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...

Co-pollinators and specialization in the pollinating seed-consumer mutualism between senita cacti and senita moths

“…Senita cacti do not increase resource allocation to initiate fruit of flowers with excess pollen, as may occur when both moths and bees visit same flowers. Also, no differences occur in seeds per fruit between senita mothand bee-pollinated flowers (Fleming et al 2001). Hence, bees are commonly, but not always, functionally redundant with senita moths.…”

“…We report Greenhouse-Geisser adjusted P-values. We re-analyzed 1995, 1996, and 1997 data (Fleming and Holland 1998;Fleming et al 2001) to account for among-plant variance and to compare results with resource limitation. Trials were not conducted in 1999 and 2000 because flowers closed prior to sunrise, naturally excluding diurnal bees.…”

“…Bee visits to senita are likely spill over from visits to flowers of saguaro (Carnegiea gigantea) and organ pipe (Stenocereus thurberi) cacti. Bees contribute more to fruit set of these cacti than to that of senita (Fleming et al 2001), likely because bees are attracted to the large quantities of pollen and nectar produced by flowers of the bat-pollination syndrome of saguaro and organ pipe. Unlike visits to saguaro and organ pipe, whose flowers remain open long after sunrise, bees were not always able to visit senita flowers.…”

Fenología reproductiva de Stenocereus thurberi (Cactaceae) en una región de transición del norte de Sinaloa, México

“…produce estructuras reproductivas de enero a julio en el centro de México (Gudiño & De la Barrera 2014, Rodríguez-Oseguera et al 2012, unos meses antes que S. thurberi en el noroeste del país. Este proceso, regulado por las temperaturas, está acoplado con los tiempos de migración de los murciélagos polinizadores (Fleming et al 2001).…”

Behavioral consequences of innate preferences and olfactory learning in hawkmoth–flower interactions

“…Whereas observational studies of flower visitation (reviewed in ref. 4) and proportionally fewer studies of the effects of changing flower composition on foraging (5) have been conducted (6), the causal mechanisms controlling floral visitation remain unclear and seldom can be demonstrated from the observed correlations. Thus, there remains a fundamental gap between the processes that occur in the field and the underlying behavioral mechanisms mediating those interactions.…”