Disentangling the Effects of Biotic and Abiotic Dimensions of Ecological Opportunity on Individual Trophic Trait Variation

Within-species individual trait variation (ITV) plays a critical role in ecological and evolutionary dynamics by influencing community structure, ecosystem functioning, and individual fitness. While the role of the biotic dimension of ecological opportunity (e.g., interspecific competition, prey availability) in shaping trophic ITV is now well established, the role of the abiotic dimension, and its interactions with biotic factors, remains critically overlooked, limiting our understanding of how individuals cope with changes in ecological opportunity. To address this knowledge gap, we investigated trophic ITV in the endangered riverine fish Zingel asper using a multi-faceted approach: (i) precise quantification of trophic ITV via faecal metabarcoding, (ii) fine-scale mapping of prey availability and habitat structure across seasons, and (iii) path analysis to assess the direct and indirect effects of prey, habitat, and their spatial heterogeneity in driving ITV. The individual niche width (INW) in Z. asper was largely determined by preferred prey availability, while between-individual variation (BIC) was largely determined by a combination of prey and habitat factors, with habitat exerting a direct effect (i.e., not mediated via prey) on trophic ITV. This study provides a mechanistic explanation of the processes underlying the shift from selective to opportunistic foraging strategies. Notably, we demonstrated that trophic ITV is altered by the interaction between predator life-history traits (in the present study, size) and four distinct dimensions of ecological opportunity: prey availability and their spatial distribution, habitat structure and seasonality.