JOSHUA TREE, Calif. — The government shutdown has left America’s national parks largely unsupervised. No one is at the gate. No one is collecting a fee. The visitor centers are closed. There are some law enforcement and emergency personnel on site, but certainly nothing as standard as a park ranger who can answer a question.

People are streaming into the parks, enjoying the free access, but they’re finding trash cans overflowing and restrooms locked. Vault toilets are not serviced, and there’s hardly a flush toilet to be found anywhere. If nature calls — well, the woods are over that way.

At Joshua Tree National Park, in particular, conditions are deteriorating.

“Once those port-a-potties fill up, there’s no amount of cleaning that will save them,” said Sabra Purdy, who along with her husband, Seth, owns the rock-climbing guide service Cliffhanger Guides in the town of Joshua Tree. “At that point, I think I’m going to have to tap out.”

The 40-year-old Purdy is among dozens of volunteers who have been collecting garbage, cleaning bathrooms and generally keeping an eye on the park. Local business owners and park supporters are donating toiletries and cleaning supplies.

“People are doing it because we love this place and we know how trashed it’ll get if we don’t,” she said.

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One of the most dramatic repercussions of the shutdown will arrive Wednesday, when the Smithsonian Institution, having depleted temporary funding, will close all of its museums and the National Zoo.

Some parks across the country have remained partially operational with state funding. Contingency plans adopted by the National Park Service last year have allowed many national parks to remain accessible, but without staffing. The result is that “closed” parks are essentially wide open. In some cases, that gives them a Wild West vibe.

The Park Service runs a constellation of parks, monuments, battlefields and historical sites. At Antietam National Battlefield in Maryland, the visitor center has a notice on the front door advising people “to use extreme caution if choosing to enter” park property. On the hallowed ground, where the federal Army of the Potomac collided with the rebel Army of Northern Virginia in the bloodiest day in American military history, the flagpole in front of the visitor center remains conspicuously bare.

The access road to Maryland’s Great Falls of the Potomac, part of C & O Canal National Historical Park, has been blocked. At midday Tuesday, hundreds of cars were lined up on nearby neighborhood roads and streets as people walked into the park, many taking advantage of the canal towpath, which remains unblocked.

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The situation is fluid: Rumors spread Monday that Joshua Tree would close the campgrounds, though the Park Service’s contingency plan indicates that campers will be allowed to stay.

Some advocates for the parks aren’t happy about this situation, fearing that visitors will do permanent damage to the parks and disrupt fragile ecosystems. They’d like to see the parks fully closed.

“The parks are supposed to be heritage sites for generation after generation. I would rather they close than be damaged,” said Joe De Luca, a sales associate at Nomad Ventures in the town of Joshua Tree.

During a government shutdown in 2013, Joshua Tree was closed to all visitors. The winter holiday season is a busy time here and important for local businesses, and some people are grateful that the park hasn’t blocked access this time, said Kenji Haroutunian, president of Friends of Joshua Tree, a nonprofit climbing organization.

But he’s not happy with the shutdown and the attendant loss of normal services in the huge park that he said is within a two-hour drive of 14 million people in Southern California.

“It’s hugely disappointing,” Haroutunian said. The border wall dispute, he said, “is not a priority that deserves to close down the government.”

At Big Bend National Park in Texas, George Cashman of Milwaukee said he was disappointed by the absence of park rangers. Last year, he said, he took his family — including four kids under the age of 10 — to Yellowstone, where the kids enjoyed the junior ranger program.

“There are no rangers to talk to and help the kids out. Last year, one of the rangers in Yellowstone let them take the temperature of one of the geysers. Those memories aren’t going to happen this year,” Cashman said.

Greg Henington, owner of Far Flung Outdoor Center in Terlingua, a town just outside the park, said he voted for President Trump but blames the president for the shutdown, which he says creates confusion and uncertainty for local businesses.

“If we are going to continue to use the federal government as a weapon for not getting what we want in the sandbox, then this is untenable for small business. We can’t make decisions, we lay off employees, we take cancellations,” he said.

Even more seriously affected is the Mexican town of Boquillas, which is just across the Rio Grande from the national park and has always enjoyed a stream of visitors this time of year. But the border crossing normally staffed by the U.S. Department of Homeland Security has been closed during the shutdown, and no one is coming or going across the river.

“It has been terrible. We were waiting for our best time of the year,” said Lilia Falcon, who owns a restaurant and a bed-and-breakfast in Boquillas. “It’s when we wait and stock up and save the money we make for the rest of the year. Christmas has not been good for us because of the shutdown, but what can we do? Nothing! Nobody will ever hear us.”

In Death Valley National Park, employees of the Oasis at Death Valley, a private resort inside the park, have joined other concessionaires in picking up trash and cleaning bathrooms, said Trey Matheu, general manager of the resort. “We’re in it for the long haul,” he vowed.

At Yellowstone National Park, access this time of year depends on grooming of snow-covered roads — but the National Park Service’s shutdown plan does not consider such activity to be essential. That means the private concessionaires have had to pool their money to pay for the grooming.

“If we don’t do this, we’re closed. That’s really not much of an option. Having said that, we’re doing this certainly in hope and anticipation that the shutdown does not go on indefinitely,” said Rick Hoeninghausen, director of sales and marketing for Xanterra Travel Collection, a concessionaire in Yellowstone and many other parks.

Back at Joshua Tree, Sabrina Krafton, 50, who traveled to the park from London for some New Year’s climbing, said she appreciates the hard work by volunteers who are collecting litter and donating toilet paper.

“I think they actually provide better roll s than the government,” Krafton said. “It’s better quality paper.”

Waters reported from Big Bend National Park; Achenbach reported from Washington. Emily Wax-Thibodeaux contributed to this report.