Nest mounds of red wood ants (Formica aquilonia): hot spots for litter-dwelling earthworms

A previously undocumented association between earthworms and red wood ants (Formica aguilonia Yarr.) was found during an investigation of the influence of wood ants on the distribution and abundance of soil animals in boreal forest soil. Ant nest mounds and the surrounding soil of the ant territories were sampled. The ant nest mound surface (the uppermost 5-cm layer) harboured a much more abundant earthworm community than the surrounding soil, the biomass of the earthworms was about 7 times higher in the nests than in the soil. Dendrodrilus rubidus dominated the earthworm community in the nests, while in soils Dendrobaena octaedra was more abundant. Favorable temperature, moisture and pH (Ca content), together with abundant food supply (microbes and decomposing litter) are likely to make a nest mound a preferred habitat for earthworms, provided that they are not preyed upon by the ants. We also conducted laboratory experiments to study antipredation mechanisms of earthworms against ants. The experiments showed that earthworms do not escape predation by avoiding contact with ants in their nests. The earthworm mucus repelled the ants, suggesting a chemical defence against predation. Earthworms probably prevent the nest mounds from becoming overgrown by moulds and fungi, indicating possible mutualistic relationships between the earthworms and the ants.