The USA was the first country to give endangered species legal status and in so doing helped define the science of endangered species. As early as 1964 the Department of Interior produced a list of 62 species at risk of extinction in the US. When the Endangered Species Act (ESA) was implemented in 1973 there were 392 endangered or threatened species on the list. The ESA itself defines endangered as a species ''in danger of extinction throughout all or a significant portion of its range.'' Although the ESA does not give any population sizes or explicit scientific criteria, it is clear from the record of listings (when species are officially called endangered or threatened and afforded legal protection) that small population size is the dominant criteria. The median population size of species when listed is 1075 for vertebrates and 120 for plants. When one realizes that all of these individuals will not be capable of reproducing (e.g., too young, too old, cannot find mate), those population sizes are disturbingly low if one is to have hope of recovering the species so that they are no longer endangered. As of 2006, there are 1272 endangered and threatened species in the USA with a very uneven taxonomic distribution. The percentage of species in the US listed as endangered or threatened are by major taxa: 19% for mammals, 15% for fish, 12% for bird, 11% for amphibians, 8% for invertebrates, and 5% for plants.
When species are listed as endangered or threatened, the US government requires a description of the major threats that are placing the species at risk of extinction. By reviewing the listed threats for all the US species it is possible to arrive at an estimate of the major sources of endangerment. As expected, habitat destruction is by far the most widely identified threat, affecting approximately 85% of all the species listed as threatened or endangered in the USA. Alien or introduced species is the second most common threat (affecting 49% of listed species), followed by pollution (24%) and overexploitation (18%). Notably underrepresented as endangered species in the US are marine organisms. Only 39 marine species (and many of these are fish such as salmon and steelhead that spawn in fresh water) are listed as threatened or endangered in the US. Only one of these marine endangered species is an invertebrate - the white abalone in California. The paucity of marine species that are listed under the ESA is not a reflection that marine organisms are not endangered. Rather marine organisms are more poorly known taxonomically, and population data are generally lacking. They are under water and hence their plight is less conspicuous than is, for example, a large terrestrial bird. Most biologists think that with further study we will find many marine species that are endan-gered-often because of habitat degradation and pollution.
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