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Terrestrial Ecoregions of North America provides a comprehensive assessment of the status of biodiversity and conservation within the United States and Canada. Part of a global program conducted by the World Wildlife Fund, it is essentially a reference work offering baseline data for conservation planning and restoration. The book emphasises the precarious condition of many natural areas in North America, at the same time illustrating the great diversity that still exists in some areas and stressing the sense of urgency required to ensure the preservation of viable plant and animal populations in their natural habitats. It is not a textbook on conservation, however, but an evaluative report based on carefully described methodology and summary analysis that integrates an index of "biological distinctiveness" and an index of "conservation status" for 116 ecoregions. Ecoregions are defined as relatively large areas of land or water that contain geographically distinct assemblages of natural communities. As part of this assessment, ecoregion maps were compared with the ranges of over 20,000 North American species of native vascular plants, birds, butterflies, mammals, reptiles, amphibians, and terrestrial molluscs. Other procedures, equally comprehensive, considered information collected in other studies and assessments by expert committees.