540 -- number of national wildlife refuges in the U.S.1 95.3 million -- size, in acres, of the refuge system2 19.3 million -- size, in acres, of Alaska's Arctic National Wildlife Refuge, the largest refuge in the country3 0.6 -- size, in acres, of Minnesota's Mille Lacs National Wildlife Refuge, the smallest refuge in the country4 20.7 million -- acres of designated wilderness in the refuge system5 59 -- number of refuges established to protect endangered species6 1.4 million -- number of volunteer-hours contributed to refuges7 22 -- percentage of all work on refuges done by volunteers8 8 million -- …