Andrea R. Litt, Justine A. Becker, Matthew J. Gould, Kerry A. Gunther, Mark A. Haroldson, Frank T. van Manen

Population trajectories emerge from the ability of individuals to acquire and allocate resources to meet energetic demands for survival and reproduction. Under density-dependent population regulation, increasing interference and exploitative competition can constrain access to high-energy foods and restructure dietary niche breadth. Individual differences in diet composition should translate into distinct movement strategies and may buffer demographic performance if behavioral plasticity compensates for variation in diet quality. We studied grizzly bears (Ursus arctos) in the Greater Yellowstone Ecosystem of the western US to evaluate the behavioral-ecological mechanisms of density-dependent population regulation. We tested these mechanisms by integrating estimates of assimilated diet from stable isotopes (δ¹³C, δ¹⁵N) of bear hair (1997–2022; n = 1270 individuals, 1906 samples) with spatially explicit indices of grizzly bear density and ungulate availability, and movement and demographic metrics. Dietary shifts were consistent with dominance-mediated interference competition: males consumed the greatest proportion of ungulates (regardless of bear density), and across cohorts, bear consumption of ungulates declined as bear density increased (low-density: male=0.60 [95% CI=0.50–0.69], female=0.47 [0.36–0.61]; high-density: male=0.32 [0.23–0.41], female=0.22 [0.15–0.32]). Dietary niches of each cohort expanded with bear density, indicating that competition increases dietary generalization. Our analyses incorporate ungulate availability to disentangle density effects from prey dynamics, quantify how diet composition aligns with movement strategies, and evaluate whether demographic performance varies with diet. This work advances a mechanistic framework linking individual foraging strategies with competition and resource availability, improving our understanding of how long-lived, behaviorally plastic and individualistic species respond to changes in their environment.