Bridging and Rewiring Mechanisms in the Deconstruction of the Functional Architecture of Fish Assemblages in the Upper Paraná River Basin by Non‐Native Species

ABSTRACT

Freshwater ecosystems harbour nearly half of all known fish species yet rank among the most threatened on the planet, with biological invasion operating as one of the primary functional destabilizers in systems where dam cascades have already dismantled natural biogeographical barriers. In the Upper Paraná River basin, which supports high fish diversity and is extensively regulated by hydropower infrastructure, the functional consequences of invasion on native assemblage architecture remain unresolved. Using an inventory of 230 species (185 native, 45 non‐native) from tributaries of the Triângulo Mineiro and Alto Paranaíba mesoregion, we quantified how non‐native fish restructure native community functional organization through complementary analytical frameworks, including (i) functional diversity indices (FRic, FDiv, FEve), (ii) bipartite species‐guild affiliation networks (modularity, centrality) and (iii) spatially explicit vulnerability mapping. Non‐native species, despite representing only 19.6% of species richness, comprised 60.6% of total abundance, a disproportionate numerical representation that left functional richness and divergence virtually unchanged while driving a 19.4% decline in Functional Evenness, indicating disproportionate monopolization of specific ecological roles rather than the introduction of new ones. Network analysis revealed a 41.5% loss in modularity, produced by generalist non‐native species acting as topological bridges between previously isolated functional compartments and a parallel reconfiguration of the piscivore guild from peripheral to central, driven by specialist predators whose low structural centrality belies the intensity of their top‐down pressure. Functional vulnerability proved spatially non‐random and highly aggregated, concentrated almost exclusively in the Araguari River basin (Moran's I = 0.744; p  < 0.001), identifying a sub‐basin where invasion pressure has crossed the threshold of functional reorganization while the broader network retains apparent resilience. These results demonstrate that invasion erodes the modular compartmentalization and functional evenness that stabilize native assemblages, producing a community that is more connected, more homogeneous and more susceptible to cascading collapse under future perturbations.