Meeting Abstract

7.4  Jan. 4  Isotopes, Cacti and Mice: Exploration of Cactus Use by a Sonoran Desert Rodent Community Across Time. ORR, T.J.*; WOLF, B.O.; Univ. of California, Riverside; Univ. of New Mexico teri.orr@email.ucr.edu

Cacti such as the saguaro (Carnegiea gigantea) are considered keystone producers due to their production of energy and water rich flowers and fruit. These plants may be of particular importance to consumers during the hottest and driest periods of the annual cycle. Their role in the nutritional ecology of consumers is, however, known mostly from anecdotal observations. Saguaro production results in a pulse of nutrients during summer months, that is expected to be reflected in rodent tissues in the form of different Carbon signatures. We investigated the importance of cactus resources to the small mammal community using stable isotopes. Cacti use a CAM photosynthetic pathway that results in tissues with carbon isotope ratios that differ from most of the plant community, which perform C3 photosynthesis (CAM approximately δ13C = -13�, C3 approximately δ13C = -26� VPDB). Because an animal�s diet is reflected in the isotopic composition of its tissues, we measured d13C of plasma and used a simple two-way mixing model to estimate the proportion of carbon derived from ingestion of cactus tissues. We also used Nitrogen 15 enrichment to assess trophic level differences among species. Hoarders such as Dipodomys merriami had isotope ratios consistent with C3 resources (δ13C between -25� and -23� VPDB) with little seasonal change. Herbivores such as Neotoma albigula consuming cactus stems had δ 13C between -18� and -13� VPDB. Several species (e.g. Chaetodipus penicilatus and C. baileyii) exhibited a dramatic increase in diet breadth during CAM fruit production (May-July). We discuss rodent community use of cacti over the period of one year at Organ Pipe Cactus National Monument.