Increasing numbers of animal and plant species face possible extinction. Endangered and threatened species—and the law that protects them, the 1973 Endangered Species Act (ESA, P.L. 93-205 , as amended; 16 U.S.C. §§1531-1543)—are controversial, in part, because dwindling species are often harbingers of resource scarcity. The most common cause of species' decline is habitat loss or alteration. Habitat loss occurs due to development, climate change, changes in land management practices, competition from invasive species, and other factors, nearly all related to economic, political, or social interests. ESA has been among the most contentious environmental laws because its substantive provisions can affect the use of both federal and nonfederal lands and resources. Congress faces the issue of how to balance these interests with the protection of endangered and threatened species and, as stated in ESA, "the ecosystems upon which endangered species and threatened species depend." Because of strong support and strong opposition, ESA has not been reauthorized since the last authorization expired in 1992. In the 109 th Congress, there were several unsuccessful attempts to enact comprehensive legislation that would have reauthorized ESA. Congressional efforts in the 110 th , 111 th , and 112 th Congresses focused on addressing specific controversial features of ESA and on oversight of concerns such as the science used for making decisions and designating critical habitat, but little legislation related to ESA was enacted. The 1973 ESA was a comprehensive attempt to protect species at risk of extinction and to consider habitat protection as an integral part of that effort. In addition, an express purpose of ESA is to "provide a means whereby the ecosystems upon which endangered species and threatened species depend may be conserved" (16 U.S.C. §1531(b)). Under ESA, species of plants and animals (both vertebrate and invertebrate) may be listed as either endangered or threatened according to assessments of the risk of their extinction. More flexible management can be provided for species listed as threatened, compared to those listed as endangered. Distinct population segments of vertebrate species may also be listed as threatened or endangered. Consequently, some populations of Chinook, coho, chum, and sockeye salmon in Washington, Oregon, Idaho, and California have been listed under ESA, even as other healthy populations of these same species in Alaska are not listed and may be commercially harvested. More limited protection is available for plant species under ESA. Once a species is listed, legal tools, including penalties and citizen suits, are available to aid species recovery and protect habitat. Use of these tools, or the failure to use them, has led to conflict. A more detailed discussion of the major provisions of ESA is provided in CRS Report RL31654, The Endangered Species Act: A Primer , by [author name scrubbed], [author name scrubbed], and [author name scrubbed]. ESA is administered by the Fish and Wildlife Service (FWS, Department of the Interior) for terrestrial and freshwater species and some marine mammals, and by the National Marine Fisheries Service (NMFS) in the Department of Commerce's National Oceanic and Atmospheric Administration for the remaining marine and anadromous species. The U.S. Geological Survey's Biological Resources Division conducts research on species for which FWS has management authority; NMFS conducts research on the species for which it is responsible. As of January 2, 2013, a total of 1,234 species of animals and 820 species of plants were listed as either endangered or threatened under the ESA, of which the majority (619 species of animals and 817 species of plants) occur in the United States and its territories (see Figure 1 and Figure 2 ). The remaining species occur only in other countries. Of the 1,436 U.S. species, 1,143 (about 80%) are covered in active recovery plans. In the most recent data available, FY2011 federal and state expenditures on endangered and threatened species totaled about $1.59 billion, of which about $1.53 billion was reported by federal agencies and about $58 million was reported by the states. The top 10 species with the highest total FY2011 expenditures (excluding land acquisition costs) included 7 subpopulations of steelhead and Pacific salmon ($338 million altogether), pallid sturgeon ($42 million), bull trout ($37 million), and red-cockaded woodpecker ($27 million). However, species do not exist in isolation, but evolve and fluctuate in abundance because of their relationships with other species and the physical environment. Conservationists increasingly are talking about not only species, but also ecosystems as the units of interest. At times, efforts