Oecologia

, Volume 139, Issue 2, pp 246–254 | Cite as

Are nectar-robbers mutualists or antagonists?

Plant Animal Interactions

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.

Keywords

Conditional outcome Indirect effects Mutualism Nectar-robbing Reproductive success 

Notes

Acknowledgements

Financial support for this work came from two grants from the Theodore Roosevelt Memorial Fund of the American Museum of Natural History, by the 1994 Cranwell Smith Pollination Award from the University of Arizona, by the Southwestern Research Station Student Support Fund, and by grants from the Department of Ecology and Evolutionary Biology at the University of Arizona. The Knight family of Paradise, Ariz., the Ritchins families of Cotton City, N.M. and Animas, N.M., J. Caron of Crown Dancer Estates, M. Kraft, and A. and J. Dominic of Portal, Ariz. were kind in allowing research to be conducted on their land. I thank N. Plotkin, J. Chesnick, S. McGraw, P. Murphy, D. Banks, N. Compton, K. Toal, K. Spörns, A. Gove, and M. Brown for assistance in the field, and S. Buchmann for instruments, dye, and advice on techniques. The comments of J. Bronstein, L. McDade, J. Rudgers, R. Hudson, S. Kinsman, S. Forsyth, A. Boyle, J. Weeks, H. Harvey, H. Reynolds, T. Rajaniemi, and J. Bever improved this study and manuscript.

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Copyright information

© Springer-Verlag 2004

Authors and Affiliations

  1. 1.Indiana UniversityDepartment of BiologyBloomingtonUSA

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