• Sahley, Catherine T.

I examined 4 components of pollination of the organ-pipe cactus, Stenocereus thurberi, by bats and hummingbirds. I quantified amount of pollen deposited per flower visit by bats and hummingbirds on stigmas of conspecifics, estimated amounts of pollen transferred per visit, examined variability in pollinator success within and among flowering seasons of S. thurberi, and determined levels of pollen-mediated gene flow. Both bats and hummingbirds deposit large quantities of pollen on a per visit basis. Pollinator exclusion experiments indicate that proportion of fruits produced by hummingbird pollination was constant in spring 1992 and 1993, but that of bats was significantly greater in 1993 than in 1992. I compared results obtained in spring of 1992 and 1993 of those obtained from a previous 2 year study, and found that, in all 4 years, proportion of fruits produced by hummingbird pollination remained constant, but proportion of bat pollination varied significantly. During 1993, fruit production attributable to both bats and hummingbirds dropped significantly in late June relative to spring, after peak flowering. Variability in fruit-set by control flowers is likely due to variability in bat pollination events. Paternity exclusion analysis of cactus seeds indicated that most paternal gametes probably came from within 75 m, although substantial amounts of gene flow came from beyond 125 and 150 m for the 2 S. thurberi populations studied.