Differential impact of two dominant Formica ant species (Hymenoptera, Formicidae) on subordinates in temperate Europe

Competition is one of the basic mechanisms shaping ant assemblages. Dominant territorial species are known to restrictively influence the traits of subordinates in various ways. However, there could be differences in the effects of dominants due to differences in their colony structure, lifestyle and also behaviour. We tested this hypothesis in natural circumstances in an area where a Formica exsecta Nyl. supercolony neighbours a strong population of the European slave-maker F. sanguinea Latr. For the purpose of our study three different sites were selected: one dominated by F. exsecta, a neighbouring site dominated by F. sanguinea, and a third site where both species co-occurred. We analyzed the structure of subordinate ant communities based on nest counts, and by recording the activity of ants at baits. Based on our findings the structure of ant communities differed significantly among the three sites. The F. exsecta site was characterized by the lowest nest density of subordinates, especially in the case of aggressive species as Lasius platythorax Seifert. At baits even the simple presence of the dominant limited the abundance and occurrence of subordinates. In contrast, no such negative effect could be revealed at the F. sanguinea site. The community of the mixed site showed intermediate features in many respects. Generally, the supercolonial F. exsecta had a bigger influence on the ant community than the slave-making F. sanguinea. On the other hand, our findings revealed a slight protective role of the territorial F. exsecta for potential slave species to F. sanguinea. The current study underlines the importance of differences between dominant ant species in shaping differentially ant communities even within the same restricted area.