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The worldwide decline in freshwater biodiversity due to rising anthropogenic pressures is of growing concern. Much evidence reveals that biodiversity loss and taxonomic homogenization results in loss of ecosystem functions. Understanding temporal and spatial patterns of biodiversity is, a major challenge in ecological conservation. Of all groups of threatened aquatic animals, freshwater mussels are responsible for critical trophic and non-trophic functions. Based on published inventories and field surveys, here we aim to evaluate whether freshwater mussel diversity, density and biomass has changed in the last 60 years, as well as the factors driving these patterns in different areas within the Poyang Lake basin, a global biodiversity hotspot in the middle reach of the Yangtze River. Results showed that alpha diversity has declined in the current period (2016–2020) as compared to diversity in the historical (1960–2003) and intermediate periods (2003–2014). Overall species richness declined by 28 % over 60 years. Density and biomass also declined by 79 % and 42 %, respectively. Population declines led to homogenization of freshwater mussel faunas. Additionally, there was a significant association between pH and alpha and beta diversity, suggesting that acidification related to intensive agriculture development may be one major factor behind mussel declines. In summary, this study provides new insights into the patterns and drivers of freshwater mussel population dynamics in the Poyang Lake basin. The results have implications for the design of freshwater protected areas to slow, and even reverse, current declines.

The Yangtze River Basin (Changjiang Basin) hosts one of the most diverse freshwater mussel assemblages on Earth. However, due to human activities this fauna is dramatically imperiled in this region, highlighting the need for urgent conservation measures. To better protect and conserve these taxa, it is crucial to understand the main drivers that control the diversity patterns, as well as the anthropogenic influence on these patterns. Here, we estimated distinct diversity metrics (i.e. species richness and alpha, gamma, and beta diversity) to analyze the spatial freshwater mussel diversity changes across 16 tributaries and lakes from the middle and lower Yangtze River Basin, in two periods: past 1962–2012 and present. For the past, mussel records were compiled from previous studies, for the present, we surveyed 565 sites for freshwater mussel presence and abundance within the 16 selected areas. We also assessed several climatic and geographical parameters to try to identify the main factors that modulate species composition across the study area. We show that from the historical to the present periods, species diversity experienced a significant decrease, registering, per river/lake, an average of 5.4 extirpations of species native to China and 3.9 of the Yangtze River Basin endemics. The gamma diversity of the whole study area and the alpha diversity of each of the studied rivers and lakes have also decreased during this period. The beta diversity values indicated a significant trend towards taxonomic homogenization over the last decades. Poyang and Dongting Lakes should be considered for protection given due to their high diversity and species loss given that nestedness in these lakes represented the main component of beta diversity.