A. M.  Klein, Vassiere, B. E., Cane, J. H., Steffan-Dewenter, I., Cunningham, S., Kremen, C., and Tscharntke, T., ?Importance of pollinators in changing landscapes for world crops?, Proceedings of the Royal Society B: Biological Sciences, vol. 274, pp. 303 - 313, 2007.