Partitioned functional spaces, collapsing redundancy: Functional extinction risks in coastal fish communities

The functional dimension of biodiversity is a critical predictor of ecosystem resilience, yet it is frequently overlooked in conservation planning for degraded coastal systems. Taking the anthropogenically pressured central East China Sea as an example, we conducted a trait-based analysis of 242 elasmobranch and bony fish species using seven years of standardized bottom trawl data (2015–2021) and quantified functional structure–ecosystem function linkages. Our analysis revealed that elasmobranchs and bony fishes occupy fully partitioned functional spaces, demonstrating complementary and non-redundant ecosystem roles. We further identified significant associations between fish functional traits and specific ecosystem functions. Spatially, offshore waters exhibited significantly elevated species richness and functional richness, whereas nearshore communities showed heightened functional originality. Crucially, simulated extinctions of key functional groups—mesopredators (76.6 % functional richness loss), nutrient transporters (61.3 %), and top predators (11.8 %)—exceeded random extinction impacts, indicating collapsing functional redundancy. Nearshore systems proved disproportionately vulnerable to mesopredator and nutrient-transporter loss. These findings necessitate targeted conservation of elasmobranchs and functionally irreplaceable species—particularly mesopredators, nutrient transporters and top predators—to mitigate disproportionate functional diversity loss and sustain key ecosystem functions under escalating anthropogenic pressures. • Elasmobranchs and bony fishes occupied distinct functional spaces with no overlap. • Nearshore areas had lower functional richness but higher functional originality than offshore regions. • The loss of mesopredators and nutrient transport and storage species greatly reduced nearshore functional richness. • Species performing top predator, mesopredator, and nutrient transport and storage functions should be prioritized for conservation.