There has been little comprehensive research undertaken to quantify resource use by small mammal communities in a nutrient limited, highly stochastic ecosystem. The most abundant small mammals in this ecosystem are Heteromyids, food-caching granivores, and Cricetids, omnivores that must utilize on board fat stores as energy reserves. Heteromyid populations co-vary with primary production whereas the cricetids can forage at multiple trophic levels reducing their dependence on primary productivity. Using isotopic values for Carbon, 13C (a ratio of 13C to 12C), of primary producers consistent within a photosynthetic pathway, C3 = -26.6 +/- 1.8‰ and C4, = -14.4 +/- 0.8‰ we can track mouse dietary assimilation of forage by plant functional type (Craig et al. 1953, Farquhar 1989). Nitrogen isotopic values 15N (a ratio of 15N to 14N) fluctuate constantly, reflecting the landscape of primary production. Therefore, tracking nitrogen values in small mammal plasma provides a landscape level tool for studying diet by trophic level and nutritional value.