Testing for pollinator recognition in multiple species of Heliconia

Plant-pollinator generalization increases the robustness of plant-pollinator communities to fluctuations in the availability of mutualistic partners. However, natural selection may reinforce floral traits that filter pollinator communities to promote pollination by efficient pollinators, which reduces the number of potential partners. We tested the generality of a recently described, cryptic pollinator filter termed ‘pollinator recognition’ (PR) which could reduce the realized number of pollinators compared to the number of floral visitors. 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. We tested for PR in four taxa spread widely across the Heliconiaceae. With aviary experiments that standardized pollen quality and minimized variation in pollen quantity, we demonstrated that visits by pollen-free hummingbirds increased pollen tube rates compared to hand pollination alone in two species; we also corroborated previous results that visits by long-billed, but not short-billed hummingbirds increased pollen tubes in H. tortuosa . Based on these results, we suggest that the PR mechanism likely exists beyond a single species in the Heliconiaceae and could promote outcrossing in two ways. First, promoting pollen tube growth following a cue from a visiting pollinator could allow a plant to invest in seed production when pollen on the stigma is less likely to be composed entirely of self pollen. Second, because hummingbirds with long, decurved bills tend to be less territorial and more mobile than those with short, straight bills, pollinator recognition could act in conjunction with physical exploitation barriers, such as long and curved corolla tubes, to filter the community of floral visitors in favor of those that are more likely to carry pollen from geographically and therefore genetically distance sources. While the mechanism underlying PR remains equivocal, cryptic pollinator filters may be more common than previously anticipated and could alter perspectives on redundancy in plant-pollinator communities.