POLLINATION BIOLOGY AND THE RELATIVE IMPORTANCE OF NOCTURNAL AND DIURNAL POLLINATORS IN 3 SPECIES OF SONORAN DESERT COLUMNAR CACTI

We studied the pollination biology of three species of night-blooming columnar cacti (cardon, Pachycereus pringlei; saguaro, Carnegia gigant ea; and organ pipe, Stenocereus thurberi) near Bahia Kino, Sonora, Mex ico, by addressing two questions: (1) What is the relative importance of nocturnal (bats, moths) and diurnal (bees, birds) pollinators for f ruit and seed set, and (2) To what extent do these cacti compete for p ollinators? Honeybees had the highest visitation rates to flowers of t he three species but are relatively ineffective pollinators. Bats had higher visitation rates to flowers of cardon than birds, which had hig her visitation rates to flowers of saguaro and organ pipe than bats. R esults of pollinator exclusion experiments indicated that visits by ba ts 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 account ed for most of the fruit set in organ pipe. Hand-pollination experimen ts suggest that fruit set in females but not hermaphrodites of cardon, a trioecious species, and organ pipe is pollen-limited, probably owin g to the scarcity of bats. Crude estimates of the per-visit effectiven ess of different groups of pollinators for fruit set indicate that bat s are somewhat more effective than birds and much more effective than bees. Competition for pollinators between the three species of cacti i s reduced by seasonal or daily phenological differences. The flowering seasons of cardon and saguaro broadly overlap, but these species diff er 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 t han those of cardon. The flowering peak of organ pipe occurs much late r in the spring than the other two species; its opening and closing an d nectar secretion schedules are similar to those of cardon. At our st udy site, the importance of bats as pollinators of columnar cacti is c urrently much lower than in other arid zone columnar cacti.