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The U.S. Environmental Protection Agency requested thernNational Research Council(which is the operating arm of the U.S.rnNational Academy of Sciences and the U.S. National Academy ofrnEngineering) to carry out a critical scientific evaluation ofrnindicators to monitor ecological changes from either natural orrnanthropogenic causes. More specifically, the National Research Council was asked to identify criteria for evaluating biological indicators, to evaluate methods of indicator development, to provide examples of ndicators that have proven useful, and to identify areas where further research is likely to yield more useful and powerful indicators. In addition, the National Research Council was to examine what aspects of environmental conditions and trends should be monitored.