An endangered species is an organism which is in danger of extinction. Species are classified as endangered for several reasons, such as dwindling numbers or a changed environment. The U.S. Endangered Species Act of 1973 created legal protections for species on the list. This, of course, angers libertarians, who see its protections as infringing upon property rights, and its expense as excessive. There have, of course, been excesses, but the overall impact of the ESA has been positive.