A short hike off a road in the Sherburne National Wildlife Refuge, near Zimmerman, he drops to his knees next to a hole in the ground that’s been covered with chicken wire, plywood, and a concrete weight. As he removes the makeshift cover, a sickeningly sweet stench wafts out. Smith, a nongame wildlife biologist with the Minnesota Department of Natural Resources, wrinkles his nose as he reaches into a buried five-gallon plastic bucket and pulls out the deflated carcasses of two very dead, very rotten rats.