干旱区生态环境知识资源中心

We studied the pollination biology of three species of night-blooming columnar cacti (cardon, Pachycereus pringlei; saguaro, Carnegia gigantea; and organ pipe, Stenocereus thurberi) near Bahia Kino, Sonora, Mexico, by addressing two questions: (1) What is the relative importance of nocturnal (bats, moths) and diurnal (bees, birds) pollinators for fruit and seed set, and (2) To what extent do these cacti compete for pollinators? Honeybees had the highest visitation rates to flowers of the three species but are relatively ineffective pollinators. Bats had higher visitation rates to flowers of cardon than birds, which had higher visitation rates to flowers of saguaro and organ pipe than bats. Results of pollinator exclusion experiments indicated that visits by bats accounted for most of the fruit set in cardon, visits by birds and bees accounted for most of the fruit set in saguaro, and birds accounted for most of the fruit set in organ pipe. Hand-pollination experiments suggest that fruit set in females but not hermaphrodites of cardon, a trioecious species, and organ pipe is pollen-limited, probably owing to the scarcity of bats. Crude estimates of the per-visit effectiveness of different groups of pollinators for fruit set indicate that bats are somewhat more effective than birds and much more effective than bees. Competition for pollinators between the three species of cacti is reduced by seasonal or daily phenological differences. The flowering seasons of cardon and saguaro broadly overlap, but these species differ in the times their flowers open and close (earlier at night and the next morning, respectively, in cardon) and in their nectar secretion schedules. Saguaro flowers are more available to diurnal pollinators than those of cardon. The flowering peak of organ pipe occurs much later in the spring than the other two species; its opening and closing and nectar secretion schedules are similar to those of cardon. At our study site, the importance of bats as pollinators of columnar cacti is currently much lower than in other arid zone columnar cacti.