Associations among alien fish dominance, biodiversity, and spatial variability of native fish assemblages in Plateau Rivers

While plateau river environments shape native fish diversity, they inherently constrain species' spatial distribution patterns; however, this equilibrium, forged through long-term evolution, may be disrupted by the introduction of alien species. Integrating spatial distribution survey data of fish communities across six rivers in the Western Sichuan Plateau, China, we applied Piecewise Structural Equation Modeling (PSEM) to disentangle the associations among physical habitats, alien species dominance, biodiversity, and community spatial variability (measured by Abundance Variation Degree, AVD). The results indicated that the rivers with high alien species dominance did not exhibit a direct decline in native diversity; rather, alien species dominance was significantly and positively correlated with native taxonomic diversity (Shannon index), but had no direct effect on functional diversity (Rao's Q). This phenomenon likely occurs because the unique habitats and highly specialized native fish assemblages in plateau rivers leave unsaturated ecological niches, thereby facilitating a temporary sympatric coexistence between alien and native species, while extreme environmental filtering strictly constrains the functional trait space of the communities. PSEM analysis further revealed that high-altitude environments strongly restricted alien species (path coefficient = −0.54), and functional diversity (Rao's Q) did not exhibit any significant association with AVD. Conversely, both alien species dominance (path coefficient = 0.52; P < 0.001) and native taxonomic diversity (Shannon index, path coefficient = 0.31; P < 0.05) were significantly and positively associated with AVD, indicating that spatial turnover in these native assemblages is highly sensitive to taxonomic shifts, whereas functional traits remain highly conserved across the plateau rivers. These findings suggest that in fragile plateau rivers, anomalously elevated diversity indices may obscure the increased spatial variability and altered compositional structure associated with the presence of alien species. Therefore, rather than merely maximizing overall biodiversity indices, conservation strategies for these high-altitude ecosystems should prioritize the spatial reconstruction of native assemblages to proactively fill unsaturated niches, thereby enhancing community resistance to alien species.