Floral display and pollination success in Achillea ptarmica (Asteraceae)

In a natural population of the self‐incompatible Achillea ptarmica , many‐headed inflorescences attracted most pollinators (flies), and each pollinator visited more heads on large inflorescences than on smaller ones. The visitation rate did not increase disproportionately with inflorescence size, as would be expected if clusters of heads had a synergetic effect on pollinator attraction. Both the calculated visitation rate per head and the percentage seed set increased slightly as a function of head number. The removal of rays from all heads in the inflorescence reduced the approach rate by 51%, but had only a small negative effect on seed set (12%). Hence, ray removal probably affected male fertility (pollen donation) to a greater extent than female fertility. The decline in pollination success was similar for all inflorescence sizes, suggesting that ray and inflorescence size have independent effects on the total display, and thus represent separate targets for pollinator‐mediated selection. Residual variation in seed set was strongly correlated with patch identity, indicating clonal or environmentally induced differences in female reproductive success.