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Urban rivers: novel ecosystems, new challenges

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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. WIREs Water 2014, 1:19–29. doi: 10.1002/wat2.1007 This article is categorized under: Water and Life > Conservation, Management, and Awareness Water and Life > Nature of Freshwater Ecosystems
Three ecosystem states as described by Hobbs et al. See text for descriptions of different states. (a) A historical state may change to a hybrid and ultimately a novel state by three pathways: (1) represents the loss of existing species and replacement by invasive species, which may be native or nonnative (usually the latter); (2) represents changes in the abiotic environment; (3) represents pathways 1 and 2 acting in tandem. (b) Potential for restoration of the different ecosystem states to a historic state. Hybrid states are more likely to be restorable, while novel ecosystems that have been created by a combination of biotic and abiotic change may be very difficult or impossible to restore to a historic state. (Reprinted with permission from Ref 10. Copyright 2009 Cell Press)
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The current ‘state’ of an ecosystem may be defined by key characteristics relating to structure and process of the system (the ‘box’). The current combination of biotic and abiotic conditions is represented by the location of the dot within the system ‘state’. The dot may move around within the box (representing disturbance and natural dynamism) and still be within the same system ‘state’, and so the state persists. In the diagram, a shift from (a) to (b) represents a change in system configuration, but not a change in system state. A disturbance or other modification that pushes the dot out of the box moves the ecosystem into another ‘state’ (c), in which the key characteristics that define the system may be altered. Moving the ecosystem back into its original state (full restoration) may be very difficult or impossible, particularly if a severe hysteretic shift has occurred and the conditions required for the original state are no longer achievable. System states must be defined based on appropriate variables for each ecosystem under consideration.
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Water and Life > Nature of Freshwater Ecosystems

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