The contribution of display size and dichogamy to potential geitonogamy in Campanula americana

An increase in floral display size may enhance fitness by increasing a plant's attractiveness to pollinators. Alternatively, a larger display may reduce fitness by increasing pollen movement within an individual. In dichogamous species the frequency of this geitonogamy also depends on floral gender ratio and pollinator response to floral gender and to inflorescence architecture. We explored the relationships of floral display size and dichogamy to potential geitonogamy in Campanula americana, a protandrous herb. We observed pollinators in a natural population, in which individuals differed in display size, gender ratio, and distance to conspecifics, and in experimental arrays, in which individuals differed only in display size. In both settings, bees responded to increased floral displays by increasing the number and length of foraging bouts. The number of potentially geitonogamous visits, those to female-phase flowers after male-phase flowers, also increased with display size. In addition, although proportion of flowers visited did not increase with the floral display, the proportion of geitonogamous visits did. There was no evidence that dichogamy reduced the potential for geitonogamy. The ratio of male-to female-phase flowers did not affect bee behavior, and bees did not display a gender preference. In contrast to other studies, the presentation of female-phase flowers below male-phase flowers on reproductive branches did not reduce geitonogamy. Variation in floral display size may cause the mating system to vary among populations. We found that display size varied among 12 populations and that geitonogamy may be infrequent in many populations as a result of small display sizes. Finally, within populations, the selfing rates of larger biennial C. americana are likely to exceed those of smaller annual individuals.