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species, and to take such steps as may be appropriate to achieve the purposes of the treaties and conventions" in which the United States has pledged its support for the conserva- tion of wild flora and fauna worldwide. This law encompasses all species of the animal and plant kingdoms, with the term species pertaining to any species, any subspecies, any smaller taxonomic unit of plant or animal, and also any viable popula- tion segment thereof. Furthermore, the law established two categories of endangerment: (1) endangered species—those species in danger of extinction throughout all or a significant portion of their range; and (2) threatened species—those spe- cies that are likely to become endangered within the foresee- able future throughout all or a significant portion of their range (Fig. 4). This law also emphasized the need to preserve critical habitats on which endangered species depend for their contin- ued existence. Individual states were encouraged to establish guidelines to complement the goals outlined in the 1973 Act. Typically, the first step is identifying which species are in danger of extinction throughout all or part of their range and adding them to an endangered species list. In the United States, species are placed on the endangered species list if one or more factors put it at risk, including habitat destruction or degradation, overutilization, disease, and predation. In addi- tion, there is an assessment of whether the species is otherwise protected from those threats. Subspecies and distinct popula- tions of species can be listed as endangered as well. The IUCN places species in categories of endangerment based on various factors, including population size, extent of population decline, and predicted population decline, either in population or in area of occupancy. From looking at patterns of where endangered species ex- ist in the United States, it is clear that areas with high species diversity and high human population tend to have the most endangered species. In the United States, the greatest numbers of endangered species occur in Hawaii, southern California, the southeastern coastal states, and southern Appalachia. These species are also highly susceptible to endangerment because they tend to have small population sizes and because they are particularly vulnerable to introduced competitors, predators, and disease. Once a species is determined to be endangered, the U.S. Endangered Species Act protects and aims to restore endan- gered populations through two main strategies: (1) halting exploitation and (2) habitat protection and restoration. It is prohibited to kill, harm, or harass an endangered species. In addition, habitat destruction is prohibited if it results in impair- ment of the animal's ability to forage, breed, or seek shelter. To complement prohibitions on harming endangered species, the act requires that critical habitat for a species be delineated and prioritized for protection. The act also requires a recovery plan for each species, which defines the status of the species, threats, measures to be taken to restore its numbers, and goals to be met so that delisting can occur. Rising numbers of cities and counties are developing conservation plans for one or more endangered species. These plans contain information on important habitat that should be preserved, as well as permits for housing development, agriculture, and other activities to occur elsewhere. These protective efforts for endangered spe- cies also serve to protect other species in the same areas that might otherwise become endangered. All of these methods, from prohibitions to large-scale planning, are aimed at recover- ing species so that they are no longer in danger of extinction. For many endangered species, a significant captive population exists in zoos and other facilities around the world. By breeding individuals in captivity, genetic variation of a species can be more easily sustained, even when the species' natural habitat is being destroyed. Some species exist only in captivity because the wild population became extinct. For a few species, captive individuals have been reintroduced into natural habitat in order to establish a population where it is missing or to augment a small population [for example, black-footed ferrets (Fig. 5) and California condors]. Depending on the species, reintroduction can be very difficult and costly because individual animals may not forage well or protect themselves from predators. For example, the golden lion tamarin has been nearly extirpated from Atlantic coastal rainforests in Brazil. These monkeys have been bred in captivity and trained for reintroduction at a nature reserve in Brazil. Although some individuals do not survive rein- troduction, others are able to breed, and a viable wild popula- tion is being reestablished. Endangered Species (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this. Fig. 5 An endangered black-footed ferret. This species has been reintroduced into its natural habitat in order to augment an otherwise small population. (Copyright © Joseph Dougherty)