River-lake connectivity, wetland, and human stress factors shape fish diversity (alpha and beta) patterns in the middle and lower Yangtze River, China

ContextAlterations of landscapes and riverscapes by humans have fundamentally altered patterns in freshwater biodiversity throughout the world's aquatic systems. Past research has demonstrated precipitous declines in small- and medium-sized streams, with much less attention given to patterns and drivers of fish biodiversity in larger riverine systems.ObjectivesBy examining alpha and beta diversity of fishes in the middle and lower reaches of the Yangtze River, China, we aimed to address the following question: What is the role of river-lake connectivity, wetland extent, and human stress factors (e.g., urbanization, fishing pressure, navigation, and shoreline modifications) in shaping fish alpha (species richness) and beta (species nestedness and turnover) diversity?MethodsWe examined associations among three classifications of fish assemblage data (i.e., all species, nonmigratory species, and migratory species) sampled along gradients of river-lake connectivity (i.e., Dongting and Poyang lakes), wetland extent, and a suite of human stress factors (urbanization, fishing pressure, navigation, shoreline modification). We conducted redundancy analyses and distance-based redundancy analyses to facilitate mechanistic interpretations of the associations between the explanatory variables and fish alpha and beta diversity.ResultsLongitudinally from the middle to the lower reaches of the Yangtze River, fish alpha diversity showed marked decreases while fish beta diversity increased. River segments with greater levels of human impact were generally associated with reduced fish species richness, while segments with higher wetland extent and greater river-lake connectivity tended to support greater species richness. River-lake connectivity, wetland extent, and fishing pressure largely influenced total fish beta diversity. Species nestedness was primarily associated with amount of wetlands, and exhibited the strongest associations with nonmigratory fish species. Turnover was primarily associated with river-lake connectivity (mainly Poyang Lake), especially for migratory species.ConclusionsOur analyses demonstrate the roles of river-lake connectivity, wetland extent, and human stress factors in shaping patterns of alpha and beta diversity for migratory and nonmigratory fishes in the middle and lower reaches of the Yangtze River. Fish diversity conservation in this large river ecosystem calls for protecting hydrological connectivity and wetland habitats, along with reducing fishing pressure. Results from this study will help better inform fish conservation efforts in the Yangtze River and have implications towards other large river systems of the world.