NECTAR-ROBBING BUMBLE BEES REDUCE THE FITNESS OFIPOMOPSIS AGGREGATA(POLEMONIACEAE)

Abstract

The removal of floral nectar by nectar robbers may change the behavior of legitimate pollinators and, consequently, the pollination and fertilization success of flowering plants. We measured the effects of a nectar-robbing bumble bee, Bombus occidentalis, on male and female reproductive success of Ipomopsis aggregata (Polemoniaceae). We artificially varied nectar-robbing rates within 20 natural I. aggregata populations in Colorado. Using powdered fluorescent dyes as pollen analogs, we estimated the amount of pollen donated (male fitness component) and the amount of pollen received (female fitness component) by plants with different levels of nectar robbing. We also measured percentage fruit set per plant, seed set per fruit, and average seed mass as additional components of female fitness. Nectar robbing significantly decreased both male and female fitness components. Heavy nectar robbing (>80% of available flowers robbed) significantly decreased dye donation, pollen receipt, and average number of seeds per fruit at the whole-plant level. Furthermore, nectar robbing significantly decreased dye donation by individual flowers. Although nectar robbing has often been regarded as unimportant to plant reproductive success, we document significant effects of nectar robbing on components of both male and female fitness of I. aggregata. Our results suggest that interactions among nectar robbers, pollinators, and their plant hosts may have important consequences for plant fitness.