Differential Outcomes of Nectar Robbing on the Reproductive Success of a Melittophilous and an Ornithophilous Species

Premise of research.Nectar robbers can affect pollinator behavior and indirectly affect plant fitness. The signal of such effects (positive, neutral, or negative) may depend on how the primary pollinators of a given plant species respond to robbed flowers. Comparative studies are important to give insight on which factors are linked to these differential outcomes of nectar robbing on plant reproductive success. Here, we investigated the effect of a common Neotropical nectar robber, the stingless bee, Trigona spinipes (Apidae), on the reproductive success of the bee-pollinated Crotalaria vitellina (Fabaceae) and the hummingbird-pollinated Besleria longimucronata (Gesneriaceae).Methodology.We compared the robbing rate (number of flowers robbed relative to total flower display) between species and evaluated the effect of robbing on plant reproductive success by comparing the seed number per fruit of robbed fruits (identified by the presence of scars in the calyx, caused by the robber) with that of intact fruits.Pivotal results.The hummingbird-pollinated B. longimucronata had a higher proportion of robbed flowers than the bee-pollinated C. vitellina. Moreover, we found a negative effect of robbing on seed number per fruit for B. longimucronata but no effect for C. vitellina.Conclusions.The differences in the outcome of nectar robbing can be interpreted based on previous studies of pollinator behavior: hummingbirds often avoid nectar-robbed flowers, while bees often do not exhibit a preference between robbed and intact flowers. Our results show that the same robber species can have differential effects on the reproductive success of distinct plant species. In this sense, the outcome of nectar robbing will depend on the interplay between plant traits and pollinator behavior.