Ecological uniqueness of fish assemblages and species contributions to beta diversity are affected by river-lake disconnection

Ecological uniqueness is an important aspect intrinsically associated with beta diversity, highlighting the relative contributions of sites (LCBD) and species (SCBD) to overall compositional variation, and has important theoretical implications and practical applications in biodiversity conservation. However, it is still unclear how human disturbances affect the ecological uniqueness of lake fish assemblages, especially at intermediate and large spatial scales. Here, we explored how river-lake disconnection affected LCBD and SCBD of fish assemblages in 51 lakes (i.e., 13 lakes connected to the mainstem river, CLs; and 38 river-disconnected lakes, DLs) scattered across the floodplains of the Yangtze River, China. We compared the relationship of LCBD with species richness, as well as of LCBD with functional alpha diversity, contrasting the CLs and DLs, and examined how environmental factors and spatial variables account for variation in LCBD. We also examined the relationships between species occupancy, functional traits and SCBD. We found that an opposite patterns between LCBD and alpha diversity (here, based on species richness, the number of rare species, and a set of functional diversity indices) in CLs and DLs. More specifically, in CLs, sites with high ecological uniqueness generally supported high alpha diversity. Conversely, in DLs, sites with high ecological uniqueness generally supported low alpha diversity. Furthermore, LCBD was explained both by environmental and spatial factors, with connectivity being the most important spatial factor. SCBD showed a hump-shaped relationship with species occupancy and was poorly explained by functional traits, although these relationships were likely influenced by the proportion of rare species. From an applied perspective, our findings suggest to (i) incorporate LCBD and alpha diversity into modern-day frameworks aimed at identifying sites of high conservation priority, and (ii) consider both SCBD values and species rarity for targeting individual species of conservation interest.