Revealing a Conservation Challenge Towards Floodplain Disconnection: Decreasing Turnover and Increasing Nestedness of Mollusc Metacommunities

Floodplain disconnection (FDC) has become a profound threat to many large river floodplains. Despite current research 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 7 lakes in the Yangtze River floodplain to evaluate how river-lake disconnection impacted taxonomic and functional alpha and beta diversity of molluscan assemblages during the past 60 years. Our results showed that five disconnected lakes experienced dramatic decline in species richness (decreased by 62.4%) and functional richness (54.6%) but increases in functional evenness (20.0%) after disconnection. Whereas such temporal changes in the two connected lakes (CLs) were much slighter. Somewhat unexpectedly, the overall taxonomic (increased by 7.0%) dissimilarities slightly increased through time, due to opposite changes in the turnover (ca. 40% decrease) and nestedness-resultant (ca.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 turnover and nestedness distinctly modified the structure of both taxonomic and functional beta diversity. The increase in nestedness-resultant component highlights the conservation priority of CLs keeping relative high levels of biodiversity. The overall correlations between changes in both alpha and beta diversity among two diversity facets were generally weak, indicating the different diversity facets would provide complementary information for biodiversity conservation and restoration. Thus, we recommended incorporating functional information of species into traditional taxonomy-based approaches in large river floodplains studies.