This barrier island on the Maryland and Virginia coast is a fragile sliver of sand and wilderness - and asphalt.

On some days, the sky belongs to the peregrine falcon and the great blue heron. When there is a brisk wind, it is crazy quilt of red, yellow and blue Mylar-clad kites.

On the beach, wild ponies, the descendants of Misty and Stormy, trot past amazed sunbathers listening to disco music on their portable radios.

On one campsite at the North Beach campground stands a blue nylon pup tent, while a few yards away a family eats breakfast in the airconditioned comfort of a 26-foot Winnebago.

Last year about 2 million tourists came to the 32-mile-long island, most of them during the 100-day summer season. They swam, beachcombed, fished, clamed, bicycled, hiked, sailed, canoed, rode horseback, hunted and water-skied.

Results of a 1977 survey showed that 40 percent of the visitors interviewed spent part of their time bird-watching. The hardiest souls backpacked to campsites on the dunes that are 15 miles beyond the last foot of asphalt.

This variety of activity was ordained by federal legislation in 1965 that created Assateague National Seashore. The legislation decreed that a road be built the length of the island. It also set aside 60 acres of the island's total 39,600-acre are for motels and other commercial development.

But in 1976 Congress scrapped both the road project and provisions for commercial development. The turnaround came after years of pressure from the environmental lobby, led by the Committee to Preserve Assateague Island.

But there was another crucial reason for the change - the gradual realization that man could not outwit nature. In the late 1960s, there were high hopes that through an ambitious "management" program. Assateague's shifting sands could be stabilized and the island converted into a playground for the 30 million people living within a 200-mile radius.

But as continuing research has shown, efforts to halt the island's retreat toward the mainland have only a temporary effect. Worse, they can actually increase the landscape's vulnerability to the winds and waves that are continually reshaping the island and moving it landward.

With these concerns in mind, the federal government is drawing up a master plan for Assateague's future. A number of proposals are being considered, ranging from putting a new lens in the old lighthouse to building a waterfowl museum. But the underlying question is: How much human traffic can Assateague bear without irreversible damage to its delicate environment?

In ordering a master plan, Congress, in its 1976 revision of the 1965 legislation, said the plan must provide "full protection . . . of the natural resources and natural ecosystems of the seashore."

What development to permit and how to fully protect the seashore have proved to be controversial issues in neighboring Worcester County, Md., and Accomac County, Va., which are becoming increasingly dependent on tourism for their sluggish economies.

Nowhere is the controversy more sharply drawn than in Chincoteague, Va., the island town that has the only bridge to southern Assateague, the area with the most attractions for tourists.

The master plan for Assateague will have a direct and major impact on Chincoteague - an implication that appears to have escaped none of its residents.

Chincoteague's early prosperity was built on seafood, primarily oysters. But as the local fishing industry declined, the oysters were replaced by tourists as an economic foundation. More than half of the 2 million tourists who visited Assateague last year came through the narrow streets of Chincoteague.

On a recent Friday night, when the weather was hot and muggy, the Redskins were on TV and "Jaws 2" was playing at the air-conditioned theater down the block, about 200 perspiring people gathered in the stuffy meeting hall of the Chincoteague Volunteer Firehouse to sound off about the master plan.

Depending on who was talking, Chincoteague is just beginning to benefit from its proximity to Assateague or is being threatened by an overwash of tourism more destructive than any hurricane. The prevailing sentiment at the meeting was that the plan, especially the proposal calling for a wildfowl museum on Assateague, would encourage more tourism than Chincoteague can absorb.

"In my mind, said Richard Vesely, who runs a real estate business in town and is an architect, "the main issue is what continuing development will do to the community of Chincoteague. We have a historic community that is being torn apart by the business of tourism."

But businessman Fred Talbert, who runs a construction business across the street from the firehouse and supports more attractions, said, "We've got a very restricted economy here. We've got seafood and tourism. Without tourists, a lot of people here wouldn't work. This is an economically depressed area."

While neighbors found themselves taking opposite sides on an important and touchy issue, a third faction - outsiders who own summer houses - also sounded off.

For the most part, they are professionals from the Baltimore-Washington area and, like many of the locals, they are opposed to the museum idea. But the locals have mixed feelings about outsiders who help carry their banner and even raise it higher.

Nathan Hill, a retired waterman who owns a mobile home park and three rental cottages, doesn't want the museum built on Assateague, but he said of the outsiders: "I have no alignment with the strangers. They're misinformed. They're too militant."

Ironically, while many residents of Chincoteague say their town is being threatened by too much tourism, Assateague has plenty of room to handle more visitors. The problem is not that there isn't enough beach, the most popular attraction, but that there isn't enough parking. When the 800 spaces are filled, as they usually are on weekends, visitors have to be turned away, even though there is room for them on the beach.

One alternative in the proposed master plan calls for more parking, but more asphalt might hasten the already heavy erosion in the area. Another alternative suggests bus or van transportation, but that has little support from the locals, who can't imagine waiting for a bus to take them across the channel to the island they consider part of their heritage.

The team drawing up the master plan (the National Park Service, Fish and Wildlife Service and State of Maryland are represented) has to contend with an array of constituencies that are often in conflict with each other and even divided internally.

Not surprisingly, the planners are trying to grope toward some middle ground.But whatever the final master plan permits in new development may be irrelevant to Assateague's future.

As J.C. Appel, manager of the wildlife refuge on Assateague, said at the hearing in Chincoteague: "More poeple are going to come . . . no matter what we do."