Where is the nature in nature-based flood management? Biodiversity is not considered enough

The rise of Nature-based Solutions (NbS) represents a shift toward design approaches that harness and facilitate natural processes for more holistic climate, biodiversity, and human wellbeing outcomes. Biodiversity and water are considered critical foundations of ecosystem function and service provision. However, without adequate measurement of biodiversity impacts, the interventions related to NbS or 'Nature-based Interventions' (NbI) risk assuming biodiversity co-benefits that may be non-existent or sub-optimal. We systematically reviewed 527 publications on NbI with physical outcomes for flood management to examine if biodiversity outcomes were measured or monitored across time at micro- to macro-scales. Only 11 % of the literature on NbI related to flood management explicitly measured biodiversity within at least one taxon. Of those studies, 41 % measured biodiversity metrics of flora, with a focus on bioengineering benefits. No specific non-vegetative taxa were documented in >10 % of studies mentioning biodiversity. Dominant non-vegetative taxa were bioindicator species (invertebrates, insects, molluscs, birds, and fish) typically associated with water quality. Few studies monitored biodiversity over time, and most monitored for <20 years. Only 35 % of studies that mentioned biodiversity measured a correlation between biodiversity and flood management outcomes. Most found a positive relationship, with apparent trade-offs for different species over time. We highlight ways for NbI to improve biodiversity inclusion and maximise potential synergies between biodiversity and flood management.