The influence of matrix type on flower visitors of Centaurea jacea L

The structure of invertebrate communities is impacted by landscape variables. Here we present a study on the influence of the surrounding matrix on pollinating flower visitors of the knapweed Centaurea jacea in a small-scale agricultural landscape (Central Hesse, Germany). The study was carried out in late summer 1998 by monitoring visits of 24 insect taxa at 15 C. jacea patches. The following matrix types were studied: (i) arable land only, (ii) arable land close to grassland (<50 m), (iii) a mosaic of arable land, grassland and forests, and (iv) grassland only. More than half of the flower visitors were bees, with honeybees (Apis mellifera) and bumblebees being the dominant taxa. The matrix type did not affect either the mean frequency of total flower visits or the mean richness of all taxa. The same applied to different size classes. Significant matrix effects were confined to five large taxa of the Apoidea: Bombus lapidarius, B. pascuorum, the B. terrestris group, large wild bees (other than bumblebees) and A. mellifera. A. mellifera significantly preferred patches surrounded by grassland, while large wild bees preferred patches surrounded by a mosaic of arable land, grassland and forests. The flower visiting frequency of all three bumblebee species was high in patches surrounded by grassland, but only the B. terrestris group showed a clear-cut preference for this matrix type. It is concluded that matrix effects on flower visitors of C. jacea are taxon- and body size-specific. Strong matrix effects on large pollinators suggest that these taxa are able to discriminate between patches. Considering the ecological services provided by pollinators, the preservation of large areas covered by interconnected grassland sites as well as by a mosaic of different land use forms should have high priority in future management strategies.