Red wood ants (Formica rufa group) impact forest ecosystems in a multitude of ways and can thus be considered as ecosystem engineers. Although they are highly efficacious predators of many arthropods, their nests host a diverse community of other arthropods. In this thesis, I wanted to highlight the diversity of red wood ant associated guest fauna in order to emphasize the role of red wood ants in the maintenance and conservation of forest biodiversity. I examined the underlying factors shaping the community composition and species richness of associates. Particularly, I studied the factors determining the occurrence of the globally vulnerable shining guest ant (Formicoxenus nitidulus), an obligate social parasite of moundbuilding wood ants. I also studied the effects of clear felling on ant nest mounds as well as on the ant-associated beetle community. 

A total of 85 different taxa (mostly beetles), including 26 myrmecophiles (species dependent on ants), were identified in the course of this thesis. One unexpected discovery was the sawtoothed grain beetle (Oryzaephilus surinamensis), which has so far only been recorded as an indoor pest species in Finland. Overall, I found that species richness and occurrence of associates were negatively associated with isolation of the nest mounds. The occurring probability of the guest ant F. nitidulus increased with decreasing isolation and increasing nest mound size. The guest ant was more likely to occur in the nests of the polydomous (multi-nest colonies) Formica polyctena than the monodomous (single nest colonies) Formica rufa. Also, local spreading along the connecting trails between nests in polydomous host colonies seems to be important to F. nitidulus. 

I found that Formica aquilonia nest mounds in clear fells had significantly lower surface layer moisture content than nests in forests, which reduces the nests’ thermal capacity. Though species richness and community composition of beetles did not greatly differ between clearings and forests, total abundance was lower in clear fell nests. Furthermore, total species richness and abundance as well as myrmecophile abundance decreased with decreasing moisture content. I investigated only the short-term effects of clear felling in active nest mounds. However, nests in clear fells have a high tendency of being abandoned, which will inevitably be detrimental to myrmecophiles. How the associate community develops in the long-term in the nests surviving clear felling needs further study. 

This thesis reinforces the status of red wood ants as hosts of a highly diverse associate community. This needs to be taken into consideration in the maintenance and conservation of arthropod diversity in temperate and boreal forests. If the whole habitat cannot be protected, forest management practices preserving dense red wood ant populations should be used. Also, colonization chances for the wood ants as well as nest density could be increased by creating small open areas in managed forests, which would also benefit many other forestdwelling species. Thus, maintaining a varied forest structure could help maintain and even increase the forest biodiversity.