Yellowstone was established by Ulysses S. Grant in 1872 as the world's first national park. Its founding began an international movement: today more than 100 countries have about 1,200 national parks or equivalent preserves. Most people know Teddy Roosevelt played a role in advocating for the early parks, but the National Park Service was created within the Department of the Interior by Woodrow Wilson in 1916, which is why this year is the centennial.

The act Wilson signed states that the fundamental purpose of the park service is "to conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations."

When it started, the park service was primarily preserving large tracts. In the 1960s there was a historic preservation movement and a conservation movement that came out of the activism of the time. This year is actually the 50th anniversary of the National Historic Preservation Act, which gave the park service the power to create the National Register of Historic Places, which is part of a national program to coordinate and support public and private efforts to identify, evaluate, and protect America's historic and archaeological resources.

Today the national park system comprises 410 sites and covers more than 84 million acres. The mission of its more than 20,000 employees is to preserve natural and cultural resources. It does this by educating the public and promoting the idea that these places are valuable.

I love the mission, this idea that there's a concentrated effort to keep intact not just natural resources for the sake of what their gold value is, but to create a sense of meaning among citizens in this country and give us pride in our land, history, and shared cultures.

Last year more than 307 million people visited the 879 park service visitor centers and contact stations. Great Smoky Mountains is the most visited. The park service is growing and evolving with the American people. We listen; we react; we make a real effort to be relevant. There has been a series of acts of Congress over the years to create new ways for the park service to preserve history and natural resources, so the park service isn't a monolithic, unchanging institution. We're always growing and looking for new ways of doing things to meet the needs of contemporary life.

As told to Mike Unger