Come May in northwestern Yukon and northeastern Alaska, as the land loosens from winter’s grip — snowpack shrinking in the valleys, lily pads of river ice rushing downstream — the Porcupine caribou herd receives an ancient cue: time to move. For millennia, caribou have been drawn north, calves cradled in utero, towards the Arctic National Wildlife Refuge, a 19.6-million-acre designated wilderness in northeastern Alaska. A refuge, created in 1980, that was meant to be protected.