Ranking the Rare Species

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After biologists determine the richness or the rarity of species in a habitat, they rank the species according to factors related to vulnerability. Put another way, various species fall into different categories based on the degree of risk of going extinct. Such rankings lead to lists such as the endangered species list and the Red List.

The Red List contains more than 40,000 species, estimated to be about 12 percent of the world’s total species. The list places today’s existing species into any of five categories based on key criteria: population size, population growth or decline, habitat fragmentation, and reproductive rate.

 

Species protections begin with tools such as the Red List and the endangered species
list to identify species facing the most severe threat of extinction. Polar bears, for
example, are expected to decline by 30 percent in the next 45 years according to the
United Nations IUCN. (Jurgen Holfort, International Arctic Research Center, University of
Alaska-Fairbanks)

The listing also takes into account how much land an individual animal needs to sustain an entire population of its species. For instance, mountain lions require areas measured in square miles, but some songbirds live their entire lives within the same acre.

 

The following table shows only the Red List’s three most at-risk classifications: critically endangered, endangered, and vulnerable. The list also includes near threatened species and species of least concern. Animals classified as near threatened are not in immediate risk, but science holds evidence that they are close to being vulnerable, endangered, or critically endangered. Widespread species in abundance in their habitat belong to a category called least concern. (Other terms used informally are critically imperiled, imperiled, vulnerable, secure, and widespread to correspond with the Red List’s terminology.) With only minor differences, the U.S. Endangered Species Act listing and the Canadian Wildlife Service’s Species At-Risk Act match the criteria used in the Red List.

The Red List provides another piece of information not included on the U.S. endangered species list: all available data on extinct species. The list contains two categories of extinct species: (1) extinct means there is no reasonable doubt that the species is gone, and (2) extinct in the wild means a species is believed to exist only in captivity or in cultivation (plants).

No list can solve the problem of biodiversity loss, but endangered species lists clarify the problem for scientists and for the public. Jane Smart, the head of the IUCN’s species program, said to the United Kingdom’s Guardian newspaper in 2007, “Our lives are inextricably linked with biodiversity and ultimately its protection is essential for our very survival.” Ecology is truly a science that discovers the interrelationships among living things.