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Unit 9: Biodiversity Decline // Section 5: Categories of Concern: Critically Endangered, Endangered, Vulnerable

What are the criteria for determining whether a species is in danger of extinction? To advise national governments, the multinational World Conservation Union (abbreviated IUCN for its formal title, the International Union for the Conservation of Nature) maintains a database of threatened species and subgroups and publishes the Red List, which catalogues species most at risk. For species in the highest risk groups—Critically Endangered, Endangered, and Vulnerable—IUCN weighs criteria including population size, geographic range, and how individuals are distributed, especially if the population is very small.

IUCN is working to improve its data on species, which currently is biased toward forests and other land ecosystems, emphasizes animals more strongly than plants, and does not cover microbes. Priority areas for the Union include better data on marine species and on arid and semi-arid ecosystems, which are expanding as a result of global climate change. Table 1 summarizes some estimates of threatened species (Critically Endangered, Endangered, and Vulnerable) from the 2006 Red List.

Table 1. Species classified as threatened by IUCN, 2006. Source: IUCN, "The IUCN Red List of Threatened Species: Summary Statistics,"
Number of described species in IUCN database Number of threatened species in 2006 Number threatened as % of described species
Mammals 5,416 1,093 20%
Birds 9,934 1,206 12%
Reptiles 8,240 341 4%
Amphibians 5,918 1,811 31%
Fishes 29,300 1,173 4%
Subtotal 58,808 5,624 10%
Insects 950,000 623 0.07%
Mollusks 70,000 975 1.39%
Crustaceans 40,000 459 1.15%
Others 130,200 44 0.03%
Subtotal 1,190,200 2,101 0.18%
Mosses 15,000 80 0.53%
Ferns and allies 13,025 139 1.0%
Gymnosperms 980 306 31%
Dicotyledons 199,350 7,086 4%
Monocotyledons 59,300 779 1%
Subtotal 287,655 8,390 3%
Lichens 10,000 2 0.02%
Mushrooms 16,000 1 0.01%
Subtotal 26,000 3 0.01%

Other biological inventories cover different sets of organisms and offer different perspectives on which species are most highly threatened. For example, NatureServe ( pools data from a network of natural heritage programs and estimates the number of threatened species in the United States to be far greater than the IUCN’s estimates. "There is no single authoritative list of the world's endangered species, because we have yet to count and describe many living species," says Harvard University biologist Anne Pringle.

Scientific evidence is central to identifying endangered species. To determine whether a species is endangered or might become so, scientists collect data to answer questions including:

These assessments draw on scientific fields including conservation biology, population ecology, biogeography, and genetics. Captive breeding programs have helped to preserve and reintroduce some species that were extinct in the wild, such as California condors. Recently scientists have successfully cloned several endangered varieties of cows and sheep, and some biologists advocate creating DNA libraries of genetic material from other endangered species. Others counter that cloning fails to address the root causes of the problem, including habitat loss and over-harvesting.

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