Are nectar robbers cheaters or mutualists?

Nectar robbers are birds, insects, or other flower visitors that remove nec tar from flowers through a hole pierced or bitten in the corolla. This pape r is a review of the effects of nectar robbers on pollinators, pollination, and fitness of the plants they rob. Charles Darwin assumed that nectar rob bers 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 neutra l effect. Several studies document that robbers frequently pollinate the pl ants that they visit. Robbers may also have an indirect effect on the behav ior of the legitimate pollinators, and in some circumstances, the change in pollinator behavior could result in improved fitness through increased pol len how and outcrossing. The effects of nectar robbers are complex and depe nd, 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 r esources available in the environment.