Nonnative Fish Species across Different Trophic Levels Shape the Trophic Niches of Native Fish

Nonnative fish threaten native species in river ecosystems, yet their effects on native trophic niches remain unclear. Here, using δ 13 C and δ 15 N stable isotope from fish and other aquatic organisms in Longyangxia Reservoir—a key regulatory reservoir in the Yellow River—we assess trophic niche shifts under nonnative species influence. The results demonstrated that nonnative fish exhibited a broader dietary range ( δ 13 C: −28.7‰ to −20.1‰; δ 15 N: 8.7‰ to 16.7‰) and wider trophic niche breadth ( SEA b : 3.6 to 6.1; SEA c : 3.4 to 6.2) compared to native fish ( δ 13 C: −28.9‰ to −22.1‰; δ 15 N: 8.9‰ to 13‰; SEA b : 2.7 to 3.6; SEA c : 2.8 to 3.1), indicating their greater ecological adaptability. Further analysis revealed that 3 native fish species ( Gymnocypris eckloni , Triplophysa scleroptera , and Triplophysa siluroides ) accounted for 3.9% to 27% of the food source of exotic top predators ( Oncorhynchus mykiss and Silurus asotus ). Additionally, the trophic niche overlap between exotic top predators ( O. mykiss and S. asotus ) and the native top predator ( T. siluroides ) was 24.9% and 33.4%, respectively, indicating that exotic top predators encroach on the ecological niches of native species through predation and competition, ultimately altering the structure of the ecosystem community. Moreover, low trophic-level nonnative fish ( Hypomesus olidus and Pseudorasbora parva ) contributed 17.8% and 10.8%, respectively, to the dietary resources for the native top predator ( T. siluroides ), thereby increasing the diversity of its food sources. This study reveals how nonnative fish reshape native trophic niches and community structures through predation, competition, and resource subsidies, offering critical insights for river ecosystem and managing biological invasions.