In simple terms, endangered species are those species whose populations have been so reduced that they are threatened with extinction. The international Union for the Conservation of Nature and Natural Resources, with headquarters in Morges, Switzerland, publishes a list of threatened mammals, birds, reptiles, and amphibians. Environmental change continues to be the principal cause of the extinction of animals and plants. Clearing land for farms and towns, lumbering, mining, building dams, and draining wetlands all alter the environment so extensively that ecosystems may be destroyed. Many endangered species have been spared extinction since 1973 when 80 nations participated in the Convention on International Trade in Endangered Species of Wild Flora and Fauna in Washington, DC. The convention delegates agreed to halt imports of endangered species. That same year the U.S. Congress passed the Endangered Species Act to protect the habitats of endangered species. This set of four stamps shows a representative sample of endangered plant species in this country.