Trophic Niche Plasticity and Differentiation Facilitate Non-Native Fish Invasion and Drive Competition with Native Fish in Erhai, a Plateau Lake

Non-native fish invasions are important drivers of freshwater biodiversity and ecosystem function loss. In this study, we compared the trophic niches of four non-native (Rhinogobius giurinus, Hemiculter leucisculus, Hypomesus nipponensis, and Tachysurus fulvidraco) and one native fish species (Carassius auratus) from April 2022 to December 2023 in Erhai Lake, a typical plateau lake on the Yunnan–Guizhou Plateau, China. We analyzed δ13C and δ15N from 766 fish samples and calculated 103 SEAb values across species, seasons, and lake regions. Stable isotope analyses revealed pronounced trophic niche differentiation between non-native and native fishes. Non-native species exhibited wider niche width (4.81 ± 0.48), lower overlap (24.43 ± 1.57), and higher within-group dispersion (2.69 ± 0.07), indicating greater trophic plasticity. In contrast, native fishes showed narrower niches (2.72 ± 0.32), higher overlap (37.32 ± 4.21), and lower plasticity (1.68 ± 0.08). Moreover, non-native and native fishes adopted contrasting trophic strategies: individual fitness increased with niche expansion in non-native fishes, whereas it declined in native fishes. Multiple linear analyses further indicated significant competitive effects of non-native fishes on native species’ niches, suggesting that niche expansion in native fishes represents a passive response to intensified competition rather than an adaptive strategy. Overall, the high trophic plasticity of non-native fishes and their asymmetric effects on native species imply a high risk of food web reorganization in Erhai Lake. These results provide guidance for the sustainable management of Erhai Lake, balancing invasive species control with native fish conservation. Our results underscored the importance of incorporating trophic interactions into invasion management and native fish conservation.