Urban rivers: novel ecosystems, new challenges

Urban rivers can be considered novel or hybrid ecosystems, and as such have experienced substantial biotic and abiotic changes that render them problematic to return to historic system states. Many urban river interventions have limited ecological benefits because such novelty is not considered, and fundamental changes in state are extremely difficult, if not impossible, to reverse while maintaining the societal functions of the system. Consequently, interventions have tended to focus on societal benefits rather than ecological improvements. But perhaps a more appropriate response is to embrace the ecological novelty inherent to urban rivers rather than trying to reverse or remove it, and find ways to enhance ecological function alongside novel conditions and species assemblages. Ecological engineering techniques have only been applied in a limited way to heavily urbanized rivers, but may represent the best options available within these highly compromised systems. The development of such techniques presents several challenges, including understanding and accepting biotic and abiotic novelty, increasing ecological functioning and resilience rather than restoring extirpated species, and rigorously testing interventions to determine efficacy. In part this will require a cultural shift in perception of urban rivers, particularly among managers, engineers and conservationists, to allow novel habitat creation and the establishment of atypical biotic assemblages. Such change sits within a wider ecological agenda to incorporate a greater range of species alongside humans in the urban environment, and improve urban ecosystem services. Urban rivers also present notable scientific opportunities, including more radical experimentation, system cocreation, and comparisons across global urban regions. (c) 2013 Wiley Periodicals, Inc.