Trophic niche interactions among native and non-native fish species vary spatially in one of the world's largest reservoirs

The invader density impact model proposes that the impact of an invasive species may increase or decrease linearly or nonlinearly as a function of its density. The Three Gorges Reservoir (TGR), located in the upper Yangtze River Basin, supports high fish diversity, yet non-native fish species have increasingly colonized and expanded their ranges. Pelagic carnivorous Coilia brachygnathus (Engraulidae), native to the middle and lower reaches of the Yangtze River Basin, has colonized and rapidly expanded into the TGR, whereas its density decreases with distance from the dam. Here, we used stable isotopes to investigate similarities in trophic position, niche width, and niche overlap of C. brachygnathus and eight native pelagic species between the lower and upper sections of the TGR. We predicted that in higher density, C. brachygnathus would broaden the trophic niche, while native species would narrow theirs, reducing interspecific niche overlap. We found that C. brachygnathus had a narrower trophic niche in the lower section of the TGR where its density was higher. In contrast, most native species had broader niche widths than those in the upper section where the density of C. brachygnathus was lower. Interestingly, C. brachygnathus has a higher trophic position than those of most native species examined, and the trophic positions of most native species were lower in the lower section. Higher niche overlap was observed between C. brachygnathus and native piscivorous Culter spp. (Cyprinidae) in the upper section. Our findings indicate that C. brachygnathus influences trophic niches of native species, and the density of C. brachygnathus mediates the effects. Under higher density, C. brachygnathus and native species adjust niche width and trophic positions, consequently reducing niche overlap to alleviate competition pressure.