Testing for pollinator recognition in multiple species of Heliconia

Abstract

Many plants have evolved floral traits that, in effect, filter pollinator communities and promote pollination by efficient pollinators. Most documented pollinator filter traits act to deter or reduce visitation rates by a subsect of the community of floral visitors. However, a recently described pollinator filter termed pollinator recognition (PR) acts at a stage after a pollinator visit. PR was first documented experimentally in Heliconia tortuosa whereby pollen tube germination - a proxy for reproduction - was enhanced following visits from morphologically specialized pollinators, but not generalists. This is thought to promote outcrossing among plants by preferentially investing in reproduction following visits by long-ranging hummingbirds with specialized bill shapes. To date, this plant behavior has only been described in H. tortuosa, but, if widespread, could have important ecological implications; given declines in abundances of specialist pollinators, visits by generalists would not buffer the loss of pollination services to plants with PR. We therefore tested for PR in four taxa spread widely across the Heliconiaceae. We corroborated previous results that visits by long-billed, but not short-billed hummingbirds increased pollen tubes in H. tortuosa with aviary experiments that standardized pollen quality and minimized variation in pollen quantity. Across species, we found great variation in pollen tube responses to experimental treatments. For one species (H. rostrata), we found increased numbers of pollen tubes in those visited by hummingbirds compared to hand pollination alone, regardless of the visiting birds bill morphology, indicating recognition of hummingbirds in general. In other cases, hummingbird visits decreased pollen tube counts compared to hand pollinations alone. Furthermore, our results could not substantiate any specific mechanism for pollinator recognition and highlight the need for further work on the complexities of and variability in reproductive strategies across plant taxa.