Niche partitioning studies are essential to understand the mechanisms that allow ecologically similar species to coexist. The Rhone streber (Zingel asper) and the European bullhead (Cottus gobio) are both benthic riverine fishes that consume macroinvertebrates. Both species are protected under European legislature. We focused on trophic niche partitioning between these species, as sufficient access to trophic resources is an important requirement for long-term coexistence. We used a combination of faeces metabarcoding dietary data and prey community data to evaluate dietary overlap, trophic niche variation and dietary preferences. Sampling was performed in three sites along the Durance River (France). Both species consumed a wide range of macroinvertebrates but neither C. gobio nor Z. asper selected the most abundant taxa (chironomids and simuliids). Both species selected larger mayfly (Baetis and Ecdyonurus) and caddisfly (Hydropsyche) taxa. Dietary overlap was generally high but declined when large mayflies were rare. Contrasting secondary prey preferences and trophic niche variation may allow these species to exploit the same preferred mayflies in periods of abundance and to avoid competition during resource scarcity. We propose that dietary partitioning may support the coexistence of these two ecologically similar, benthic predators.

Community ecology has so far struggled to integrate both deterministic and stochastic processes into a global model of community variation. To address this issue, we aimed to characterise the ecological conditions that determine the relative importance of deterministic and stochastic community variation in benthic macroinvertebrates. We sampled macroinvertebrate and algal periphyton communities, and microhabitat conditions at monthly intervals over a five year period in two sampling sites in the regulated Durance River and one site in the Asse River (a natural tributary of the Durance). The relative importance of stochastic and deterministic processes was estimated over time as functional and taxonomic spatial β‐deviation (i.e. β‐diversity_obs − mean β‐diversity_null/SD β‐diversity_null) for each sampling date and site. We additionally quantified deterministic variation as the trait–environment relationship and predicted a positive correlation with β‐deviation metrics. We found evidence of overdispersal in both functional and taxonomic β‐diversities compared to null expectations. Spatial β‐deviation was highly variable over temporal scales and was jointly explained by invertebrate community size, disturbance (i.e. flooding), periphyton diversity and to a lesser extent, microhabitat diversity. The key factor that explained β‐deviation was recent river discharge (< 90 days), which had a strong negative effect on community size but was also directly associated with higher β‐deviation, likely related to recolonization processes post‐flood. Lastly, we found that when β‐deviation was high, the hydraulic trait–environment relationship was stronger. This indicates that spatial heterogeneity in hydraulic conditions was the main driver of deterministic variation in invertebrate community structure. This study demonstrates that the importance of stochastic processes can vary greatly over short (~ months) and long (~ years) temporal windows with significant consequences for trait–environment relationships. Our study provides an example of how stochastic community variation can be modelled to better interpret deterministic patterns of community structure in natural communities.

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.

Prey preferences are important drivers of predator–prey interactions and trophic network structure. We present a species‐level selection analysis for the endangered Zingel asper (L. 1758) and its prey within the Baetis (Ephemeroptera) genus. By combining robust diet metabarcoding data with fine‐scale prey community data, we revealed that Z. asper selected four Baetis species differently, despite being largely considered as ecologically similar species. Our results suggest that fine‐resolution analysis of prey selection may be key for understanding trophic interactions and for improving the conservation and management of fish.

Within-species Individual Trait Variation (ITV) is now recognised as an important source of variability in ecological communities. Individual variation in trophic niche traits (i.e. individual niche width and between-individual variation) can greatly modify top-down and/or competitive interactions. Trophic traits vary according to variation in ecological opportunity, which represents the range of exploitable resources. While the role of prey availability in driving trophic traits is well-established, abiotic drivers (e.g. habitat structure) are rarely accounted for, or are solely considered via their effect on the prey community. We aimed to disentangle prey-mediated from direct habitat effects on trophic ITV in a critically endangered riverine fish, the Rhone streber (Zingel asper). We quantified individual trophic traits using high-resolution diet data obtained from faeces metabarcoding, and quantified prey availability and habitat structure using a fine-scale sampling protocol. Trophic traits were driven by distinct mechanisms: the individual niche width was driven by prey availability, while between-individual variation was largely driven by habitat conditions. Habitat conditions acted both directly on trophic traits and indirectly via their effect on the prey community. By simultaneously accounting for biotic and abiotic drivers of trophic ITV, we obtained a more complete understanding of how prey availability and habitat structure jointly determine ecological opportunity. Furthermore, by accounting for fine-scale variation in prey community and habitat conditions, the importance of spatial heterogeneity factors was highlighted. This study demonstrates how robust metabarcoding data, combined with detailed prey community and habitat information can be used to reveal the mechanistic pathways that drive trophic traits.

The relationship between a predator’s diet, its prey community and its habitat : perspectives for the conservation of the Rhone streber: Dans le contexte de l’augmentation des facteurs de stress anthropiques sur les poissons d'eau douce, il est essentiel de mieux comprendre leur écologie trophique pour guider les actions et les politiques de gestion et de conservation. Cette thèse vise à améliorer la compréhension des processus à l’origine des variations de la niche trophique d’une espèce menacée d’extinction l’apron du Rhône (Zingel asper (L.)). J'ai ainsi relié la variation des traits trophiques individuels à l'ontogénie et la compétition intraspécifique. En combinant 1) des données robustes de régime alimentaire obtenues par metabarcoding, 2) un échantillonnage et une caractérisation détaillés de la communauté de proies et de la structure de l'habitat, et 3) des approches de modélisation et simulation individu-centrées, j’ai pu mettre en évidence les relations complexes liant la variation intra-spécifique aux changements spatio-temporels des opportunités écologiques et de la structure de l’habitat. L’approche bayésienne et les analyses causales ont permis d’identifier et de caractériser les liens de causalité entre variation temporelle et spatiale des opportunités écologiques et de la structure de l'habitat et la variation intraspécifique des traits trophiques de l’apron. Alors que les proies préférées de l’apron sont le moteur principal de la variation de sa largeur de niche individuelle, les analyses causales ont révélées que la variation de niche inter-individuelle est déterminée principalement par la structure de l'habitat. Enfin, à partir de cette base de connaissances nouvelles, j'ai développé un indice de qualité de la ressource trophique pour orienter la gestion et la conservation de l’apron