Extreme Drought Restructures Floodplain Fish Assemblages Through Trait Convergence and Assembly Mechanism Shifts

ABSTRACT Aim Extreme droughts, intensified by climate change, threaten floodplain ecosystems. However, the mechanisms underlying biotic responses remain insufficiently understood. We assessed drought‐induced changes in the taxonomic and functional structure of fish communities, identified key environmental and biotic drivers during the drought period, and evaluated post‐drought community recovery trajectories. Location Poyang Lake, Yangtze River Basin, China. Methods Fish assemblages were surveyed across four floodplain lakes of Poyang Lake from 2022 to 2024 during the same seasonal window to isolate drought effects. Community composition and traits were analysed in multivariate trait space to assess changes in functional identity and the abundance of key species, and diversity indices at multiple scales were calculated. Key environmental drivers were identified using ordination and trait‐environment association analyses. Results Extreme drought induced substantial shifts in fish community structure and functional composition, favouring small‐bodied, benthopelagic omnivores and causing a 94% decline in the abundance of the apex predator Culter alburnus . Simultaneously, taxonomic and functional α‐diversity declined significantly, while β‐diversity increased, driven primarily by species turnover. The mechanisms underlying community assembly also shifted, with physicochemical factors predominant before the drought, but macrophyte growth form and macrobenthic biomass becoming the dominant drivers during drought conditions. Although hydrological connectivity was restored in 2024, community recovery remained incomplete, characterised by only partial functional trait rebound and persistent suppression of taxonomic richness, particularly among habitat specialists. Main Conclusions Extreme drought functioned as a basin‐scale filter, selectively favouring fish assemblages with drought‐tolerant trait combinations while leading to the decline or local extinction of intolerant taxa. The resulting habitat contraction and fragmentation reduced the local capacity to support biodiversity and promoted niche partitioning along gradients of habitat structure and food availability. Even after the restoration of hydrological connectivity, drought‐induced habitat legacies and priority effects constrained specialist recovery, impeding the re‐establishment of pre‐drought community composition.