WOLF, B.O; MCKECHNIE, A.E.; Univ. of New Mexico, Albuquerque; Univ. of New Mexico, Albuquerque: Nutrient Dynamics in a Desert Bird Community; the Functional Importance of Columnar Cacti

Columnar cacti are prominent features of arid and semi-arid ecosystems in the Neotropics. Cacti are unusual in that they offer an abundance of succulent, energy rich fruit to vertebrates in an environment where water and nutrient abundance may limit animal function. Identifying the importance of these resources to consumers provides important insight into the role that cacti play in structuring communities and food webs. We are looking at one such system in the Sonoran Desert of Arizona, where the saguaro cactus Carnegiea gigantea provides extensive water and energy resources to the bird community during the hottest and driest periods of the annual cycle. Between May and August, the saguaro releases a huge pulse of nutrients (400,000+ kJ/ha) into the ecosystem in the form of floral nectar, pollen and fruit pulp. This nutrient pulse can be tracked into the bird community because the saguaro uses a form of photosynthesis that differentiates it isotopically from other resources in the environment. Plants such as saguaro use CAM photosynthesis, and have a carbon isotope signal that differs strongly from the C3 photosynthetic signal produced by the majority of plants in this community (saguaro -13.0� VPDB versus -25.0� VPDB average for seeds of seven C3 food plants). During the peak period of fruit production, stable isotope analysis of avian plasma indicates that saguaro fruit represents approximately 43% of the bird community�s carbon intake. The saguaro resource penetrates deeply into both insectivorous and granivorous foraging guilds where it provides water, energy and nutrients. These studies provide the first insights into the functional importance of cacti to communities and ecosystems.