to protect and recover listed species are controversial; declining species often function like the proverbial canary in the coal mine, by flagging larger issues of resource scarcity and altered ecosystems. Past resource debates in which ESA-listed species were part of larger issues include Tennessee's Tellico Dam (water storage and construction jobs versus farmland protection and tribal graves, as well as snail darters); Pacific Northwest timber harvest (protection of logging jobs and communities versus commercial and sport fishing, recreation, and ecosystem protection, including salmon and spotted owls); and the management of the Apalachicola Basin in Alabama, Florida, and Georgia (allocation of water among metropolitan, agricultural, and industrial users along with commercial and recreational fishing interests, as well as one listed fish and three listed mussel species). ESA is the domestic implementing legislation for the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES; TIAS 8249), signed by the United States on March 3, 1973; and the Convention on Nature Protection and Wildlife Preservation in the Western Hemisphere (the Western Hemisphere Convention; 50 Stat. 1354; TS 981), signed by the United States on October 12, 1940. CITES parallels ESA by dividing its listed species into groups according to the estimated risk of extinction, but uses three major categories (called appendices), rather than two. In contrast to ESA, CITES classifies species based solely on the risk that trade poses to their survival. Under ESA, violations of CITES are also violations of U.S. law if committed within U.S. jurisdiction (16 U.S.C. §1538). ESA also regulates import and export of controlled products and provides some exceptions. Both ESA and CITES address illegal trade in wildlife. International illegal wildlife trade is estimated to be worth more than $10 billion annually and has been associated with the decline of species, spread of disease, and proliferation of invasive species, among other things. In addition, FWS's Multinational Species Conservation Fund (MSCF) benefits tigers, the six species of rhinoceroses, Asian and African elephants, marine turtles, and great apes (gorillas, chimpanzees, bonobos, orangutans, and the various species of gibbons). This fund supports conservation efforts benefitting these species, often in conjunction with efforts under CITES. In the 112 th Congress, H.Res. 47 would have expressed the sense of the House of Representatives regarding the contributions of CITES and urged that CITES adopt stronger protections for the polar bear, sharks, and bluefin tuna. H.R. 50 and Section 245(a-c) of S. 3525 would have reauthorized certain provisions of the MSCF related to elephants, rhinoceroses, and tigers. H.R. 1456 and Section 246 of S. 3525 would have reauthorized the Neotropical Migratory Bird Conservation Act through FY2017. H.R. 1760 and Section 245(d) of S. 3525 would have amended and reauthorized the Great Ape Conservation Act of 2000 through FY2017. H.R. 1761 and Section 245(e) of S. 3525 would have amended and reauthorized the Marine Turtle Conservation Act of 2004 through FY2017. On July 28, 2011, the House Natural Resources Subcommittee on Fisheries, Wildlife, Oceans, and Insular Affairs held a hearing on H.R. 50 , H.R. 1760 , and H.R. 1761 . H.R. 3510 , S. 3208 , and Section 244 of S. 3525 would have extended the authorization for the Multinational Species Conservation Funds Semipostal Stamp Act. ESA reauthorization has been on the legislative agenda since the funding authorization expired in 1992, and bills have been introduced in each subsequent Congress to address various aspects of endangered species protection. Below are descriptions of some of the issues that received attention in the 112 th Congress. The answer to this question depends on what is measured. Because a major goal of ESA is the recovery of species to the point at which ESA protection is no longer necessary, this may be a useful starting point. In the 39 years since ESA was enacted, 56 U.S. and foreign species or distinct population segments thereof have been delisted. The reasons cited by FWS are (a) recovery (28 species); (b) extinction (10 species; however, some may have been extinct when listed); and (c) original data in error (18 species). Recovered species include the American alligator, bald eagle, brown pelican (two areas), peregrine falcon (two subspecies), gray wolf (four areas), gray whale (except the Western Pacific Ocean), and three species of kangaroo. Extinct species include the dusky seaside sparrow, Guam broadbill (a bird), and two small fish living in desert springs. However, it can be quite difficult to prove whether extraordinarily rare species are simply that or, in fact, are already extinct.