محرک های متعدد در اکوسیستم های آب های شیرین Multiple stressors in freshwater ecosystems

SUMMARY ۱٫ The fundamental importance of freshwater resources, the rapid extinction rate among freshwater species and the pronounced sensitivity of freshwater ecosystems to climate change together signal a pre-eminent need for renewed scientific focus and greater resources. Against this background, the Freshwater Biological Association in 2008 launched a new series of ‘summit’ Conferences in Aquatic Biology intended to develop and showcase the application of ecological science to major issues in freshwater management. 2. This collection of studies arose from the first summit entitled ‘Multiple Stressors in Freshwater Ecosystems’. Although freshwater science and management are replete with mutiple-stressor problems, few studies have been designed explicitly to untangle their effects. 3. The individual case studies that follow reveal the wide array of freshwaters affected by multiple stressors, the spatial and temporal scales involved, the species and ecosystem processes affected, the complex interactions between ecology and socioeconomics that engender such effects, the approaches advocated to address the problems and the challenges of restoring affected systems. The studies also illustrate the extent to which new challenges are emerging (e.g. through climate change), but also they develop a vision of how freshwaters might be managed sustainably to offset multiple stressors in future. 4. More generically, these case studies illustrate (i) how freshwaters might be at particular risk of multiple-stressor effects because of conflicts in water use, and because the hydrological cycle vectors stressor effects so effectively and so extensively; (ii) that dramatic, nonlinear, ‘ecological surprises’ sometimes emerge as multiple-stressor effects develop and (iii) that good ecology and good ecologists add considerable value to other freshwater disciplines in understanding multiple stressors and managing their effects. Introduction The need to communicate the importance of water as a resource, and as an integral component of all ecosystems, has never been more acute. From a resource perspective, the prospects for billions of people faced with growing water scarcity, and associated food scarcity, are already grim (Falkenmark, 1997). From a conservation perspective, the recognition that freshwater ecosystems contribute disproportionately to global biological richness is being eclipsed by the growing realisation that extinction risks in freshwaters could be among the greatest of all (Revenga et al., 2005; Strayer & Dudgeon 2010). On their own, these are already major concerns that illustrate how the exploitation and impairment of freshwaters have outpaced our best attempts at management. Consider further, however, the growing evidence from several continents that freshwaters are also highly sensitive to climate change through effects that will complicate many existing problems (Daufresne et al., 2004; Beˆche & Resh, 2007; Chessman, 2009; Durance & Ormerod, 2009). Because the risks of management failure are so large, this combination of factors places freshwater ecosystems second to none among the environmental priorities for science funding. It was against this background in 2008 that the Freshwater Biological Association (FBA) launched its new series of ‘FBA Conferences in Aquatic Biology’ to draw together the very best in aquatic biology and its application to environmental management. Essentially ‘summits’ of the world’s foremost leaders in the field, these workshops are intended to act as a forum for the exchange of ideas, developing new concepts, raising the profile of the contribution made by freshwater ecologists and advancing freshwater science at its frontiers. Both the FBA and WileyBlackwell have taken a further significant step by ensuring that the studies from this first conference are freely accessible to all as a virtual issue of Freshwater Biology. As guest editor (SJO) and convenors of the first conference and its output, we are delighted to bring you the studies presented and discussed at the FBA’s Windermere headquarters in September 2008. The theme of ‘multiple stressors’ was intended to epitomise the challenges facing freshwaters while bringing together some of the subject’s leading scientific voices. We were aware both that there had been no other major aquatic meeting on this topic, since the American Society of Limnology and Oceanography reported on it in 1999 (e.g. Hughes & Connell, 1999), and that explicit studies of multiple stressors were only thinly distributed in the major freshwater journals. However, this is paradoxical, because freshwater science and management are replete with mutiple-stressor problems. For example, in evolutionary ecology, the fitness of organisms seldom reflects selection pressure from single sources, while the processes linking populations, communities and ecosystems are invariably multivariate. In freshwater management, too, problems almost always involve simultaneous challenges either, because human pressure typically alter more than one environmental factor (e.g. urbanisation affects runoff quantity, water quality, thermal regimes, habitat availability, the dispersal of invasive species…), and because pressures from several sources often coincide. If this were not argument enough to take a fresh look at multiple stressors, climate change is now exacerbating, confounding or complicating many existing problems. Not only are climatically mediated shifts in thermal regime or runoff moving conditions outside those previously experienced by species in their current geographical range, these same factors alter the severity of most other pressures. Human responses to climate change are a further major source of ecological change, for example, via altered patterns of abstraction, water transfer, flood-risk alleviation or catchment use and management (Strayer & Dudgeon 2010).