AYER'S Main Street is bustling now, but businesses in this rural town 30 miles northwest of Boston are preparing for a bleak future - the possible loss of up to 70 percent of their customers.Home of Fort Devens, one of 34 United States military installations selected by a special base-closing commission to be shut down, this town of about 6,500 residents and surrounding communities will be forced to cope with the loss of a major economic center which provides jobs for 9,000 military and civilian employees. The closure will also significantly affect the area's schools; 70 percent of Ayer's students come from the base. "Take away that military presence and we would not have a viable school system on our own," says Timothy Higgins, Ayer's executive secretary. The demise of Fort Devens, New England's only permanent Army installation, is expected to impact this area more severely than many other communities where bases are slated to close, says Jonathan Gill, a policy analyst at Business Executives for National Security Education Fund in Washington, D.C. "Each base is unique, and it impacts the regional economies in remarkably different ways. In some areas it's not going to make that much difference; in other areas it could make it a lot worse," he says. This area will be particularly hard hit: It is a rural community whose businesses and schools depend on the base - thousands of military retirees rely on its services. In addition, the state's poor economy exacerbates the problem. Closure of Fort Devens or any of the other bases is not final yet - President Bush must approve or reject the Defense Closure and Realignment Commission's package by July 15. If he approves it, it would then go to Congress, which has 45 days to approve or veto the list. If approved, the base closings would shrink the United States military by 25 percent in the next five years. Ayer residents, however, don't hold out much hope that the decision will be overturned. And although closure of the facility would be gradual over the next few years, the town realizes it must start now to deal with the situation. While specific plans will be decided once the Army gives a time frame for the closing, local officials have announced their intention to create an economic development task force to consider civilian uses for the base. Some ideas include bringing in an industry or a college. But making the transition from a military facility to one used by civilians often takes three to five years, with complete reuse often taking two decades, Mr. Gill says. "The hardest time is the three to five years after a closure." Edward Kelley, owner of The Studio Shop, a store that sells cards and gifts, says part of his strategy to keep business going for that interim period will be to work more hours and diversify products. "It's almost like I have to start all over again," he says. Neville Markham, superintendent of Ayer's public schools, says the expected loss of two-thirds of his student body means a drastic change in the size and shape of the school system. "It obviously will result in the downsizing of the school department," he says. Another complication Ayer and many other communities have to grapple with is toxic waste. Fort Devens has a number of hazardous waste sites scattered around the base. "Environmental problems and hazardous waste contamination is a major issue. It can slow down the reuse of a base significantly.... Some communities are potentially looking at 10 to 12 years for that cleanup," Gill says. Current Environmental Protection Agency regulations forbid reuse of bases until all areas are cleaned. Still, Ayer residents plan to band together and insist that the area has a lot of potential. Because the town is near many high-tech companies along Route 128, residents hope to attract industries and cite low taxes as one selling point.