by Ledbetter et al.
|Colorado columbine (Aquilegia caerulea)|
Pollinators are declining worldwide, a topic of huge interest and study at present. Flowers, though, are fed upon by a wide variety of less beneficial and less well-studied animals: some consume the flowers, others the developing seeds, and still others feed on floral nectar without transferring pollen among flowers. All of these visitors can, at least in some situations, harm plants. Loss of pollinators is clearly detrimental to plants – but would the loss of these floral antagonists, in contrast, be good for them? We took advantage of a precipitous decline in abundance of a floral antagonist in a well-studied plant-pollinator system to address this question. During the 1970s, floral visitors of the Colorado columbine, Aquilegia caerulea (Ranunculaceae), was studied near Gothic, Colorado. At that time, Bombus occidentalis, the Western Bumble bee, was one of its many pollinators. More commonly, though, it acted as a nectar robber: it drilled holes in columbine’s spectacular, long floral spurs and fed on the nectar through them, neither picking up nor depositing pollen in the process. The Western Bumble bee has declined dramatically throughout the Western USA since the 1970s. In 2016, we documented floral visitors at sites near those used in the original survey. We then used experiments to measure how detrimental nectar robbing is to columbine. With this information, we could estimate how the plant would fare if the Western Bumble bee were to disappear. We found that visitors to columbine flowers was dramatically different in 2016 compared to the 1970s. The Western Bumble bee was only very rarely seen, and nectar robbing, which had been quite frequent in the 1970s, was virtually absent. What might have been the consequences to columbine of losing this antagonist? Our experiments indicate that a high level of nectar robbing would lead to significantly reduced fruit set, although not seeds per fruit, suggesting (although not proving) that its rarity might benefit columbine. We note, however, that columbine’s pollinators have also declined, a loss that likely far exceeds in impact the decreased floral antagonism the plant faces. In the year of the study, houseflies (Muscidae) were present in almost overwhelming numbers at our sites. They were common seen in large numbers sitting on the flowers (and, of course, on us). Taking advantage of the situation, we explored how important these visitors are to the plants, specifically, whether they transfer pollen. If they are good pollinators, it is possible that they can help make up for the decline in other pollinator species. However, we found that housefly visits dramatically reduce columbine fruit production. An increase in housefly abundance thus would be one more blow to the success of this charismatic plant. Although our field observations were conducted in a single year, when interpreted in combination with our manipulative experiments, they suggest how columbine might fare in a changing visitation landscape.