Rodents provide a unique glimpse into the environmental conditions at specific geographic ranges and timescales and play an important role as primary consumers. Investigating their diet using thermal ionization mass spectrometry reveals resource availability across periods of precipitation fluctuation. Museum specimens and live small mammals were sampled for this study to analyze stable isotopes in correlation with Palmer Drought Severity Indices. Small mammal trapping occurred at the Jack and Laura Dangermond Preserve in Lompoc, California for two consecutive summers to infer diet under varying levels of drought severity. Animal tissues were analyzed in collaboration with the University of New Mexico’s Center for Stable Isotopes to determine the diet of small mammals on the Dangermond Preserve and a historical analysis of Neotoma specimens from Central California. Variability in the ratios of stable isotopes indicates changes in resource availability during periods of drought across many decades of California’s history. Diet changed in response to drought on spatially explicit scales and is subject to habitat-specific pressures as well. Understanding the previous and current ecological niches of native rodents may further our understanding of resource use and conservation efforts.