Under the federal Endangered Species Act, the U.S. Fish and Wildlife Service is authorized to designate land (or water) that a species occupies, i.e., lives (or could naturally live or grow) as “critical habitat” for an endangered species.  16 U.S.C. § 1533 (a)(3)(A)(i).  But Fish and Wildlife can only designate unoccupied land as critical habitat for a species if the agency can show that “such areas are essential for the conservation of the species.” 16 U.S.C. § 1532 (5)(A)(ii).