A map of environmental threats to the Great Lakes shows humans\' impact on an ecosystem containing 20 percent of the world\'s fresh water, U.S. scientists say. Three years of work by researchers at the University of Michigan and other institutions has created the most comprehensive map to date of Great Lakes\' stressors, the first to explicitly account for all major types of stresses on the lakes in a quantitative way, a university release reported Monday. \"Despite clear societal dependence on the Great Lakes, their condition continues to be degraded by numerous environmental stressors,\" aquatic sciences Professor David Allan said. U.S. and Canadian scientists analyzed the latest and best data from federal and state agencies, non-governmental organizations and individual researchers. Among the 34 stressors examined were coastal development, pollutants transported by rivers from agricultural and urban land, fishing pressure, climate change, invasive species and toxic chemicals, the researchers said. \"Current efforts to conserve, manage and restore the Great Lakes often take a piecemeal approach, targeting threats one by one,\" Allan said. \"We need to recognize that the Great Lakes are affected by multiple environmental stressors, and devise strategies based on a full reckoning.\"