Pollinator Sharing and Seed Set of Stellaria pubera: Competition for Pollination

Abstract

I used field experiments to examine the effect of pollinator sharing on seed set in the understory herb Stellaria pubera. In piedmont North Carolina the solitary bee and fly pollinators of this species are also frequent visitors to Claytonia virginica. Removals of nearby flowers of C. virginica increased seed set of S. pubera. Examination of seed set in populations of potted plants indicated that the effect was due to change in species composition rather than plant density. Variation in seed production was high, however, and even when flowers were provided with excess pollen through supplemental hand pollinations, only 20% of ovules produced seeds. Competition for pollination appears to be one of several selective forces that act on blooming time of S. pubera. Stellaria pubera blooms slightly later than C. virginica. Whereas the seeds set of early flowers was increased 13% by removal of C. virginica and 19% by hand pollination, seed set of late flowers was unaffected. Plants forced to bloom early also tended to be more pollinator—limited than control plants. Removal of C. virginica did not, however, increase the seed set of S. pubera plants forced to bloom early significantly more than that of control plants. Other forces are probably also involved in the maintenance of blooming time.