MAMMOTH HOT SPRINGS, WYOMING - A record number of visitors flocked to Yellowstone National Park last year despite fewer hotel rooms and campsites being available because of the coronavirus pandemic and construction projects.

About 4.86 million visits were tallied in 2021, breaking the prior record set in 2016. It's a million more people than visited in 2020.

Known worldwide for its wolves, bears and other wildlife and thermal features such as the Old Faithful geyser, Yellowstone will mark its 150th anniversary in 2022. It straddles the borders of northwestern Wyoming, southern Montana and eastern Idaho.

Visits to national parks across the U.S. have been trending up in recent years. Others such as Utah's Zion National Park also set new visitor records in 2021 as tourism bounced back from the shutdowns imposed during the early days of the pandemic.

At Yellowstone, a rush of people from May through September last year strained employees and park services. It came as the park was understaffed through the summer because of worker housing caps and difficulty recruiting new employees, park officials have said.

There were also 20% fewer campsites and hotel rooms in 2021 compared to previous years. That meant hundreds of thousands of visitors left the park at night and would re-enter after staying elsewhere. Each time they entered the park counted as a separate visit.

Park officials said they are trying to find a way to differentiate between new visits and people who enter the park multiple times on the same trip.

Yellowstone's road corridors and parking lots can get crowded, but they make up less than one-tenth of 1% of its 8,903 square kilometers (3,400 square miles) - an area about 150 times the size of New York's Manhattan Island.

Most visitors stay within a half-mile of those roads, according to park officials. Park crowds drop sharply during winter when much of it is inaccessible except by snowmobile or skiing.