Are nectar-robbers mutualists or antagonists?

Abstract

As "exploiters" of plant-pollinator mutualisms, nectar-robbers remove rewards (nectar) without providing pollination services. Though one might expect nectar-robbing to be costly to plants, it may instead benefit plants by indirectly increasing pollen dispersal. I investigated the direct effects of nectar-robbing bees ( Xylocopa californica) on floral rewards and behaviors of pollinators visiting desert willow ( Chilopsis linearis) and indirect effects of robbing on the reproductive success of the plant. Nectar-robbers reduced nectar; while unrobbed and robbed flowers were equally likely to contain nectar, nectar volumes were smaller in robbed flowers with nectar. Apis mellifera (honeybees), ineffective pollinators in terms of pollen deposition, avoided robbed flowers. In contrast, Bombus sonorus (bumblebees), effective pollinators, did not avoid robbed flowers. While bumblebees tended to spend less time in robbed flowers, the time that they spent in flowers was not correlated with pollen deposition. Using powder mimicking pollen, I found that on some days, powder was dispersed farther or to more flowers from robbed flowers, indicating that robbing may sometimes benefit plants by increasing male reproductive success. Powder movement suggested that the effect of robbing on male reproductive success ranged from costly to beneficial. The outcome for flowers that were marked early each morning was a function of prevalence of robbing and abundances of effective pollinators, but not a function of spatial variability among trees in prevalence of robbing or the abundance of ineffective honeybees. Unlike powder dispersal, female reproductive success, measured by fruit set and the number of pollen tubes growing in styles, was not affected by robbing. Thus, robbers did not reduce plants' female reproductive success either directly by damaging flowers or indirectly by reducing pollen deposition by pollinators. Overall, this study indicates that nectar-robbers were not often costly to plants, and sometimes even benefited plants.

Nectar robbing of a carpenter bee and its effects on the reproductive fitness of Glechoma longituba (Lamiaceae)

“…Our observations in the field indicated that after robbing the flowers in one patch, carpenter bees fly greater distances before landing to rob in another patch than the distance flown by the other main pollinator species between 2 pollination bouts. This behavior is similar to that of bumblebees and may probably increase the distance of pollen flow and also the out-crossing rates (Zimmerman and Cook 1985;Gliddon and Saleem 1984;Fenster 1991;Richardson 2004; but see Irwin 2003). In this case nectar robbing may be beneficial to G. longituba by indirectly contributing to increased heterozygosity and genetic diversity in its populations.…”

Polinizadores de Bertholletia excelsa (Lecythidales: Lecythidaceae): interações com abelhas sem ferrão (Apidae: Meliponini) e nicho trófico

“…Bertholletia excelsa é uma espécie de planta com sistema reprodutivo auto-incompatível (Maués 2002), sendo, portanto, efi cientemente polinizada somente se seus polinizadores visitarem outros indivíduos na população (Goulson 1999, Singer & Koehler 2003, Richardison 2004, Tangmitcharoen et al 2006. Desse modo, é intrigante considerar que as abelhas sem ferrão, embora estejam afetando a atividade de forrageio dos polinizadores em questão, possam também, até certo ponto, contribuir para a polinização cruzada da castanheira.…”

Notes on the flowering and pollination of the endemic grassland <i>Aloe reitzii</i> var. <i>reitzii</i> (Asphodelaceae)

“…While it is possible that several of these were nectar robbers (Richardson 2004), honeybees were recorded in this study and have been shown as important pollinators of other Aloe species, (Botes et al 2009a;Symes et al 2009). They made up by far the greatest proportion of recorded insect visitors so, like in A. greatheadii var.…”