Revealing a conservation challenge towards floodplain disconnection: Decreasing turnover and increasing nestedness of mollusc metacommunities

Abstract Floodplain disconnection (FDC) has become a profound threat to many large river floodplains. Despite current researches examining the impacts of FDC on aquatic biodiversity, little is known about how FDC affected freshwater molluscs from multifaceted biodiversity perspectives, especially at long-term temporal scales. We selected the 7 lakes in the Yangtze River floodplain to evaluate how river-lake disconnection impacted taxonomic, functional and phylogenetic alpha and beta diversity of molluscan assemblages during past 60 years. Our results showed that five disconnected lakes (DLs) experienced dramatic decline in species richness (decreased by 62.4%) and functional richness (54.6%) but increases in functional evenness (20.0%) and Δ + (5.5%) after disconnection. Whereas such temporal changes in the two connected lakes (CLs) were much slighter. Somewhat unexpectedly, the overall taxonomic (increased by 7.0%) and phylogenetic (12.2%) dissimilarities only slightly increased through time, due to opposite changes in the turnover (40%-50% decrease) and nestedness-resultant (100%-110% increase) components through time. The functional nestedness experienced more significant increase (162.0%), leading to relatively considerable increase in functional overall dissimilarity (64.0%). Such opposite changes in multifaceted turnover and nestedness distinctly modified the structure of beta diversity and highlight the conservation priority of CLs keeping relative high levels of biodiversity. The overall correlations between changes in both alpha and beta diversity among different facets were generally weak, indicating the different diversity facets would provide complementary information for biodiversity conservation and restoration. Thus, we recommended incorporating functional and phylogenetic information of species into traditional taxonomy-based approaches in large river floodplains studies.