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How Rarity is Determined
Which species are thriving and which are on the brink of extinction? These are critical questions for targeting conservation efforts toward those species and ecosystems in the greatest need.
Rare species are particularly vulnerable to both human-induced and natural hazards. As a result, rarity is a key predictor of extinction potential. Although rarity may seem a straightforward concept, it is actually complex to characterize. For this reason, Natural Heritage biologists evaluate four distinct characteristics of rarity for each species when assessing its conservation status:
- Total population size (number of individuals of the species)
- Number of different populations
- Extent of species habitat
- Breadth of the species geographic range
Considerations other than rarity are also factored into conservation status determinations.
Population trend - whether a species numbers are increasing, stable, or declining - is a key factor for rare and common species alike. Extinction, after all, is simply the ultimate decline in population numbers.Threats to species - both human and natural, must also be considered since these are important predictors of future decline.
The conservation status of a species or community is designated by a number from 1 to 5, preceded by a letter reflecting the appropriate geographic scale of the assessment (G = Global and S = State). The numbers have the following meaning:
1 = critically imperiled
2 = imperiled
3 = vulnerable to extirpation or extinction
4 = apparently secure
5 = demonstrably widespread, abundant, and secure.
For example, G1 would indicate that a species is critically imperiled across its entire range (i.e., globally). In this sense, the species as a whole is regarded as being at very high risk of extinction. A rank of S3 would indicate the species is vulnerable and at moderate risk within a particular state, even though it may be more secure elsewhere.
Extinct or missing species and ecological communities are designated with either an "X" (presumed extinct or extirpated) if there is no expectation that they still survive, or an "H" (possibly extinct or extirpated) if they are known only from historical records but there is a chance they may still exist.
Conservation status assessments must be continually reviewed, refined, and updated. ANHC biologists rely on the best available information in making and documenting conservation status determinations, including such sources as natural history museum collections, scientific literature, and documented sightings by knowledgeable biologists. To augment this existing knowledge, ANHC biologists conduct extensive field inventories and population censuses, especially targeting those species thought to be imperiled or for which few existing data are available. Most changes in status assessments tend to reflect this improved scientific understanding of the actual conditions of the species.
Designed to assist in setting research and protection priorities, these conservation status ranks are biological assessments rather than legal categories. They do not confer legal protection, as do listing under the U.S. Endangered Species Act. Natural heritage status ranks are, however, among the factors evaluated by the U.S. Fish and Wildlife Service in identifying which species warrant consideration for protection under the act.
