Bumblebee floral neighbors promote nectar robbing in a hummingbird-pollinated plant species in Patagonia

Abstract

Nectar robbers are common cheaters of plant-pollinator mutualisms by making holes in flower tissues to attain floral rewards often without providing pollination service. Most studies have focused on the consequences of nectar robbing on plant reproduction, whereas the underlying drivers of spatiotemporal variation in nectar robbing have been comparatively less explored. We assessed variation in nectar robbing of Campsidium valdivianum, an endemic hummingbird-pollinated climber species from the temperate forests of Southern South America, which currently is subjected to nectar robbing by the alien short-tongued Bombus terrestris, and determined if this variation is related to characteristics of the floral neighborhood. We located plants of C. valdivianum and estimated the proportion of flowers with holes. We recorded the presence, identity and distance to the nearest bumblebee-pollinated plants with open flowers. Results showed that the proportion of robbed flowers in C. valdivianum increased almost seven times in the presence of bumblebee flowering plants in the neighborhood. No evidence was found that the proportion of robbed flowers differs between neighborhoods with Berberis darwinii only vs. B. darwinii and Cytisus scoparius, the co-flowering plant species typically visited by bumblebees during the study. Finally, the proportion of robbed flowers increased not only with the presence but also with the proximity of these bumblebee-pollinated plants. Our results suggest that floral neighborhoods attractive to bumblebees can operate as magnets, potentially increasing the intensity of nectar robbing on nearby hummingbird-pollinated species. This study provides novel insights into understanding spatio-temporal variation in nectar robbing.