Niche conservatismthe retention of ecological traits across space and timeis an emerging topic of interest because it can predict responses to global change. The conservation of Grinnellian niche characteristics, like species-habitat associations, has received widespread attention, but the conservation of Eltonian traits such as consumer-resource interactions remains poorly understood. The inability to quantify Eltonian niches through space and time has historically limited the assessment of Eltonian niche conservatism and the dynamics of foraging across populations. Consequently, the relative influence of endogenous factors like phylogeny versus exogenous features like environmental context has rarely been addressed. We tested Eltonian niche conservatism using a paired design to compare foraging among four populations of American martens Martes americana and Pacific martens Martes caurina, morphologically and ecologically similar sister taxa that are allopatrically distributed throughout western North America. We developed a three-stage isotopic framework and then quantified dietary niche overlap between the sister species and paired island-mainland sites to assess the relative influence of endogenous (i.e., species) versus exogenous (i.e., environment) factors on Eltonian niches. First, we calculated pairwise dietary overlap in scaled -space using standard ellipses. We then estimated proportional diets (p-space) for individuals using isotopic mixing models and developed a novel utilization distribution overlap approach to quantify proportional dietary overlap. Lastly, we estimated population-level proportional diets and quantified the differential use of functional prey groups across sites. We detected no pairwise overlap of dietary niches in -space, and distributions of individual diets in p-space revealed little overlap in core diets across populations. All pairwise comparisons of individuals revealed significant differences in diet, and population-level comparisons detected contrasting use of functional prey groups. We developed a multi-faceted isotopic framework to quantify Eltonian niches and found limited evidence of Eltonian niche conservatism across carnivore populations. Our findings are consistent with the growing recognition of dietary plasticity in consumers and suggest that consumer-resource dynamics are largely driven by exogenous environmental factors like land cover and community composition. These results illustrate the context-dependent nature of foraging and indicate consumer functionality can be dynamic.