Extinction and Endangered Species - How Many Species Are Endangered?

Since 1960 the International Union for the Conservation of Nature and Natural Resources has compiled the IUCN Red List of Threatened Species, which aims to examine the status of biological species across the globe. The so-called "Red List" categorizes species based on the level of risk of their extinction in the wild, as follows:

• Critically endangered—extremely high risk
• Endangered—very high risk
• Vulnerable—high risk
• Near threatened—likely to qualify for a risk category in the near future

The IUCN refers to species in all of these categories as "threatened" species.

Determining how many species of plants and animals are threatened or endangered is difficult. In fact, only a small fraction of the species in existence have even been identified and named, let alone studied in detail. Various estimates of the total number of threatened/endangered species on Earth range from five million to 100 million with most estimates figuring around ten million species worldwide. Of these, about 1.5 million species have been named and described. Mammals, which are probably the best-studied group—and the one that includes humans—make up less than 1% of all known organisms. Insects are a particularly rich biological group—over 900,000 insect species have been identified, with countless more to be described.

Worldwide, in the IUCN's 2004 Red List, a total of 15,503 species were listed as threatened. Just over 38,000 species were examined out of the 1.5 million species that the IUCN considers "described species." Thus, only 2.5% of all known species were evaluated by the IUCN.

The listed species were as follows:

• 1,101 of 4,853 mammal species evaluated (23%)
• 1,213 of 9,917 bird species evaluated (12%)
• 304 of 499 reptile species evaluated (61%)
• 1,770 of 5,743 amphibian species evaluated (31%)
• 800 of 1,721 fish species evaluated (46%)
• 559 of 771 insect species evaluated (73%)
• 974 of 2,163 mollusk species evaluated (45%)
• 429 of 498 crustacean species evaluated (86%)
• 30 of 55 other species evaluated (55%)

In addition, the IUCN listed 8,321 plant species as threatened in 2004 out of 11,824 species evaluated. More than 280,000 plant species are known to the IUCN.

According to the IUCN nearly all described bird, amphibian, and mammal species were evaluated during 2004. The other species have not yet been thoroughly assessed. Further study will likely result in many more species being added to the Red List.

Table 1.2 lists the number of species identified as threatened or endangered under the Endangered Species Act (ESA) as of February 1, 2006. Of the 1,090 animal species listed, 527 are found in the United States. Among these, 398 are endangered and 129 are threatened. Among animals, the greatest numbers of listed species occur among fish, birds, and mammals. Of the 748 plant species listed, 745 are found in the United States. Among these, 599 are endangered, and 146 are threatened. Nearly all of the endangered plants are flowering plants.

Figure 1.2 shows the number of listed U.S. species per calendar year from 1980 through 2005. Table 1.3 shows the breakdown by species group and year. The number of listed species increased dramatically between 1980 and the mid-1990s. The plateau in listings during the late 1990s and early 2000s reflects, in part, budgetary constraints on listing activity at the U.S. Fish and Wildlife Service.

Within the United States, endangered and threatened species are not evenly distributed but are clustered in specific geographical areas. Figure 1.3 shows the number of federally listed endangered and threatened species in each state on February 1, 2006. Regions where the number of listed species is particularly high include southern Appalachia, Florida, the Southwest, California, and Hawaii. Hawaii harbors more threatened and endangered species than any other state, despite its small size. This is due largely to the fact that a significant proportion of Hawaiian plant and animal life is endemic—that is, found nowhere else on Earth. Endemism is very dangerous for imperiled species for a variety of reasons. A single calamitous event, such as a hurricane, earthquake, or disease epidemic, could wipe out the entire population at one time. The likelihood of interbreeding and resulting genetic problems is also higher for a species that is so geographically limited