Loss of distinct functionality during fish community disassembly

Human-induced environmental changes increase species turnover, typically characterized by native species extirpation and non-native species (NNS) invasions, leading to multiple functional consequences owing to varying species roles and whether losses from extirpation are compensated by newly established NNS. We analysed community functional roles among fish species that persisted, were extirpated or were newly established over 75 years (1940-2015) in 15 lakes in southwest China, using functional diversity indices and trait-based network analyses. While species extirpation rate increased strongly through time, earlier extirpations caused the greatest losses to novel functional diversity. Functional losses from native species extirpation were not compensated by NNS. Functional patterns of extirpations were mainly random or overdispersed, whereas invasions were almost always random species replacements. Synthesis and applications. Our findings highlight that the loss of distinctive functional diversity persists even when extirpation rates are low, due to the incomplete functional replacement of native species by NNS, which are typically functionally random relative to extirpated species. This underscores the importance of prioritizing the protection of functionally distinct species to preserve community integrity. Additionally, our use of trait-based network analysis provides a novel perspective for understanding the functional implications of species turnover, and could be a valuable tool for researchers and conservation practitioners to evaluate community assembly processes and functional structure dynamics.