Individual diet specialization is prevalent in wild populations and scales up to drive ecological processes at the population and community levels. As the population trophic niche emerges from the combination of foraging decisions made by individuals, specific individuals with particular niches can disproportionately contribute to the niche of their population, i.e., keystone niche individuals. We investigated whether individuals of South American fur seals and sea lions differ in their contribution to the trophic niche width of their populations. To quantify trophic variation within and between individuals over long time frames, we analyzed stable isotopes (δ13C and δ15N) of serially sampled whiskers (18 individuals of O. flavescens and 20 of A. australis females; average of 33 measures per individual). We used these repeated observations per individual to model population-level trophic niches in the isotopic bivariate space, accounting for between and within-individual variation. Then, we estimated niche width, position, and orientation for each individual in the population. Finally, we quantified the relative contribution of each individual to the niche of its population by estimating how niche width changes when a given individual is removed from its population. While most conspecifics make negligible contributions, a few keystone niche individuals contribute disproportionately to population niche width. Individuals with niches located close to the population centroid promote the contraction of the population trophic niche, while individuals with niches located far from the population centroid promote its expansion. Further, wide individual niches with a divergent orientation compared to the population niche tend to expand the population niche width. Our findings reveal how intraspecific niche variation can shape population niches from the inside out, highlighting that individuals differ substantially in their contribution to patterns at higher levels of biological organization.