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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.

Disparities in nutrient content (nitrogen and phosphorus) between herbivores and their plant resources have lately proven to have major consequences for herbivore success, consumer-driven nutrient cycling, and the fate of primary production in ecosystems. Here we extend these findings by examining patterns of nutrient content between animals at higher trophic levels, specifically between insect herbivores and predators. Using a recently compiled database on insect nutrient content, we found that predators exhibit on average 15% greater nitrogen content than herbivores. This difference persists after accounting four variation from phylogeny and allometry. Among herbivorous insects, we also found evidence that recently derived lineages (e.g., herbivorous Diptera and Lepidoptera) have, on a relative basis, 15-25% less body nitrogen than more ancient herbivore lineages (e.g., herbivorous Orthoptera and Hemiptera). We elaborate several testable hypotheses for the origin of differences in nitrogen content between trophic levels and among phylogenetic lineages. For example, interspecific variation in insect nitrogen content may be 1) directly traceable to differences in dietary nitrogen (including dilution by gut contents), 2) selected for directly in response to the differential scarcity of dietary nitrogen, or 3) an indirect consequence of adaptation to different feeding habits. From some functional perspectives, the magnitude rather than the source of the interspecific differences in nitrogen content may be most critical. These findings have implications for both the trophic complexity of food webs and the evolutionary radiation of herbivorous insects.