Second, some environmentalists worried that construction of the site would adversely affect the endangered population of the Mount Graham Red Squirrel. The Mount Graham subspecies was thought to have been extinct in the 1950s, but small numbers of squirrels were "rediscovered" in the 1970s. The squirrel was added to the federal endangered species list in 1987 by the U.S. Fish and Wildlife Service, when the estimated population in 1986 was fewer than 400. The Mount Graham subspecies has been isolated from other subspecies of red squirrels since the end of the Pleistocene glacial periods. It is still rather unclear if the Mt. Graham red squirrel is distinct or not from red squirrels elsewhere. Studies on genetic data are in progress.