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. Under the act, species are to be listed as threatened or endangered based solely on the best available scientific information, without regard to economic considerations. Congress faces the issue of how to balance these economic 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 few bills related to ESA were enacted. In the 113 th Congress, no measures have been introduced to reauthorize the act. A few bills have been introduced to limit its reach or to prevent listing of one or more species. 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. In addition to an entire species, 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 to protect habitat. Use of these tools, or the failure to use them, has led to conflict. 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 (NOAA) for the remaining marine and anadromous species. The U.S. Geological Survey's Ecosystems Division conducts research on species for which FWS has management authority; NMFS conducts research on the species for which it is responsible. As of December 26, 2013, a total of 1,268 species of animals and 877 species of plants were listed as either endangered or threatened under the ESA, of which the majority (645 species of animals and 874 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 1,519 U.S. species, 1,144 (75%) are covered in active recovery plans. In the most recent data available, FY2011 expenditures on endangered and threatened species, by all federal and state agencies together, totaled about $1.39 billion, of which about $1.34 billion was reported by federal agencies and about $57.8 million was reported by the states. The top 10 species with the highest total FY2012 expenditures (excluding land acquisition costs) included 7 subpopulations of steelhead and Pacific salmon ($352.5 million for all 7 together), pallid sturgeon ($53.8 million), bull trout ($37.7 million), and red-cockaded woodpecker ($37.9 million). Th ese expenditures can cover a wide variety of activities, such as streamside cleanup, land acquisition, creation of artificial nest sites, etc. 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 because a declining—even seemingly insignificant—species is a flag that signals both an ecosystem in danger and a scarcity of economic resources. These actions have been especially controversial where they affect resource-based economies. Controversies in which ESA-listed species are or have been part of larger debates include water supply in central California (water for some users vs. effects of water supply conveyance and other factors on commercial and recreational fishing), 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 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). For foreign species, the Secretary of the Interior is authorized to consider actions taken by foreign governments or agencies on species when determining if a foreign species should be listed. Further, the Secretary must make determinations for listing species based on the best available scientific and commercial data (which may include foreign sources) and taking into account efforts by foreign governments and agencies. FWS reviews foreign species under the ESA's listing criteria, with a required regulatory and public comment process before listing. Foreign governments and scientific organizations can and do comment on proposed listings. Foreign species are also addressed by international wildlife treaties. ESA is the domestic vehicle to implement 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 focuses on extinction due specifically to trade. It parallels ESA by dividing its listed species into groups according to the estimated risk of trade-caused extinction, but uses three categories (called appendices), rather than two. CITES does not have enforcement authority and regulates the international trade of species through permits. Under ESA, violations of CITES are also violations of U.S. law if committed within U.S. jurisdiction (16 U.S.C. §1538), by a person subject to U.S. jurisdiction. ESA also regulates import and export of controlled wildlife products made from listed species (e.g., sport hunted trophies) and provides some specified exceptions to import and export regulations. According to U.S. government estimates, illegal trade in endangered wildlife products, including elephant ivory, rhino horns, and turtle shells, is worth at least $7 billion annually. (This figure does not include illegal logging and illegal fishing, which can account for roughly an additional $30 billion to $100 billion annually and $10 billion to $23 billion annually, respectively.) Such figures may place illegal wildlife trafficking among the most lucrative criminal activities worldwide, behind only narcotics, counterfeiting, and human trafficking. Prosecutions by the Department of Justice of ESA and CITES violations have, in some cases, led to the dismantling of wildlife trafficking rings in the United States and throughout the world. Recent activity in the 113 th Congress has addressed the illegal wildlife trade, in particular elephant and rhino poaching and the trade of ivory. 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 have or are likely to receive attention in the 113 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 40 years since ESA was enacted, 58 U.S. and foreign species or distinct population segments thereof have been delisted. The reasons cited by FWS are (a) recovery (30 species); (b) extinction (10 species); (c) taxonomic revisions (7 species); and (d) original data in error or legislative changes to ESA (11 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.