The Endangered Species Act of 1973 was passed in an attempt to prevent the further elimination of animal species in the United States and to help such animal populations to grow.  Called the ESA, the Act was a combination of and an addition to two previous laws, the Endangered Species Preservation Act and the Endangered Species Conservation Act.  The ESA has been amended several times since it first came into effect.

The Endangered Species Preservation Act was passed in 1966.  It stated that the Secretary of the Interior was to produce a list of vertebrates that were in danger of extinction.  These animals were to be protected whenever possible and convenient.  The Act prohibited the killing of these animals only within the national wildlife refuge system; it also permitted the acquisition of critical habitat for these animals.

The Endangered Species Conservation Act contained amendments to the Preservation Act.  Under the new act, if species were in danger worldwide, they could be listed as endangered in the United States.  However, this provision dropped some species from the list that were only endangered in the U.S. Another change was that mollusks and crustaceans could now join the list; trading of these animals was henceforth prohibited. 

The Endangered Species Act added further legal provisions.  Its goal was to bring endangered species back to their natural numbers. According to the Act, all plants and animals, except pests, could be listed as endangered.  The Act also allowed and required the consideration of petitions by the public to list or unlist particular species.  The law made a distinction between endangered and threatened animals: those endangered were close to extinction, those threatened were close to endangerment.  Endangered species could never be killed, while those threatened could only sometimes be killed.  The Act no longer required listed species to be endangered worldwide.  In addition, the Act split responsibility for these animals between the Fish and Wildlife Service and the National Oceanic and Atmospheric Administration, and it provided for agreements between federal and state level governments.  Finally, it forbade federal action that could cause risk to any endangered species. Because this last provision of the Act proved an impediment to many important government projects, the Endangered Species Committee was established several years later.  This Committee could grant a government agency an exemption from the ESA, if the benefit of the project seemed to outweigh the loss of species.

Later amendments to the ESA included the requirement to review the condition of animals on the list to determine if their condition was improving, the development of recovery plans for  such animals, and rules to shorten the listing process.

The text of the law