Effects of flower production and predispersal seed predation on reproduction in Cirsium purpuratum

Researchers have often assumed that the reproductive success of a plant increases with flower production. However, if predispersal seed predation also increases with flower production, this may counteract the increase in reproductive success expected with such increased flower production. To investigate this, we examined the effect of flower number and predispersal seed predation on seed production in two field populations of Cirsium purpuratum (Maxim.) Matsum. At both sites, the proportion of seeds or heads preyed upon per plant increased with the number of flowers (florets or heads) on each plant, while the proportion of mature seeds per plant was independent of flower number per plant. Based on these results, we predicted that an increased level of seed predation at the population level would reduce the annual flower production of each plant. The observed pattern of flower production supported this prediction. Our results suggest that increased flower production may not always improve plant reproductive success under the influence of predispersal seed predation.