The structure of ant assemblages in an urban area of Helsinki, southern Finland

We collected ants in ten replicated habitat types of an urban island and described their assemblages using Non-metric Multidimensional Scaling and Multivariate Regression Trees. Lasius niger was the most abundant species, followed by Myrmica rubra, Formica fusca, L. flavus and L. platythorax; these species comprised 87% of all 1133 nests of the 16 species found. Ant assemblages changed gradually from open habitats to sites with closed tree canopy. Species most tolerant to urban pressure were L. niger, L. flavus and M. rugulosa, whereas forest-associated species were scarce or absent. Successful urban species had extensive (Palaearctic) or more limited (Euro-Siberian) distribution. Common and abundant habitat generalists were overrepresented relative to rural areas; these were also efficiently dispersing pioneer species with independent colony founding. Lack of suitable (micro)habitat apparently hampered colonization of many species. Competitively superior, territorial species were rare or absent, as were species dependent on dead wood, and many species depending on other ants. The indicated reorganization of interspecific competitive relationships may be due to selective impoverishment of the urban species pool. Comparison with other urban studies suggested that in ants, faunal homogenization has not taken place on a European scale.