A long-term case study indicates improvements in floodplain biodiversity after river restoration

Natural floodplains are complex systems embodying an interwoven network of biodiverse aquatic and terrestrial ecosystems. Many floodplains have been impaired and disconnected from the river, leading to declines in biodiversity. River restorations often affect floodplains, yet monitoring programs rarely investigate effects on floodplain organisms. This study examined a river restoration project, which improved river hydromorpholgy and partly reconnected the adjacent floodplain. We applied a 'control-impact' design to investigate changes in floodplain community composition and diversity four, six, eight, and ten years after the restoration of the Nidda River in central Germany. Investigated taxonomic groups included ground beetles, birds, dragonflies, and spi-ders. We examined trends of abundance, richness, Shannon diversity, Shannon evenness, and temporal turnover over time post-restoration by comparing restored sites to a non-restored control site. Abundance and diversity increased in bird, dragonfly, and spider communities after the restoration. Effect sizes between restored and unrestored sites were mainly positive, indicating that restoration benefitted the investigated taxonomic groups. However, initially high abundance and diversity numbers of ground beetles declined at the restored sites over time, indicating differing responses across taxonomic groups. Our study accentuates and underlines the necessity for ongoing post-restoration monitoring of both floodplain and aquatic biota.