EFFECTS OF VARIATION IN FLOWER NUMBER ON POLLINATOR VISITS IN CIRSIUM-PURPURATUM (ASTERACEAE)

We examined the functional relationships between floral display and tw o types of bumble bee response, the visitation rate per plant and the number of flowers visited on a plant, in an artificially arranged fiel d population of Cirsium purpuratum. To reduce the variance in data, we collected data for each day separately and adopted a Latin square des ign in selecting the focal plants within a day. We then tested several types of regressions to each set of data to find the best-fitting lin e accounting for the observed relationship between pollinator response and display size. We found that the visitation rate of bumble bees pe r plant was a decelerating function of floral display, and that the nu mber of flowering heads visited on a plant increased linearly with dis play size. Predicted from the above two functions, the visitation rate per head was independent of floral display and nearly constant within each day. Our results suggest that conventional methods in collecting and analyzing data on pollinator visitation may yield large variance in data derived from temporal and spatial heterogeneity and that impro ved methods employed here are effective in reducing the variance and e stimating patterns of pollinator response to floral display more accur ately.