ARTICLE doi:10.1038/nature09440 Global threats to human water security and river biodiversity C. J. Vo ¨ro ¨smarty 1 *, P. B. McIntyre 2 *{, M. O. Gessner 3 , D. Dudgeon 4 , A. Prusevich 5 , P. Green 1 , S. Glidden 5 , S. E. Bunn 6 , C. A. Sullivan 7 , C. Reidy Liermann 8 & P. M. Davies 9 Protecting the world’s freshwater resources requires diagnosing threats overa broad range of scales, from global to local. Here we present the first worldwide synthesis to jointly consider human and biodiversity perspectives on water security using a spatial framework that quantifies multiple stressors and accounts for downstream impacts. We find that nearly 80% of the world’s population is exposed to high levels of threat to water security. Massive investment in water technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas less wealthy nations remain vulnerable. A similar lack of precautionary investment jeopardizes biodiversity, with habitats associated with 65% of continental discharge classified as moderately to highly threatened. The cumulative threat framework offers a tool for prioritizing policy and management responses to this crisis, and underscores the necessity of limiting threats at their source instead of through costly remediation of symptoms in order to assure global water security for both humans and freshwater biodiversity. Water is widely regarded as the most essential of natural resources, yet freshwater systems are directly threatened by human activities 1–3 and stand to be further affected by anthropogenic climate change 4 . Water systems are transformed through widespread land cover change, urb- anization, industrialization and engineering schemes like reservoirs, irrigation and interbasin transfers that maximize human access to water 1,5 . The benefits of water provision to economic productivity 2,6 are often accompanied by impairment to ecosystems and biodiversity, with potentially serious but unquantified costs 3,7,8 . Devising interventions to reverse these trends, including conventions 9 and scientific assessments 10 to protect aquatic biodiversity and ensure the sustainability of water delivery systems 11 , requires frameworks to diagnose the primary threats to water security at a range of spatial scales from local to global. Water issues feature prominently in assessments of economic development 6 , ecosystem services 3 , and their combination 12–14 . However, worldwide assessments of water resources 2 rely heavily on fragmented data often expressed as country-level statistics, seriously limiting efforts to prioritize their protection and rehabilitation 15 . High-resolution spatial analyses have taken understanding of the human impact on the world’s oceans 16,17 and the human footprint on land 18 to a new level, but have yet to be applied to the formal assessment process for freshwater resources 2 despite a recognized need 19,20 . The success of integrated water management strategies depends on striking a balance between human resource use and ecosystem pro- tection 2,9,10,21 . To test the degree to which this objective has been advanced globally, and to assess its potential value in the future, requires systematic accounting. An important first step is to develop a spatial picture of contemporary incident threats to human water security and biodiversity, where the term ‘incident’ refers to exposure to a diverse array of stressors at a given location. Many stressors threaten human water security and biodiversity through similar pathways, as for pollution, but they also influence water systems in distinct ways. Reservoirs, for example, convey few negative effects on human water supply, but substantially impact on aquatic biodiversity by impeding the movement of organisms, changing flow regimes and altering habitat. Similarly, non-native species threaten biodiversity but are typically inconsequential to human water security. Here we report the results of a global-scale analysis of threats to fresh water that, for the first time, considers human water security and biodiversity perspectives simultaneously within a spatial accounting framework. Our focus is on rivers, which serve as the chief source of renewable water supply for humans and freshwater ecosystems 2,3 . We use river networks to redistribute the distinctive impacts of stressors on human water security and biodiversity along a continuum from head- waters to ocean, capturing spatial legacy effects ignored by earlier studies. Our framework incorporates all major classes of anthro- pogenic drivers of stress and enables an assessment of their aggregate impact under often divergent value systems for biodiversity and human water security. Enhancing the spatial resolution by orders-of- magnitude over previous studies (using 309 latitude/longitude grids) allows us to more rigorously test previous assertions on the state of the world’s rivers and to identify key sources of threat at sub-national spatial scales that are useful for environmental management. Finally, we make the first spatial assessment of the benefits accrued from tech- nological investments aimed at reducing threats to human water secur- ity, revealing previously unrecognized, global-scale consequences of local water management practices that are used extensively worldwide. Global patterns of incident threat Using a global geospatial framework 22 , we merged a broad suite of individual stressors to produce two cumulative incident threat indices, one for human water security and one for biodiversity. The resulting 1 The Environmental CrossRoads Initiative, City University of New York, The City College of New York, New York, New York 10035, USA. 2 School of Natural Resources and Environment, University of Michigan, Ann Arbor, Michigan 48109, USA. 3 Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, and Institute of Integrative Biology (IBZ), ETH Zurich, 8600 Du ¨ bendorf, Switzerland and Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), 16775 Stechlin, Germany. 4 Division of Ecology and Biodiversity, School of Biological Sciences, The University of Hong Kong, Hong Kong SAR, China. 5 Water Systems Analysis Group, University of New Hampshire, Durham, New Hampshire 03824, USA. 6 Australian Rivers Institute, Griffith University, Nathan, Queensland 4111, Australia. 7 School of Environmental Science and Management, Southern Cross University, New South Wales 2480, Australia. 8 School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98195, USA. 9 Centre of Excellence in Natural Resource Management, The University of Western Australia, Albany 6330, Australia. {Present address: Center for Limnology, University of Wisconsin, Madison, Wisconsin 53706, USA. *These authors contributed equally to this work. 30 SEPTEMBER 2010 | VOL 467 | NATURE | 555 Macmillan Publishers Limited. 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