SCHADE, John D.; KYLE, Marcia; HOBBIE, Sarah E.; FAGAN, William F.; ELSER, James J.; Arizona State University, Tempe, AZ; Arizona State University, Tempe, AZ; University of Minnesota, St. Paul, MN; University of Maryland, College Park, MD; Arizona State University, Tempe, AZ: Stoichiometric Interactions Between Desert Soils, Plants and Herbivores

We studied C, N and P dynamics in soils, plants, and their insect herbivores over the course of two years along a topographic gradient from desert scrub to riparian zone in the Sonoran Desert. Our objective was to describe the relationship between soil nutrient availability, plant nutrient concentrations and insect abundance and biochemical composition, both within zones along the gradient, and between zones. We focused on mesquites (Prosopis velutina) and two species of weevil (Curculionidae). Study years differed greatly in amount of precipitation during spring leaf-out, with 2000 drier than normal and 2001 wetter than normal. Foliar C:P ratios were significantly lower in 2001, and within both years foliar C:P was significantly related to soil P. We found a significant positive relationship between foliar C:P and insect C:P and abundance in 2000. Weevil RNA and P content increased significantly between 2000 and 2001, in association with increases in plant P content. These results provide rare evidence of an effect of small-scale changes in soil P availability on insect biochemistry and abundance, suggesting weevils may be P limited. Furthermore, the wet spring in 2001 had a strong effect on desert productivity, in part due to concomitant increases in nutrient availability. Since mesquites have the capacity to fix atmospheric N, they respond more strongly to soil P, and we observe more variation in foliar C:P than C:N. These data suggest a link between patterns of precipitation and insect populations, mediated through the interaction between rainfall, soil P availability and mesquite foliar C:P.