Background/Question/Methods It has been proposed that the pollination system of chiropterophilous columnar cacti in Mexico show latitudinal geographic differentiation. Current hypothesis state than most columnar cacti in tropical areas depend on nectar-feeding bats for their reproduction (specialists), while species from extratropical areas are pollinated by diverse assemblages of species (generalists). To test this hypothesis, the effects of pollinator guild and the variation in time and space on the reproductive success of a widespread species were examined. During two years, we recorded the temporal variation in pollinator assemblages and their effect on fruit and seed production in three widely separated populations of Stenocereus thurberi in NW Mexico. Variation in breeding system, pollen limitation, and controlled pollinator exclusion experiments were carried out by exclusion experiments.
Results/Conclusions Significant differences were found in the type, timing of activity, and effectiveness of pollinators among sites. In the northern and central populations most of the reproductive success depends on the bat Leptonycteris curasoae whereas in the southern population a combination of pollinators was more effective. No differences between the open and the hand cross-pollination treatments were found in the northern and central populations, indicating that there was no pollen limitation.  However, significant differences between these treatments were detected in the southern population indicating temporal differences in pollinator abundance and/or the timing of their arrival. Variation in pollinator assemblages and reproductive success suggests that local processes could greatly affect the evolution of specialized pollination systems. The pattern of generalist pollination in the southernmost populations and specialized pollination in the central and northern population contradicts the current hypothesis of latitudinal variation of generalist-specialist pollination. The temporal variation in pollinator timing (timing of arrival and intensity of the activity of pollinators) could lead to a bet hedging strategy caused by the gamble on reproductive success based on the unpredictable availability of pollinators on some populations.