Are the national parks in danger of exceeding their "carrying capacity?"

The concept, a population biologist's term often used to describe the Earth's capacity to support human life, has lately been applied to a more prosaic matter: the ability of the national parks to handle crowds without harming wildlife, vegetation or other qualities the parks were designed to protect.

In some respects, of course, the question of crowds and parks is as old as Yosemite and Yellowstone. But with population growth pushing the number of park visitors to record highs, conservationists are increasingly concerned that the National Park Service has failed to address the issue of how many are too many.

In a recent report on carrying capacity by the National Parks and Conservation Association (NPCA), researchers documented a variety of cases in which excessive crowds have jeopardized natural values, from soil erosion to raised heart rates in deer exposed to snowmobiles and cars.

The study also examined the effects of crowding on visitors' use and enjoyment of the parks -- a category known as the "social" carrying capacity.

"We felt the Park Service needed some tools," said Terri Martin, the association's Rocky Mountain regional director. "We have to deal with these growing numbers or we'll sacrifice the very reasons that people go to the parks."

The Park Service has begun its own studies of social carrying capacity, but officials contend that for now the parks are functioning tolerably well.

"My sense is it's not a major problem except during the heavy visitation periods of the year," said Richard Briceland, assistant to the director for science and technology.

Still, park visits last year reached 266 million, up from 195 million in 1979, and the numbers are expected to climb. "I take it very seriously and I think it's something we're going to have to spend a lot more time on," Briceland said. "We aren't getting more parks, but we are getting more visitors, so something has to be adjusted."

But conservationists contend that some parks already have reached their carrying capacity. Visitors to the magnificent sandstone arch at Utah's Rainbow Bridge National Monument, for example, discover not solitude but graffiti, trampled vegetation and engine noise from crowded tour boats.

Last month, the NPCA sharply criticized a Park Service proposal that would allow a 300 percent increase in the number of visitors to the monument, as well as paved trails and a 1,600-square-foot "congregating area." Deriding the proposal as a "how-many-people-can-you-fit-in-a-phone-booth" approach to crowd management, the association said "the NPS sets carrying capacity by determining how many people can fit on the Monument's walkways at one time."

The Park Service arrived at the carrying capacity, the association noted, by assuming that "people will walk in small groups of eight, two abreast, six feet apart, with 35 feet between groups."

"This is like sizing a roof beam for a snow load on your house," said Martin. "What it reflects is an attitude that Rainbow Bridge is a sideshow to be gawked at rather than a powerful place to be experienced."

Park Service officials said the Rainbow Bridge plans are not final and that the number of visitors could well be scaled back. "What we're trying to do is lessen the impact, not increase it," said John O. Lancaster, superintendent of the Glen Canyon National Recreation Area, which encompasses the monument. "It is a lot noisier around there than I would like."

But as the NPCA study makes clear, crowding is more than just a matter of noise and litter. Human effects on the national parks are subtle and varied -- and the subject of an extraordinary volume of scientific research. Consider, for example, that a pack horse destroys up to eight times as much vegetation in prairie grasslands as a hiker does, or that car tracks are 7.4 times more damaging than footprints.

Crowding also takes its toll on wildlife. Birds show evidence of reproductive declines. Small mammals can choke on litter. Feeding patterns of large animals can be disrupted by park traffic. There is even some evidence that animals are disturbed by brightly colored clothing.

The relationship between crowding and visitor enjoyment is harder to measure, but that hasn't deterred social scientists and park officials from trying. For example, the NPCA cited the case of Montana's Flathead River, where researchers suggested that in order to provide for a genuine wilderness adventure, access should be limited so that someone floating the river encounters no more than three other rafting parties in a single day.

Briceland suggested that preventing ecological damage is often a relatively simple matter, such as fencing off a trampled meadow at Yosemite or a patch of rare orchids in the Everglades. Keeping visitors happy is a more complex challenge, Briceland said, but he expressed confidence that "common sense" solutions will suffice.

For example, the Park Service is trying to persuade people to take advantage of the so-called shoulder seasons, avoiding the busy summer months. Another approach is to segregate visitors -- such as snowmobilers and cross-country skiers -- who might otherwise object to each other's presence.

"It's so easy to deal with -- you put them in different places," Briceland said. "My whole focus is not to put restrictions on anybody."