DENSITY-DEPENDENT AND FREQUENCY-DEPENDENT SELECTION BY BUMBLEBEES BOMBUS-TERRESTRIS (L) (HYMENOPTERA, APIDAE)

The behaviour of bumblebee workers Foraging on arrays of artificial fl owers of two colour morphs was observed. Experiments were conducted on arrays of varying morph frequencies and at three different total flow er densities. Bumblebees consistently shelved a preference for the com monest colour morph, and this behaviour was not significantly affected by changing density. In contrast, frequency-independent preferences c hanged significantly with density. At low densities, there was a stron g bias towards the more conspicuous colour, whereas at higher densitie s there was no overall colour bias. Flight distances between flowers d ecreased significantly at high density. Bumblebees also visited flower s of similar colours sequentially, but this behaviour was not density- dependent. It is suggested that as densities increase, there is an inc reased probability that bumblebees detect yellow flowers, which were p robably less conspicuous compared xith blue flowers, and that this mig ht be caused by changes in flight speed with flight distance. Where th ere is a positive relationship between pollinator visitation and the r elative fitness of a floral morph, the observed behaviour would induce positive frequency-dependent selection on a plant population with two corolla colour morphs on which the bumblebees were foraging, which wo uld result in stabilizing selection for a single corolla colour, irres pective of density. There was no indication that rare colour morphs wo uld be preferred at high density. The probability of different corolla colour morphs going to fixation would, however, be affected by densit y. (C) 1997 The Linnean Society of London.