Sex and the Single Mustard: Population Density and Pollinator Behavior Effects on Seed‐Set

Abstract

The local density of a plant population can influence both the number of pollinators attracted and the behavior (and thus pollination efficiency) of those that are. I present the results of a field experiment performed on Brassica kaber, a self—incompatible annual crucifer, designed 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 manipulate 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 resources were provided (three treatments), and displayed a mixture of behaviors in two background plantings where dissimilar flowers were available. Pollinator visitation rates to focal B. kaber plants declined sharply 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 effects occurred where pollinators were induced to behave as generalists, suggesting that density—related declines in pollinator quality are more important than parallel declines in the quantity of visits.