What are the plant reproductive consequences of losing a nectar robber?

Abstract

Pollinator declines worldwide are having strong negative consequences for plants. In many communities, antagonistic flower visitors, including nectar robbers, have likely declined in abundance as well. Given the negative effects that these visitors can sometimes inflict, might declines in their populations benefit plants? During the 1970s, the floral visitor community of the Colorado columbine, Aquilegia caerulea (Ranunculaceae), was documented near Gothic, Colorado. At that time, Bombus occidentalis, the Western Bumble bee, was one of its many pollinators, but more commonly acted as its only known nectar robber. Bombus occidentalis abundance has declined precipitously throughout the Western USA since the 1970s. In 2016, we documented the floral visitor community in sites near to those used in the original survey. We then experimentally quantified the effects of nectar robbing, allowing us to estimate the reproductive consequences of losing B. occidentalis. We also quantified the potential pollination services of muscid flies (Muscidae, Diptera). The floral visitor community was dramatically different in 2016 compared to the 1970s. Bombus occidentalis, a frequent A. caerulea visitor from 1969-1976, was infrequently observed visiting the plant, and nectar robbing was negligible. Our experiments suggested that a high level of nectar robbing would lead to significantly reduced fruit set, although not seeds per fruit. Fly visits to flowers were dramatically higher in 2016 compared to the 1970s. We show that, in the absence of bumble bee pollinators, muscid flies significantly reduced fruit set below the self-pollination rate. The negative effect of the increase in these flies likely outweighed any positive effects A. caerulea experienced from the absence of its nectar robber. Although the field observations were conducted in a single year, when they are interpreted in combination with our manipulative experiments, they suggest how A. caerulea may fare in a changing visitation landscape.