Adaptive Significance of Ipomopsis Aggregata Nectar Production: Pollination Success of Single Flowers

Floral nectar rewards are expected to contribute to plant fitness by influencing several aspects of pollinator behavior. For example, when flowers have large standing crops of nectar, pollinators may take longer to probe them, thereby increasing the amounts of pollen deposited and removed. In addition, pollinators may return more often to rewarding plants and/or probe more of their flowers during each visit. We experimentally investigated how each of these behaviors influences pollination success of the hummingbird—pollinated herb Ipomopsis aggregata (scarlet gilia). Captive hummingbirds were trained to probe flowers containing known volumes of artificial nectar (25% mass/mass sucrose solution). Increasing the nectar standing crop of a flower from 1 to 5 μL significantly increased probe duration, but this had no detectable effect on pollination success through the female sexual function (pollen deposited on stigmas) or male sexual function (pollen removed from anthers and number of fluorescent dye particles exported to other flowers). In contrast, both male and female pollination success increased with the number of times a hummingbird probed a flower. For male function, there was at best a weak tendency for pollen export to diminish with successive probes, in contrast to strongly diminishing returns reported for bee—pollinated flowers. This difference may reflect foraging mode; hummingbirds hover and remove a small fraction of available pollen with each floral probe whereas bees land and remove most pollen with the first probe. For plants such as I. aggregata whose pollination success is most sensitive to the number of probes per flower, selection on nectar production may be strongest when there are cues pollinators can use to identify rewarding plants, such as phenotypic correlations between nectar production and floral morphology.