ASHLAND,ORE. — IN the years since Richard Nixon signed it into law in 1973, the Endangered Species Act has proven to be, in the words of the United States Supreme Court, "the most comprehensive legislation for the preservation of endangered species ever enacted by any nation.""It is an extraordinary piece of legislation," says Lewis and Clark College law Prof. Daniel Rohlf, who has written extensively on the subject. "It elevates the goal of conservation of listed species above virtually all other considerations." An "endangered" species is one "in danger of extinction throughout all or a significant portion of its range.Threatened" species are those thought likely to become endangered in the near future. The law defines "species" as species, sub-species, or (for vertebrates) a specific population. Thus, a population of grizzly bears in the coterminous 48 states may be listed, while grizzlies in Alaska may not be. The Secretary of Commerce (through the National Marine Fisheries Service) lists marine mammals; the Secretary of the Interior (through the Fish and Wildlife Service) lists all other species. Section 7 of the act forbids any activity on federal land that would jeopardize a listed plant or animal species or its habitat. Section 9 addresses such species on private lands. Endangered fish and wildlife (and plants on federal land) may not be "taken," which means to "harass, harm, pursue, hunt, shoot, wound, kill, trap, capture or collect, or to attempt to engage" in such conduct. Amendments to the law passed in 1978 require officials to designate "critical habitat" when listing a species and also to draft recovery plans. While economic considerations do not play a role in listing species, they may in the designation of critical habitat or design of recovery plans. The act also provides for an Endangered Species Committee to review listings and grant exemptions based on economic factors. This group (which has met only three times in the history of the law) is made up of the secretaries of Agriculture, the Army, and the Interior; the chairman of the Council of Economic Advisors; the administrator of the EPA, the administrator of the National Oceanic and Atmospheric Administration, and one individual from each affected state. This group is now considering exemptions for 44 timber sales on US Bureau of Land Management land in western Oregon that includes northern spotted owl habitat.