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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

This study was supported by Contract/Grant No. SMA-1019816 between the National Academy of Sciences and the National Science Foundation; Contract/Grant No. N01-OD-4-2139, TO #231, between the National Academy of Sciences and the National Institutes of Health; Contract/Grant No. G104P00159 between the National Academy of Sciences and the U.S. Geological Survey; Contract/Grant No. 59-9000-0-0093 between the National Academy of Sciences and the U.S. Department of Agriculture; Contract/Grant No. EP-11-H-001414 between the National Academy of Sciences and the Environmental Protection Agency; Contract/Grant No. DE-SC000614 between the National Academy of Sciences and the Department of Energy; Contract/Grant No. NNH10CC488,TO #5, between the National Academy of Sciences and NASA. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the organizations or agencies that provided support for the project

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Cover: The cover design incorporates a feature of the 1924 National Academy of Sciences building in Washington. Sculpted by Lee Lawrie, the bronze cheneau, running the length of the roof, features alternating figures of owls and lynxes, symbolizing wisdom and alert observation, respectively.

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