Evolutionary Options for Maximizing Pollen Dispersal of Animal-Pollinated Plants

On the average, nectar-collecting bumble bees deposited 0.6% of the pollen removed from the flowers of Erythronium grandiflorum (Liliaceae) onto the stigmas of subsequently visited flowers. Because the proportion deposited declined as the amount removed increased, an individual plant would maximize its total pollen dispersal by relying on many pollen-removing visits while limiting the pollen removed by each pollinator. This restriction of pollen removal could be achieved by a plant presenting only a small portion of its pollen at one time (packaging) and/or by limiting the amount of presented pollen that a pollinator removes during a single visit (dispensing). The restriction of pollen removal required to maximize the expected total deposition on stigmas depends on the number of pollinator visits a plant receives, variation in the frequency of visits, and the pattern of pollen removal during a series of visits. Many aspects of floral biology contribute to a plant's ability to restrict pollen removal, including inflorescence size, flower morphology, anthesis patterns, nectar production, and dichogamy. Selection increasing paternal fitness of animal-pollinated plants could therefore elicit one of a variety of evolutionary responses; the specific response will depend on characteristics of both the plant and the pollinator.