Future limits on the number of people who can enter Yosemite National Park are inevitable as the park grapples with a growing crush of visitors and increasing pollution, a park official said at a symposium in Concord, Calif.

Century-old Yosemite Park, one of America's natural jewels, is faced with choices that could have an effect on other national parks, participants in Monday's symposium said."Yosemite has always been the crucible of experimentation and evolution," said John Reynolds, assistant director for the National Park Service. "Very often, what happens first in Yosemite becomes the eventual norm everywhere."

The park's future is a focus of the weeklong symposium, sponsored by Yosemite officials and the non-profit Natural Areas Association.

Yosemite and other parks are undergoing dramatic changes as they shift from being mere keepers of scenic or scientific wonders to active guardians of natural resources, such as plants and animals, he said.

"The change is slow and painful," Reynolds said. "Even today, integrated, sustainable efforts to inventory natural-area resources are few and far between."

He said the park has lacked consistent planning and management over the past decade.

"The sustained, unwavering direction in the Everglades stands in counterpoint to the seeming lack of consistency of purpose actions over the past decade in Yosemite Valley," he said.

Lack of movement on the park's master plan, which calls for removing many buildings from crowded Yosemite Valley, has disillusioned park supporters and delayed action on Yosemite's biggest problem - congestion - Reynolds said.

Last year, 3.4 million people visited the park.

Acknowledging that "some kinds of limits on visitation are inevitable," Yosemite official Len McKenzie said, "We do recognize that park resources are finite."

McKenzie said the greatest threats to the park were external. He cited air pollution from the San Joaquin Valley, evidence of acid rain, effects of the shrinking ozone layer and a global warming trend.

At the same time, there are bright spots, McKenzie said. About 45,000 seedlings of native vegetation have been planted, and a million more plantings are planned. Bighorn sheep and peregrine falcons have made a comeback, and officials hope to restore the Merced River to its free-flowing state.