Niche conservatismâthe retention of ecological traits across space and timeâis an emerging topic of interest because it can predict responses to global change. The conservation of Grinnellian niche characteristics, like speciesâhabitat associations, has received widespread attention, but the conservation of Eltonian traits such as consumerâresource 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 We detected no pairwise overlap of dietary niches in δâspace, and distributions of individual diets in pâspace revealed little overlap in core diets across populations. All pairwise comparisons of individuals revealed significant differences in diet, and populationâlevel comparisons detected contrasting use of functional prey groups. We developed a multiâfaceted 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 consumerâresource dynamics are largely driven by exogenous environmental factors like land cover and community composition. These results illustrate the contextâdependent nature of foraging and indicate consumer functionality can be dynamic.
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