The Ecology of Flowers of Chalk Grassland and their Insect Pollinators

(1) Competition for pollinators and avoidance of interspecific pollen transfer have been proposed as selective factors influencing flowering time and floral specializations. Their importance in a community of chalk grassland plants was studied. (2) The flowering periods of the important insect-pollinated herbs were determined. The most prominent flowering species were Helianthemum nummularium, Hippocrepis comosa and Onobrychis viciifolia during June and early July, and Centaurea nigra, C. scabiosa, Knautia arvensis and Scabiosa columbaria from late July to September. (3) Bumblebees, particularly Bombus lapidarius, were the most abundant visitors to most of these plant species; Apis mellifera, some Lepidoptera and Syrphidae were of lesser importance. (4) Some insect species differed in the flowers they visited most frequently. (5) In the early-flowering groups of plants there was some specialization for different pollinators and divergences in flowering time, suggesting that competition for pollinators or avoidance of interspecific pollen transfer has exerted selection pressure in the past. (6) In the late-summer group all species overlapped considerably in flowering time and their insect visitors, which suggests that there has not been any strong selection against an overlap. There appeared to be a saturation of insect visitors in late summer, with particularly large numbers of Bombus lapidarius.