Mound-building ants, and especially wood ants (Formica rufa group), are wide-spread key species that affect ecosystem functioning in several ways in the boreal forests of Eurasia. I evaluate how wood ants affect biodiversity of arthropod communities in boreal forests. Studies conducted on mountain birch (Betula pubescens ss. czerepanovii) showed that the impact of wood ants on the structure of invertebrate community living in canopy was mainly negative. Predation by wood ants reduces especially the numbers of leaf-chewing herbivores and thus biodiversty in the canopy of birches growing near the ant mounds. Predation by wood ants affected more seriously the early season invertebrate community. The impact of hairy wood ants on the guild structure of herbivorous insects on birch (Betula pubescens) was similar. Total species richness of the herbivorous insect community on birch was reduced by ants. Wood ants also affect soil animal community structure. Increased ant density affected both the taxonomic and trophic structure of soil invertebrate community. However, the direct effects of wood ants on soil animal food web is likely to be small. Summarizing. Several studies indicate that when the densities of wood ants are high they may have remarkable negative effects on invertebrate communities and thus they reduce the biodiversity of local arthropod communities.