Floral specialization without trade-offs: Optimal corolla flare in contrasting pollination environments

Most models of floral specialization assume that adaptations to a particular type of pollinator incur a fitness trade-off in reduced effectiveness of others. To examine this assumption and characterize the form of trade-offs, I manipulated floral form in Dudleya greenei (Crassulaceae), a species that is pollinated by hummingbirds, bumble bees, and other insects. Existing pollination literature predicts that pollination by hummingbirds should favor narrower flowers than pollination by humble bees. When hummingbirds alone visited experimental populations, narrow flowers indeed performed best by most measures. Hummingbirds showed a slight preference for flowers of intermediate width but deposited more pollen per visit and more pollen overall, and they exported more dye (a pollen analogue) as corolla flare decreased. Bees were more effective than birds at pollinating wide flowers but did not generate overall performance trade-offs. Bees also preferred flowers of intermediate width and deposited more pollen per visit and more pollen overall as corolla flare decreased, but the overall efficiency with which bumble bees exported dye was independent of corolla flare. This demonstrates that phenotypic specialization for hummingbirds might evolve without trading-off the effectiveness of bumble bees. However, both birds and bees tended to transfer dye most efficiently between like phenotypes, suggesting that the form of selection and presence of trade-offs may be frequency dependent in natural populations.