased under high wood ant density, while reproduction of Dendrobaena octaedra was unchanged. The overall effects of wood ants on soil fauna was relatively weak: despite the 15-fold difference in wood ant density between the low and the high density treatments, only some taxa were affected. Consequently, the direct effects of wood ants on soil animal food web is likely to be small. (C) 1999 Elsevier Science Ltd. All rights resented.