Floral display and pollination success in Senecio jacobaea (Asteraceae): Interactive effects of head and corymb size

The self-sterile Senecio jacobaea (Asteraceae) presents its rayed heads in large, compound inflorescences (corymbs). I examined the role of head and corymb size in pollinator attraction, and whether the positive effect of intact rays (if any) depends on the size of the corymb. Using female fertility as a measure of pollination success, I assessed the performance of stems representing four experimentally produced character combinations: (1) few heads without rays, (2) many heads without rays, (3) few heads with rays, and (4) many heads with rays. The proportion of flowers setting fruit was higher for intact stems (treatments 2, 4) than for stems on which the majority of the heads had been removed (treatments 1, 3), suggesting selection for maximum inflorescence production. By contrast, experimental removal of all rays had a relatively weak negative effect on fruit set, with few-headed stems (treatment 1) experiencing a greater reduction than stems with many heads (treatment 2). These results suggest that clusters of heads produce a synergistic effect on pollinator attraction, allowing plants to maintain high visitation rates even if there are drastic reductions in the basic attraction units. Hence, the number of heads and the attractiveness of the individual heads interacted in their effect on pollination success. Fruit set per flower differed greatly between sites and was positively correlated with plant density.