Sonoran Desert Columnar Cacti and the Evolution of Generalized Pollination Systems

Abstract

We studied variation in flowering phenology, fruit and seed set, and the abundance of the pollinators of four species of night-blooming Sonoran Desert columnar cacti for up to eight years at one site in Mexico and one year at one site in Arizona. We determined how spatiotemporal variation in plant-pollinator interactions affects the evolution of generalized pollination systems. We conducted pollinator exclusion and hand pollination experiments to document annual variability in pollinator reliability and to determine whether pollination systems were redundant (different species are partially or totally substitutable) or complementary (different species have an additive effect on fruit set). The cacti we studied included three species with generalized pollination systems involving bats, birds, and bees (cardon, Pachycereus pringlei; saguaro, Carnegiea gigantea; and organ pipe, Stenocereus thurberi) and one specialized moth-pollinated species (senita, Lophocereus schottii). We predicted that the migratory lesser long-nosed bat, Leptonycteris curasoae, is a less reliable pollinator than birds and bees, and that cacti with generalized pollination systems have more variable flowering phenologies than the specialized species.Annual time of peak flowering and mean size of flower crops were relatively invariant in saguaro and organ pipe. Time of peak flowering in cardon varied by as much as six weeks, and mean flower crop size varied threefold over six years. In senita, peak flowering varied by as much as 5-8 wk among years. Peak numbers of the nectar bat L. curasoae varied among years, and bat density (0.9/ha) was an order of magnitude lower than that of cactus-visiting birds at both study sites. The abundance of migratory hummingbirds was also highly variable among years.Pollinator exclusion experiments indicated that bats were major pollinators of cardon, whereas diurnal visitors accounted for most fruit set in saguaro (except in 1995 when bats were most important) and organ pipe at our Mexican site; honeybees accounted for 64-87% of diurnal fruit set in these species. Annual variation in the contribution to fruit set by bats was substantially higher than that of diurnal pollinators in saguaro and organ pipe, but not in cardon. There was little geographic variation in the relative importance of nocturnal vs. diurnal pollinators in saguaro and senita, but bats were much more important for fruit set in organ pipe in Arizona than in Mexico. We generally detected no effect of different pollinators on number of seeds per fruit in any species.Annual variation in fruit set was lowest in saguaro, the species with the most diurnal pollination system, and highest in organ pipe, the species with the most generalized pollination system. Fruit set was strongly pollen limited only in females of cardon (a trioecious species) and in organ pipe (at both sites). The ''missing'' pollinators in both species are likely Leptonycteris bats. The pollination systems of saguaro and cardon were partially redundant, whereas that of organ pipe was...

Historical Vicariance and Postglacial Colonization Effects on the Evolution of Genetic Structure in Lophocereus, a Sonoran Desert Columnar Cactus

“…Mature fruits contain approximately 180 small black seeds imbedded in a red pulp with the seeds presumably being largely bird dispersed. Flowering occurs from April through August, with the fruits maturing within 30 days (Holland and Fleming 1999a;Fleming et al 2001). In addition to pollen and seed dispersal, vegetative reproduction occurs by rooting from stem fragments in some populations (Parker and Hamrick 1992), perhaps more so toward the northern limits of the geographic range.…”

Exploring the boundary between pollination syndromes: bats and hummingbirds as pollinators of Burmeistera cyclostigmata and B. tenuiflora (Campanulaceae)

“…Once the 'correct' pollinator for a particular plant has been identified, researchers may consciously or unconsciously overlook other potential pollinators. Recent studies suggest that generalization on multiple taxa of pollinators may be more common than previously thought (Waser et al 1996;Fleming et al 2001;Mayfield et al 2001).…”

Resource availability, population dynamics and diet of the nectar-feeding bat Leptonycteris curasoae in Guaymas, Sonora, Mexico

“…We conducted periodic evening-exit censuses of the colony (7 censuses from February 15th, 2000 to August 17th, 2000) using the Guaymas roost following the methods of Horner et al (1998) and Fleming et al (2001). Every time we visited the Guaymas roost we stayed after sunset and estimated the number of bats using the cave by counting the individuals leaving the roost and crossing a red light set at the entrance of the cave.…”