Interspecific Interactions Constrain Individual Niche Divergence and Reveal Energetic Cost of Niche Expansion

ABSTRACT

Individuals vary in resource use, generating trophic niche variation that shapes a population's niche width (PNW). Interspecific interactions are predicted to constrain PNW, though individual responses are often heterogeneous. The keystone niche individual (KNI) framework identifies individuals that disproportionately expand PNW by exploiting resources beyond the population's realized niche, but whether interspecific interactions limit their emergence is unknown. We tested this by comparing an allopatric and sympatric population of brook trout ( Salvelinus fontinalis ) and rainbow trout ( Oncorhynchus mykiss ). Fatty acid profiles from repeatedly sampled individuals were used to quantify individual trophic niches and their contributions to PNW. Individuals in sympatry exhibited more negative contributions to PNW (−2.70% ± 1.7%) than individuals in allopatry (−0.47% ± 1.8%) and showed a continuum of contributions rather than discrete KNI, indicating that interspecific interactions constrained niche expansion by reducing disproportionate contributions to PNW. Across both populations, individuals with greater contributions to PNW exhibited reduced trophic overlap with competitors and lower body condition, revealing energetic costs of niche expansion. These findings suggest that, in this study system, interspecific interactions can limit niche differentiation by reshaping how individuals contribute to population‐level trophic diversity, highlighting the need to evaluate the generality of these mechanisms across ecological contexts.