WALPOLE, N.H. — THOUSANDS of people are expected to gather at the north entrance to Yellowstone National Park on Aug. 25 to celebrate a uniquely American idea. The National Park Service turns 100 years old, and its birthday party will take place under a stone arch emblazoned with the words “For the benefit and enjoyment of the people.”

No nation had ever set aside such a magnificent place for that reason. Wild reserves had been the exclusive property of nobility or the rich. Decisions by Congress to protect Yellowstone and other wonders reflected a different idea: In a democracy, such landscapes should belong to everyone.

But that idea included a challenge: Places like Yellowstone must also be “preserved,” as President Theodore Roosevelt urged his fellow citizens, for our “children and their children’s children forever, with their majestic beauty all unmarred.”

That tension between access and preservation has become ever more strained today.

In 1916, when the National Park Service was created, there were a dozen national parks, all of them in the West, visited by 326,506 people. Today, 412 parks, national monuments and historic sites cover more than 84 million acres and were visited more than 307 million times last year. Attendance is setting records this summer, and by the time the year ends, the number of visits over the last 100 years is expected to crest 13.5 billion.