Natural Heritage Program model works by assigning conservation priority ranks to “elements of natural diversity”, which are plant and animal species, subspecies, and natural communities or habitats. In practice, ranking scheme is used primarily with vertebrates and plant species. However, a standardized natural community classification system has been completed for western United States and soon will be for eastern part of the country.
Thus, Natural Heritage Program in the United States increasingly focuses on community and habitat elements of diversity. Species ranks are based on information of species including known or estimated number, quality, and condition of element “occurrences” (subpopulations of species), estimated number of individuals, narrowness of range and habitat, trends in population and habitat, threats to element, element’s fragility and other factors (TNC, 1988).
This information is then used to assign a rank of 1 to 5 (with 1 representing extreme vulnerability) to species on three separate scales viz., global, national, and state or province. When global, national, and sub national ranks are combined, system allows for a rapid assessment of species’ known or probable threat of extinction or extirpation in a particular jurisdiction.
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TNC and its partners neither do not use ranking system alone to set priorities. With each species or element record, recommendations are made for protection, inventory, research, and management.
These recommendations are frequently based on a number of site-specific facts and qualitative assessments of species’ conservation needs. Ranking system has limitations for some conservation objectives.
This system does not distinguish between species on basis of economic, ecological, or taxonomic value. Another limitation is that system has been used primarily to identify and purchase or otherwise conserve small “ecologically unique” habitats rather than conserve large diverse landscapes.