National parks across the United States have welcomed millions of visitors this summer to enjoy their fragile beauty, wildlife and ecosystems. But at least one-fifth of the country’s 433 parks have been significantly strained and understaffed because of steep cuts mandated by the Trump administration, according to internal government data obtained by The New York Times.

Staffing at the National Park Service had been steadily shrinking in the past decade because of tightened budgets. But it has lost 24 percent of its permanent employees since President Trump took office, according to data compiled by the nonprofit National Parks Conservation Association. Among those who left: more than 1,800 park workers who accepted the administration’s resignation initiatives, devised to quickly reduce the size of government.

Despite the reduced staffing, the Trump administration has directed all national parks to remain open. In July alone, more than 41 million people visited a park, down slightly from last year.

More than 90 national parks reported problems between April and the end of July stemming from departures, cuts and a hiring freeze, according to internal Interior Department data. Routine tasks like cleaning and stocking the bathrooms have gone undone. Fewer rangers have given tours and lectures. Visitor centers have reduced hours. And parks have lost millions of dollars because they are unable to staff entrances and collect visitor fees.

“Budget cuts and staff reductions have set our national parks on an unsustainable and dangerous path,” said Phil Francis, a 40-year veteran of the park service and the chair of the Coalition to Protect America’s National Parks, an advocacy group of current and former national park employees. “Some of the impacts of the staff cuts are visible to the public, but many are not yet. And all of this is only going to get worse.”