. . . . . . . . . "Source: Freebase - The World's database". "Ecological Systems of the United States: A Working Classification of U.S. Terrestrial Systems"@en. "2003". . . . . . . "http://www.natureserve.org/getData/ecologyData.jsp". . "Download Ecological Systems data"@en. . "http://www.natureserve.org/publications/usEcologicalsystems.jsp". . "NatureServe Ecological Systems website"@en. . "http://www.natureserve.org/library/usEcologicalsystems.pdf". . "Download pdf of report"@en. .