Flowering plant density and pollinator visitation in Senecio.

It has commonly been assumed that pollinator energy intake increases with flowering plant density, and visitation to flowers should therefore be higher in denser stands. I therefore investigated the relationship between flight distance and flight time for bumblebees and butterflies foraging on Senecio integerrimus and S. crassulus in the Colorado Rocky Mountains. I also compared patterns of pollinator visitation and seed set in two adjacent 15x 15 m plots in a population of S. integerrimus; one plot was experimentally thinned of flowering stalks, while the other was left at natural density. Mean flight distance had no effect on mean flight time or the number of florets or heads visited per unit time. There were no significant differences between the two plots in the rate at which plants received visits, although visitation rates varied through the season, with greatest activity at peak flowering. Pollinators were more selective in their visits to plants in the high-density plot, however. Bumblebee-visited plants in the dense plot had a lower variance in stalk height than the plant population average, while butterfly-visited plants in the dense plot had more heads than the population average. Plant density had no effect on number of heads visited per plant, but number of heads visited by bumblebees was correlated with number of heads per plant. Efficiency of visitation (percentage of flowering heads visited), declined with inflorescence size. Flowering plant density had no effect on seed production, and inflorescence size did not affect the percentage of florets setting seed. In Senecio, flowering phenology patterns and differences among pollinators in foraging behavior may have more important consequences for seed set and gene flow patterns than plant density or plant size.