A global systematic review of the effects of hydromorphological floodplain restoration on biodiversity

Abstract River floodplains are among the most threatened ecosystems on Earth. Efforts at hydromorphological restoration by enhancing lateral connectivity between the river channel and former floodplain are increasingly popular as nature‐based solutions (NbS) for hydrometeorological risk, including flood management and drought resilience, while potentially improving habitat for fish and wildlife and performing other ecosystem services. However, the biodiversity conservation benefits of such interventions have not been comprehensively assessed. A comprehensive review of hydromorphological floodplain restorations and their conservation evidence is needed to guide managers in selecting interventions and understanding potential outcomes. Here we report findings from a systematic review of 100 studies monitoring ecological responses to hydromorphological floodplain restoration across 22 countries from four continents and a broad range of taxonomic groups including birds, fishes, invertebrates, plants and amphibians. Most research comes from temperate climates, especially in Europe, and assesses restoration success at local scales by comparing restored to reference sites, with less than 20% of studies using a full Before‐After‐Control‐Impact (BACI) Design. The most common interventions used were side‐channel excavation, lowering of the floodplain, levee setbacks and environmental flows, and these yielded generally positive effects on metrics of biodiversity. Studies showed variation in biodiversity response among river systems, intervention types, spatiotemporal scales and among taxonomic groups. Studies were biased towards invertebrates, plants and fish, with little research on terrestrial vertebrates (birds, amphibians, and small mammals, including bats) and soil microorganisms. Synthesis and applications . Our evidence synthesis reinforces the notion that hydromorphological floodplain restorations, as a biodiversity conservation measure or NbS, can significantly enhance local floodplain biodiversity. However, such benefits were often compromised at least in part by invasive species post‐restoration, highlighting the importance of sustained management.