CONSEQUENCES OF NECTAR ROBBING FOR REALIZED MALE FUNCTION IN A HUMMINGBIRD‐POLLINATED PLANT

The effects of nectar robbers on plants and their mutualistic pollinators are poorly understood due, in part, to the paucity of studies examining male reproductive success in nectar‐robbed plants. Here we measured the effects of a nectar‐robbing bumblebee, Bombus occidentalis, on realized male reproductive success (seeds sired) in a hummingbird‐pollinated plant, Ipomopsis aggregata. To determine the effects of nectar robbing on paternity, we used a series of experimental populations of plants containing a known allozyme marker. In each population, we experimentally controlled the levels of nectar robbing on each I. aggregata plant by cutting a hole in the corolla with dissecting scissors and removing nectar with a micro‐capillary tube. We measured hummingbird‐pollinator foraging behavior and fruit and seed production (maternal function) for each plant. We then genotyped seeds for the allozyme marker to determine the number of seeds sired by plants with known levels of robbing. Heavy nectar robbing (> 80% of flowers robbed) significantly reduced the number of seeds sired, as well as the number of seeds produced due to pollinator avoidance of heavily robbed plants. Total plant reproduction, both male and female contributions, were reduced by 50% due to high levels of robbing. To date, no other studies have measured the effects of nectar robbing on realized male function (number of seeds sired). Ours is the first study to demonstrate that robbing can simultaneously decrease realized male reproductive success as well as female reproductive success, and that the effects are incurred indirectly through pollinator avoidance of robbed plants.