Issue link: https://wardsworld.wardsci.com/i/1411487
degraded or changed, the altered conditions that are no longer suitable for native species can be advantageous for invasive species. In the United States, approximately half of all endan- gered species are adversely affected by invasive species. Pollution Pollution directly and indirectly causes species to become en- dangered. In some cases, pesticides and other harmful chemi- cals are ingested by organisms low on the food chain. When these organisms are eaten by others, the pollutants become more and more concentrated in plant and animal tissues, until the concentration reaches dangerous levels in predators and omnivores. These high levels cause reproductive problems [for example, birds carrying high levels of the pesticide DDT (dichlorodiphenyltrichloroethane) cannot reproduce] and sometimes death. In addition, direct harm often occurs when pollutants make water uninhabitable. Agriculture and industrial production cause chemicals, such as fertilizers and pesticides, to reach waterways. Lakes have become too acidic from acid rain, which contains previously airborne chemicals. Other human activities, including logging, grazing, agriculture, and housing development, cause siltation (another form of pollu- tion) in waterways. Largely because of this water pollution, two out of three freshwater mussel species in the United States are at risk of extinction. Pollution has also led to other phenomena that present risks to species. Most scientists agree that pollu- tion is also causing climate change (often referred to as global warming), as well as greater exposure to ultraviolet radiation from ozone-layer depletion. Overexploitation Many species have become endangered or extinct from hu- mans killing them for food or population control throughout their ranges. For example, the passenger pigeon, formerly one of the most abundant birds in the United States, became extinct largely because of overexploitation. Overexploitation is especially a threat for species that reproduce slowly, such as large mammals and some bird species. Overfishing by large commercial fisheries is a threat to numerous marine and fresh- water species. Strategies for protection Habitat destruction and other threats to species worldwide increased in the twentieth century and have continued into the twenty-first century; however, steps are under way to reduce those threats. Many people have realized that endangered species can signal current or eventual threats to human health and safety. Efforts to save species focus on ending exploitation, halting habitat destruction, restoring habitats, and breeding populations in captivity. Internationally, endangered species are protected from trade that depletes populations in the wild through the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). More than 180 member countries and states act by banning commercial international trade of endangered species and by regulating and monitoring trade of other spe- cies that might become endangered. For example, the inter- national ivory trade was halted in order to protect elephant populations from further depletion. In addition, the Interna- tional Union for Conservation of Nature and Natural Resources (IUCN) identifies which species are in danger of extinction and initiates international programs to protect them. In the United States, the Endangered Species Act of 1973 protects endangered species and the ecosystems upon which they depend. This law seeks "to provide a means whereby the ecosystems upon which endangered species and threatened species depend may be conserved, to provide a program for the conservation of such endangered species and threatened Endangered Species (continued) Fig. 3 Invasive reptiles, such as the Burmese python, are often introduced via the pet trade. (Credit: Everglades National Park, Florida/National Park Service) + ward ' s science Fig. 4 West Indian or Florida manatee. Because of recent conservation efforts, this species of manatee, formerly considered to be endangered, has been recategorized as a threatened species. (Credit: Keith Ramos/U.S. Fish and Wildlife Service)