The local density of a plant population can influence both the number
of pollinators attracted and the behavior (and thus pollination effici
ency) of those that are. I present the results of a field experiment p
erformed on Brassica kaber, a self-incompatible annual crucifer, desig
ned to test for the existence of density effects in pollination and to
separate the two major component effects: reductions in the quantity
and in the quality of pollinator services. B. kaber plants were grown
in fan-shaped density arrays in six floral backgrounds chosen to manip
ulate pollinator foraging behavior. Pollinators visiting focal plants
behaved as generalists in backgrounds of the similar-flowered Brassica
hirta, behaved as floral specialists where no alternative floral reso
urces were provided (three treatments), and displayed a mixture of beh
aviors in two background plantings where dissimilar flowers were avail
able. Pollinator visitation rates to focal B. kaber plants declined sh
arply at low density in all backgrounds except the similar flower (B.
hirta) treatment. Widely spaced plants displayed significantly reduced
seed-set in all backgrounds studied. The most dramatic density effect
s occurred where pollinators were induced to behave as generalists, su
ggesting that density-related declines in pollinator quality are more
important than parallel declines in the quantity of visits.