The scene was improbable: James W. Rouse, savior of decaying inner cities and dressed in a business suit, lying in a ditch near his planned town--and watching, of all things, the dance of courting woodcocks.

But there he was, about 10 years ago, with wildlife expert and longtime Howard County resident Al Geis at his side.

"Jim wasn't wearing the best clothes for a field trip," Geis recalled recently. "But it was great. He was like a kid with an ice cream cone."

Geis is a researcher for the U.S. Fish and Wildlife Service, an expert on the effects of urbanization on dozens of animal species across the state. When he isn't braving poison ivy patches for a glimpse of rare birds, Geis roams Patuxent River marshes, Carroll County forests and the deer trails of central Maryland to chart the ebb and flow of animal populations against suburban sprawl.

He is often troubled by what he sees, but no more so than when he contemplates the future of his own neighborhood near Columbia--nearly 2,000 acres of largely unspoiled forests and fields that are to be the site of up to 3,300 new homes.

The Howard Research and Development Corp., which Rouse established in the 1960s to develop Columbia, is now planning to build the 10th and final section of the "new town" on a triangle-shaped tract between Columbia and Clarksville, a small farming community in south central Howard. HRD's request to change Columbia's original zoning ordinance to allow apartments and town houses in some sections of the last "village" have fueled an increasingly bitter debate between developers and Columbia residents on one side, and county residents on the other.

Clarksville area residents say the 8,000 to 10,000 new Columbians will strain school resources, burden water and sewer lines, snarl traffic on the area's winding roads and could spoil the tranquil rural life many of them lead. Others, like Geis, whose street would bisect the new development, say the plan could have devastating effects on water quality and animal life as far away as the Chesapeake Bay.

HRD officials, meanwhile, said the village will be a showcase of the kind of planning that has made the 16-year-old development a model of its kind.

"Obviously, we'll do the best we can by the neighboring community," said Alton J. Scavo, the HRD vice president in charge of planning Columbia's final phase. "At the same time, though, our goal for the next 20 years is to make a profit with the village." HRD already has invested $200 million in planning and acquiring the land for the expansion.

Although the corporation has set aside a 1,000-acre swath along the Middle Patuxent River as open space excluded from development, completion of the final village could disrupt the fragile ecological balance that makes the area distinctive, environmentalists say.

With more than 140 bird species, large deer herds, other wildlife and 300-year-old oaks, the valley remains "extraordinarily rich, one of the state's most dynamic natural areas," said Geis, 53, an Illinois native who has lived near Trotter Road since 1960. Maryland officials have designated the valley as one of several areas of "critical environmental concern," he said.

"...What is unknown is the environmental impact, the kind of disruptions almost certain to come with development."

Housing construction would almost certainly disrupt the deer herds that now run between Clarksville and the Middle Patuxent, and could lead to further overgrazing in the valley, Geis said.

Also, treated sewage from new Columbia homes would probably tax the already overburdened Patuxent River--the natural pipeline that carries effluent from Howard County southeast to the Chesapeake Bay, he added.

In a meeting last week with Geis and others, Savo and another HRD planner said the villages low-density housing--two detached houses per acre over most of the site--would minimize the environmental impact. If Howard officials allow high-density housing--plans now call for such units over a small percentage of HRD's land--an extra 240 acres would be designated open space.

Like dozens of Clarksville residents who planned to form a citizens association tonight to fight HRD's requested zoning change, Geis plans to testify against the request at county hearings later this summer.

"I deeply regret what will happen to this area," he said. "One of the truths that is beginning to emerge . . . is that it will never be the same again."