Abstract. 1. Grassland fragmentation is expected to influence the abundance of different invertebrate species to a different extent. Fragmentation-related effects are of particular importance in species that interact with many other species.
2. The density and spatial distribution of nests of 15 ant species in experimentally fragmented calcareous grasslands at three sites in the Northern Swiss Jura mountains were examined. Fragments of different size (0.25 m2, 2.25 m2, and 20.25 m2) were isolated by a 5-m wide strip of frequently mown vegetation. Control plots of corresponding size were situated in adjacent undisturbed grassland.
3. Three years after initiation of the experiment, ant nest density did not differ between fragments and control plots. Six years after initiation of the experiment, however, ant nest density and forager abundance were higher in large fragments than in large control plots. Ant nests tended to occur more frequently along the edge of fragments than in the core area. Persistence time of nests of the most abundant species, Lasius paralienus, tended to be shorter in fragments than in control plots. Furthermore, persistence time was longer in nests situated close to the fragment edge than in nests in the core area.
4. Effects on nest density, edge effects on the spatial distribution of nests, and the relationships between nest density and environmental factors were more pronounced when only nests of L. paralienus were considered. The implications of these findings for plant and other invertebrate species are discussed.