Quantifying restoration success across biomes by linking biodiversity, multifunctionality and hydromorphological heterogeneity

Abstract

Context: Rivers and streams are among the most degraded ecosystems worldwide, yet they play a crucial role in providing clean water, supporting biodiversity, storing carbon, and reducing flood risks. In response, restoration approaches that aim to improve in-stream hydromorphology, such as channel structure, flow variability, and habitat diversity, are increasingly implemented worldwide. However, many of these restoration efforts fail to achieve good ecological status or to fully recover biodiversity and ecosystem functioning. Scientific evidence suggests that hydromorphology strongly influences both biodiversity and ecosystem processes in rivers, indicating that the potential of hydromorphological restoration remains underexploited. RESTOLINK was built on the premise that restoration often underperforms because it does not sufficiently account for the spatial scales of river structure and flow that are most relevant to organisms and ecosystem functioning. In addition, traditional indicators of restoration success, which primarily focus on diversity and community composition, may show recovery trajectories different from those of key ecosystem functions, such as nutrient cycling. Objectives: RESTOLINK aimed to move beyond purely structural and biodiversity-based assessments of river restoration and to identify the ecological processes that underpin the success and resilience of stream and river restoration. The main objectives were to: a) Understand how physical habitat diversity and flow variability influence biodiversity and ecosystem functioning; b) Test whether restoring hydromorphology also restores key ecosystem functions; c) Examine biodiversity–ecosystem functioning relationships across contrasting climatic regions; d) Assess how current restoration approaches perform under climate change; and e) Translate scientific findings into knowledge relevant for practitioners, managers, and policy makers. Methodology: RESTOLINK combined field measurements, laboratory analyses, and large-scale data synthesis across four countries representing tropical, Mediterranean, temperate, and boreal river systems. At each site, we studied restored, degraded, and near-natural stream reaches using a harmonised sampling design. Measurements included stream flow patterns and habitat structure, biodiversity ranging from microorganisms to aquatic invertebrates, and ecosystem functions such as nutrient uptake and organic matter processing. In addition, RESTOLINK analysed existing scientific studies and conducted stakeholder surveys to better understand how restoration is currently planned and evaluated, especially in the context of climate change. Main results and impacts: RESTOLINK showed that habitat diversity and flow variability are key drivers of biodiversity and ecosystem functioning. Rivers with more diverse physical conditions support richer biological communities and more efficient ecosystem processes. At the same time, the project demonstrated that biodiversity often recovers faster than ecosystem functioning, meaning that the return of species does not automatically translate into fully restored ecosystem performance. By comparing rivers across different climates, RESTOLINK confirmed that positive links between biodiversity and ecosystem functioning are widespread, while also revealing that many restoration strategies remain focused on past environmental conditions and are poorly adapted to increasing droughts and floods. These findings highlight the need for climate-resilient restoration approaches that explicitly target ecosystem functioning. Societal relevance and use: RESTOLINK provides practical guidance for river restoration by showing how projects can be designed to better support biodiversity, ecosystem functioning, and long-term resilience. The results are directly relevant for river basin management, environmental agencies, and policy frameworks such as the EU Water Framework Directive and the EU Biodiversity Strategy for 2030. By improving how restoration success is assessed, RESTOLINK supports more effective and sustainable use of public investments in nature.