A crowd of about 120 year-round and seasonal residents sat before a large screen at the new agricultural hall in West Tisbury on Wednesday to witness an eye-opening computer simulation which chronicled the history of development on the Vineyard over the past 400 years.

On the screen was a detailed map of the Island from the late 1600s, the earliest years of European colonization. Small dots showed which lots had been developed. At first, the dots were sparse and spread out evenly across the Island. But as the simulation moved forward through the 17th, 18th and 19th centuries, the red dots began to resemble chicken pox. Development through the 1960s closely mirrored the year-round population, and the rate of growth by all accounts was gradual and sustainable.

But when the simulation hit the early 1970s, the dots suddenly resembled a quickly-spreading rash.

By the 1980s, the rash covered large portions of Edgartown, Oak Bluffs and Vineyard Haven and also had spread to formerly unblemished portions of open space in the up-Island towns. The simulation then ventured into the future. Using projected development rates to 2030, then 2050 and then 2083 - the Island finally looked more like a New Jersey suburb than a rural farming community.

The effect on the standing room-only crowd was audible. One woman whispered to her friend in a back row: "The Island can't take much more of this."

Blunt concern about the Island's future in the face of continued development was the prevailing sentiment on Wednesday, the fifth and final meeting in a series of public forums to discuss the Island Plan, an ambitious planning initiative that will attempt to chart the Island's course for the coming decades.

The plan will seek to find a way for the Vineyard to manage its economy and resources in a manner that better sustains its environment, character and quality of life. Architects of the plan have broken into six groups, each of which has focused on specific planning concerns like water resources, housing, natural environment, livelihood, commerce, energy and waste and growth and development - the subject of this week's forum.

Wednesday's forum reached high water marks in both attendance and emotion.

"People say it is too late to save the Vineyard, but I don't agree with that. I think we can save the Island, and I think it is absolutely worth saving," said James Athearn, a member of the commission and chairman of the Island Plan. "We need to think about how many people can live here before we can no longer protect this small patch of land that is our home," he said.

Tom Chase of The Nature Conservancy said continued development will threaten the Island's fragile ecosystem, alter the natural character of the Island and destroy the habitat of native species. He said the Island is dangerously close to the threshold of minimal viability, a sort of point of no return.

"It's not just a matter of protecting the few remaining scraps of open space, it's asking why the ponds are being polluted, it's about stopping habitat fragmentation where parcels become too small for native plants and animals to survive," he said. Mr. Chase promoted the concept of undevelopment, where a town or other entity buys the development rights to a property while the owner is still alive.

Henry Stephenson, an architect and member of the Tisbury planning board, said town planners have already allowed opportunities for smart growth to slip away. He showed a picture of a pristine ocean view on Main street in Vineyard Haven that has been spoiled by a large three-story home under construction. He also showed several homes that were either too large or out of character with their neighborhood surroundings.

"These homes and structures are technically allowed under current zoning, although they clearly contribute nothing to their environment. New England is famous for structures that fit in with their surroundings . . . but I fear here on the Vineyard we are not doing nearly enough to uphold that tradition," he said.

Using a computer program, Mr. Stephenson showed images of what several properties would like with better planning: a large building on State Road Vineyard Haven shielded by trees, a bank on Vineyard Haven Road with shrubs and large oaks shielding it from view.

The audience applauded when he showed what downtown Vineyard Haven might look like without overhead power lines. "Believe me it's easy to do on Photoshop - its a little harder to do in real life," he said.

Kurt Gaertner, director of land use policy for the state Executive Office of Energy and Environmental Affairs, suggested the Island change its zoning to allow for cluster development and smart growth.

Some said the biggest threat is from the oversized homes being built on the Island.

"You look at these homes some of these seasonal residents build here and ask yourself if these people really care about the Island's environment or the impact of [that home] on the surroundings," said one man.

West Tisbury resident William Stewart expanded on the theme.

"We must try to change this mentality that assumes privilege automatically trumps the interest of the community as a whole," he said.