Interior Secretary Sally Jewell commemorated the National Park Service’s 100 birthday in a speech late Thursday, calling their creation “one of the nation’s most revolutionary ideas — that these lands, our iconic historic sites and our culturally significant places should belong to every American.”

Standing on a stage erected near the Roosevelt Arch at the north entrance of Yellowstone National Park near Gardiner, Mont., Jewell said, “I can think of no better place to commemorate this milestone than here, at America’s first national park, under a big sky, on a crisp night, in the shadows of beautiful mountains and on the shoulders of conservation giants who came before us.”

About 6,000 people gathered to hear country bands play and numerous speakers, including a Theodore Roosevelt impersonator who introduced Jewell. Roosevelt has gone down in history as a titan of conservation, but it was President Woodrow Wilson who created the service in 1916 that now oversees more than 410 parks, monuments and historic sites on 85 million acres in the 50 states and territories.

But there was concern in the night air even as Jewell spoke about the future of the parks. Will the Park Service survive a second century?

The natural beauty of the parks is unquestioned, but the human touches that make them accessible aren’t all pretty. The system faces a $12 billion maintenance shortfall that has left infrastructure as big as bridges and small as restrooms in disrepair. Yellowstone’s backlog alone is $603 million, facing crumbling roads, buildings and wastewater systems. Congress has declined to provide funding needed for fixes that have lingered for more than a decade.

Another looming challenge lies in who comes to the parks. The average age of its visitors is as high as 63 years old at some sites, and the Park Service is unsure how to entice younger people away from cities and the Internet.

Climate change is making matters worse. Before attending the celebration at Yellowstone, Jewell scaled a summit at Glacier National Park in Montana and met with scientists to discuss how rising temperatures have caused Grinnell Glacier, the most accessible one in North America, to virtually disappear. Rising temperatures and and sea-level rise are grinding away at the Assateague Island National Seashore. A decrease in snow and rain has stunted the growth of vegetation in several parks, including Grand Canyon National Park in Arizona and Mojave National Preserve in California, leaving bighorn sheep with little to eat.

It is projected to be a banner year for the Park Service, with attendance topping 330 million for the first time — a 23 million increase over last year. The top national park draws are Great Smoky Mountain National Park in North Carolina and Tennessee, with 10.7 million visitors in 2015; Grand Canyon, with 5.5 million visitors; and Rocky Mountain National Park in Colorado and Yosemite National Park in California, both with about 4.15 million visitors.

The most stalwart park visitors are disappearing, though, because of aging and death. The question of how to draw more young people and minorities who were historically alienated from parks is unsolved. Jewell wants to diversify the visitors and ensure that “the service is relevant to all Americans and engaging the next generation,” according to an announcement of Thursday’s events.

The splendor of the parks is tough to oversell. Visiting national parks, Americans sometimes find themselves face to face with bison and within shouting distance of bears. They walk across earth charred by lava and watch it flow down cliffs into the Pacific Ocean.

There are also pulsing geysers, eye-popping views from cliffs, canyons the size of big cities, and rich animal diversity. Many of the millions of people who visit the Washington Monument, Lincoln Memorial and Martin Luther King Jr. National Historic Site are unaware that they’re all maintained by the Park Service.

More visitors add to the numbers of people who encounter problems. “Restroom facilities have been closed, trails have not been maintained because there’s no money so visitors can’t take hikes,” said Theresa Pierno, president and chief executive of the National Parks Conservation Association.

“Sometimes campgrounds and services are lacking,” all while more people are coming to parks, Pierno said. “It’s a very serious issue.”

For years, Congress has declined to increase the Park Service’s appropriation above about $3 billion. Republican members instead called on the Government Accountability Office to investigate whether the Park Service was collecting enough visitor fees and membership dues to address the problem on its own.

In a December report, the GAO concluded that Congress’s $3.1 billion appropriation over about a decade amounted to an 8 percent funding drop when adjusted for inflation. Lawmakers who called on the service to create a higher revenue stream overlooked one major obstacle: Congress. It virtually barred the agency from increasing rates and must pass a law to change that.

But even if the Park Service could increase fees, will there be enough visitors to pay them a few decades from now?

Many park visitors are older than 65, and at that age, entrance is free. The bulk of paying visitors are between 50 and 60, paving the way for a revenue crash in the next decade. The Park Service desperately needs new visitors as it moves into its new century.

That’s where Sangita Chari comes in. As the program manager for the Office of Relevancy, Diversity and Inclusion, her job is to increase the number of African American, Latino, Native American and Asian employees. The hope is that they, with the help of a five-year-old recruitment program for more diverse visitation, will become a beacon for minorities.

It’s been a hard slog, Chari said. “The issue we have with our minority employees is our turnover rates mirror our recruitment rates,” she said, meaning that they lose as many as they recruit. Traditionally, “it’s expected that to move up, you move from park to park,” Chari said, and postings in remote locations may make minorities feel particularly isolated.

“We also have a challenge retaining millennials,” Chari said. “Unless we stem our retention issues … build a more inclusive environment, we will continue to remain stable.”

At Yosemite, John Jackson, a park ranger who is black, said he goes out of his way to make members of underrepresented groups feel welcome when they show up at the park.

“If you like reading a book, I tell them you can sit by the river. It could be a good place to take a nap,” he said. “I let them know the park is an open space for many different activities. But don’t try to do too much. If you have one day or one hour, just do one thing.”

Jackson visited Yosemite while living in Los Angeles in 1978 and fell in love. He worked there off and on for a few years before joining the staff permanently eight years ago. “If I see a horse, I want to ride it. If I see water, I want to swim. If I see snow shoes, I want to use them,” he said.

Not everyone shares his sense of adventure. “People come here and say I’m scared of bears … how are they going to enjoy the place if they think there’s a bear around every corner?”

Fear, he said, can be overcome, but he said the Park Service as yet isn’t doing enough to lure people to its wide-open spaces for that to happen. He said the Park Service doesn’t do enough to tell the story of how people who weren’t white helped to build Yosemite and other parks.

Yosemite dwells too much on the contributions of John Muir, whose love for the Sierra Nevada led to the creation of the park in 1890. “We keep talking about him,” Jackson said. “If we spent more time talking about American Indian contributions, maybe we would get them. If we talked about African Americans, maybe we would get more. The Chinese built roads here.

“We know John Muir,” he said. “Multiple groups made this place famous historically, not just one group.”