Are nectar robbers cheaters or mutualists?

Nectar robbers are birds, insects, or other flower visitors that remove nectar from flowers through a hole pierced or bitten in the corolla. This paper is a review of the effects of nectar robbers on pollinators, pollination, and fitness of the plants they rob. Charles Darwin assumed that nectar robbers had a negative impact on the plants that they visit, but research done in the last 50 years indicates that they often have a beneficial or neutral effect. Several studies document that robbers frequently pollinate the plants that they visit. Robbers may also have an indirect effect on the behavior of the legitimate pollinators, and in some circumstances, the change in pollinator behavior could result in improved fitness through increased pollen flow and outcrossing. The effects of nectar robbers are complex and depend, in part, on the identity of the robber, the identity of the legitimate pollinator, how much nectar the robbers remove, and the variety of floral resources available in the environment.