Pollination of two species of Ferocactus:: interactions between cactus-specialist bees and their host plants

1. Resolving the controversy over the prevalence of generalization in plant-pollinator interactions requires field studies characterizing the pollination effectiveness of all a plant's floral visitors. Herein, the pollination effectiveness of all visitors to two species of barrel cactus (Ferocactus) was quantified. 2. Flowers of both species were pollinated almost exclusively by cactus-specialist bees: 99% (F. cylindraceus (Engelm.) Orcutt) and 94% (F. wislizeni (Engelm.) Britt. and Rose) of all seeds produced in this study resulted from cactus bee visits. 3. For F. cylindraceus, the cactus-specialist Diadasia rinconis was the most abundant visitor. For F. wislizeni, three cactus-specialists (including D. rinconis) plus generalists in the family Halictidae (which did not act as pollinators) each accounted for a quarter of all visits. 4. Diadasia rinconis visits to F. wislizeni flowers were more effective (per-visit) than visits by the other two cactus-specialists. 5. Pollen-collecting and nectar-collecting visits were equally effective. Nectar-collecting visits were the most abundant. 6. Apart from the non-pollinating halictids, floral visitors surprisingly did not include commonly co-occurring generalist bees. 7. These data suggest that, just as apparently specialized flowers may be visited by a diverse assemblage of generalists, so apparently generalized flowers may be visited predominantly by specialists, and that these specialists may perform virtually all of the pollination.