The effects of nectar-robbing on a plant-pollinator mutualism and the evolution of nectar-robbing and sociality in bees

Abstract

How will the intrusion of other species that remove rewards without providing reciprocal services affect the interaction between mutualists? How do costs and benefits from these "cheaters" compare to costs and benefits from potentially mutualistic visitors? Finally, did nectar-robbing, one kind of cheating, promote the evolution of complex levels of sociality by allowing bees access to a wider range of resources? I investigated these questions in the research described below. I found that pollinators visiting Chilopsis linearis (Bignoniaceae) spent less time visiting robbed flowers than visiting unrobbed flowers, and did not visit them as often as expected. Thus, robbing appeared to have a negative effect on pollinators visiting Chilopsis linearis . I compared costs and benefits of floral visitors to Chilopsis linearis (desert willow). Chilopsis had sensitive stigmas that closed immediately upon touch and may have reopened later. I found that the probability of stigma reopening depended on the source and number of pollen grains deposited. I compared visitors by number of pollen grains deposited, viability of pollen that they deposited, and their effect on stigmas. Nectar-robbers did not benefit plants by pollen deposition, but they also did not cost plants by causing stigmas to close without adequate pollen having been deposited. I investigated the effects of robbing on pollinator behavior and plant reproductive success. Nectar volumes were lower in robbed flowers than in unrobbed flowers. However, the most effective pollinators, bumblebees, did not avoid robbed flowers. In investigating male reproductive success, I found that on some days, dye mimicking pollen traveled farther from robbed flowers, indicating that robbing may sometimes be beneficial to plants. In investigating female reproductive success, I found that there was no difference in pollen tube number between robbed and unrobbed flowers. Thus, a negative effect on one mutualist may not affect the other mutualist. I hypothesized that the evolution of robbing in bees was associated with a broad diet breadth and the evolution of complex sociality. Using phylogenetically independent contrasts for taxa within three geographical regions, I found that in some cases, a broad diet breadth was associated with sociality and robbing.