Spatiotemporal variation in the role of hummingbirds as pollinators of Silene virginica

Pollination restricted to a guild of visitors is central to the concept of pollination syndromes. However, there is limited quantitative evidence that the pollination of plants exhibiting certain pollination syndromes is actually dominated by the expected guild of pollinators. We determined the importance of the sole hummingbird species in eastern North America, the Ruby-throat, Archilochus colubris, and invertebrate visitors as pollinators of Silene virginica, which exhibits traits commonly associated with hummingbird pollination. We quantified the role of either type of visitor as pollinators by comparing female reproductive success of plants that were allowed free access by all visitors to female reproductive success of caged plants that were restricted to pollination by invertebrates (small bees and flies, and in one year Bombus spp.). Spatial and temporal variation in the role of either type of pollinator was documented by comparing the female reproductive success of caged and control plants in two nearby sites for five years. In general we found that hummingbirds are the predominant pollinator of S. virginica, but there is considerable spatiotemporal variation in their role. We discuss the importance of understanding the role of hummingbirds as potential selective agents of S. virginica floral traits within the greater community context of other potential pollinators.