It took one visit to Yellowstone National Park for Mike Skelton, 60, to decide that he should move closer.

But as trash started to pile up in the park during the partial federal government shutdown, Mr. Skelton, who moved to Gardiner, Mont., from Dallas in 2015, felt he could not sit idly by. So he assembled a cleanup crew — “local area residents turned gardeners,” he said.

“We all live here,” said Mr. Skelton, who runs a sightseeing company providing wildlife and scenic park tours. “When it gets down to it, it is our park and it belongs to all of us in this country.”