Effects of Cascade Dam Removal on the Longitudinal Dispersal of Macroinvertebrates in the Jiuchong River

Dispersal is crucial for the distribution of macroinvertebrate populations and communities. However, research on the dispersal of macroinvertebrates after dam demolition remains limited. Studying the impact processes and driving factors of dam demolition on the dispersal capacity of macroinvertebrate communities can help in the development of effective strategies for river ecosystem restoration and biodiversity conservation. Therefore, this study conducted two continuous monthly surveys, monitoring drifting macroinvertebrates in the Jiuchong River of Shennongjia National Park from January to August, 2022 and January to August, 2023 (before and after dam removal), and analyzed the impact process and driving mechanism of cascade dam removal on the dispersal of macroinvertebrates. The results showed that before dam removal, there were significant differences in the community composition of drifting macroinvertebrates between the reference and impaired sites, indicating the significant cumulative effects of multiple dam barriers. After dam removal, the representation of the classes Diptera and Ephemeroptera increased at the impaired sites, whereas that of Lepidoptera and Tubificida disappeared. The dominant species changed from Rhagionidae and Heptageniidae to Ephemerellidae, and the number of species and individuals approached that of the reference sites, indicating that dam removal has a significant ecological restoration effect on rivers. After dam removal, strong dispersers increased, whereas weak dispersers decreased. Additionally, in most sites, the dispersal capacity metric (DCMc) of the impaired sites was higher than that before dam removal. Temporal variation patterns showed significant changes before and after dam removal, and in the initial stage after removal, the DCMc at each site increased with the increase in the number of “fugitive species," those with strong dispersal capacity but weak competition capacity in the initial stage. Piecewise structural equation models revealed that Flow-Velocity and Flow-CPOM (course particulate organic matter) were positive feedback paths that affected DCMc, whereas Flow-Width and Flow-FPOM (fine particulate organic matter) were negative feedback paths. Our findings provide foundational support for the study of future freshwater ecosystem communities and biodiversity maintenance mechanisms as well as a scientific basis for the protection and management of cascade dams.