Synergistic mechanisms foster declines in native fish community stability following the introduction of non‐native species

Non‐native species introductions profoundly alter freshwater communities, yet the mechanisms by which non‐native fishes affect native community stability remain unclear. Using a multi‐decadal dataset on fish community composition and abundance collected across France, we quantified native community stability and decomposed this metric into its main constituents: population variability and synchrony. We examined how non‐native fish richness, eco‐evolutionary features of non‐native and native fishes, and environmental gradients influence the two components of community stability. Our results revealed that increased non‐native richness elevated population variability by expanding interspecific functional trait divergence and through persistent disruptive effects on native species resulting from intensified interspecific competition. At the same time, the presence of non‐native species amplified temporal synchrony through a contraction in phylogenetic distances of resident communities. Population variability was further regulated by environmental gradients, whereas synchrony was unrelated to the environmental template. In summary, declines in community stability associated with non‐native fishes' stem from the synergistic interplay of multiple mechanisms: increased richness of non‐native fish species fostered a functional reorganization of native communities that amplified population fluctuations, whereas reduced interspecific phylogenetic distances enhanced temporal synchrony, with environmental gradients exerting additional modulation on these relationships. The study highlights that non‐native fishes undermine freshwater community stability through interacting functional, phylogenetic and environmental mechanisms, providing insights for addressing biodiversity changes associated with the establishment and spread of non‐native species.