River restoration: seeking ecological standards. Editor’s introduction

1. In the face of continuing anthropogenic stressors to freshwater systems worldwide , the natural resilience and resistance of the freshwater ecosystems that enables them to cope with or recover rapidly from certain levels of perturbation is under significant threat. In many systems, the changes brought about through human activities have significantly altered the physical habitat and ecological functioning of the natural systems. 2. Given the importance of freshwater systems in the provision of ecological services and diverse habitats for a huge range of species, there is a clear need for restoration that can maintain sustainable ecological services whilst reinstating ecosystem function and habitat range. 3. While restoration has been attracting huge financial investment in recent times, to date there has been little or no consensus as to what constitutes successful ecological restoration. The studies highlighted in this Special Profile attempt to meet this challenge. The Forum paper establishes a set of criteria or standards against which restoration projects can be evaluated, and these criteria are discussed by both practitioners and researchers in two Comment papers. 4. The five criteria proposed in the Forum paper include the establishment of a dynamic ecological endpoint to the restoration, providing for a 'guiding image' of a healthier river, and an improvement in the river's ecological condition, that ideally leads to a more self-sustaining and resilient system. No lasting harm should be inflicted on the system during the restoration, and pre-and post-assessment and monitoring must be incorporated into the overall restoration project. An additional sixth criterion is proposed in one of the Comment papers whereby a description or prediction of the ecological mechanisms should be determined through which the intended restoration strategy will achieve its goals 5. The kinds of mechanisms that might be involved are explored in three research articles in the Special Profile, centred on river systems that have been subject to regulation or channelization. The mitigation of the effects of river regulation measures and the successful restoration of floodplain ecosystems has a focus on re-establishing river flow dynamics (temporal heterogeneity) and connectivity of the river with the floodplain. Restoration of channelized systems is centred on increasing the structural (spatial) heterogeneity of the system. 6. Synthesis and applications. River restoration is a worldwide phenomenon of growing importance as we attempt to redress the problems that have arisen from our use and misuse of freshwater habitats and resources. The adoption of standards for …