WOLF, B. O.; MCKECHNIE, A. E.; WARNE, R. ; MATHIASEN, C.; Univ. of New Mexico; Univ. of Witwatersrand; Univ. of New Mexico; Univ. of New Mexico: The functional importance of columnar cacti as resources for a desert bird community; an assessment using stable isotopes

Columnar cacti are prominent features of arid and semi-arid ecosystems in the Neotropics. Cacti are unusual among arid zone plants in that they offer an abundance of succulent, energy rich fruit to vertebrates in an environment where water and nutrient abundance may constrain animal function. Quantifying the importance of these resources to consumers provides important insight into the role that cacti play in structuring communities. In the Sonoran Desert of Arizona, saguaro, Carnegiea gigantea and organ pipe cacti, Stenocereus Thurberi , provide extensive water and energy resources to the bird community during the hottest and driest periods of the annual cycle. Between May and August, saguaro and organ pipe cacti release a huge pulse of nutrients (250,000+ kJ/ha) into the ecosystem in the form of floral nectar and fruit pulp. This nutrient pulse can be tracked into consumers by means of its stable isotope signal, which differentiates cacti from other plant resources in the environment. Cacti such as saguaro, use CAM photosynthesis, and have tissue carbon isotope ratios that differ strongly from the isotopic values of the majority of desert plant species, which use C3 photosynthesis (saguaro/organ pipe -13.0 per mil VPDB versus -25.0 per mil VPDB average for C3 plants). During June, the peak period of fruit production, stable isotope analysis of avian plasma indicates that saguaro fruit represents more than 40% of the bird community�s carbon intake. Our data show that the saguaro resource penetrates deeply into both insectivorous and granivorous avian guilds where it provides water, energy and nutrients. This study provides the first insight into the functional importance of cacti to animal communities in hot subtropical deserts.