Exploring the 'most effective pollinator principle' with complex flowers: Bumblebees and Ipomopsis aggregata

The 'most effective pollinator principle' implies that floral characteristics often reflect adaptation to the pollinator that transfers the most pollen, through a combination of high rate of visitation to flowers and effective deposition of pollen during each visit. We looked for the expected positive correlation between quantity and quality of visits in lpomopsis aggregata, whose red, tubular flowers are considered to be adapted to hummingbirds. Hummingbirds were indeed the most common floral visitors in 5 years of observation. However, long-tongued bumblebees deposited on average three-times as much outcross pollen per visit to virgin flowers, and elicited four-times as much seed production, as did hummingbirds. Hence visitors that are relatively infrequent, and unexpected given the 'pollination syndrome' of the plant, can be surprisingly good pollinators. One interpretation of this observation is that natural selection favours a specialized floral morphology that excludes all but a single type of visitor, but that there are constraints on achieving this outcome. An alternative is that selection favours some degree of floral generalization, but that flowers can retain features that adapt them to a particular type of pollinator in spite of this generalization. (C) 2001 Annals of Botany Company