Loss and Gain: Temporal Succession in Different Facets of Fish Diversity Over a Half Century Under Cascade Dam Construction

Abstract

Freshwater biodiversity is increasingly imperilled by human activities, with dam construction posing significant threats to fish communities. Species composition changes through introductions and extinctions have been widely reported, yet the long-term consequences of cascade dam construction on multiple facets of biodiversity remain poorly understood. Moreover, the compensatory effects of species introductions on extinction have received limited attention. This study presents a comprehensive evaluation of the impact of extinction-introduction successions, triggered by cascade dam construction, on the taxonomic, phylogenetic, and functional diversity of fish assemblages in the upper Yellow River over five decades. Our results reveal that shifts in species composition significantly increased phylogenetic and functional diversity but not taxonomic diversity, suggesting a greater sensitivity of the former to cascade dam construction. However, introduced species only partially compensate for approximately 50% of phylogenetic and functional diversity losses caused by extinctions. Furthermore, the timing of significant increases in phylogenetic and functional diversity is not synchronised, with all measures gradually stabilising post-dam construction. Cumulative reservoir capacity, reservoir age, and individual reservoir capacity were identified as key determinants of multifaceted diversity change after dam construction, with cumulative reservoir capacity and reservoir age generally having positive effects, while individual reservoir capacity tended to have a negative impact. These findings stress the urgent need to reassess the compensatory effects of introductions on extinctions under global change, emphasise caution in interpreting short-term data due to non-linear diversity patterns, and highlight the importance of using long-term monitoring and multifaceted diversity metrics in biodiversity conservation actions.