How distances between plants and inflorescences of Phyteuma nigrum affect pollinator behaviour was examined in a natural situation as well as experimentally. Patches of Phyteuma in two densities were set up in a field in Drenthe. In the natural population flight distances of bumblebees and syrphids were examined. Migration of pollinators was demonstrated between two populations of Phyteuma, which in previous studies had seemed completely isolated. The largest distance between recaptures of bumblebees on Phyteuma on the same day was 200 m. In the experimental patches bumblebees made longer foraging bouts in high-density patches than in low-density patches. They stayed longer per inflorescence and visited more inflorescences per plant in low density, while the number of inflorescences visited per minute did not differ between densities. In low density bumblebees flew more often to nearest neighbours and visited more inflorescences per plant. Plants with four inflorescences were preferred to plants with one in both densities, and the preference was stronger in low density. Implications for the fitness of the plants are discussed.
