Lloyd Raleigh is bent double , trying to negotiate his way through a dense thicket of catbriar in the moist wetands of Brookside Farm. As thorns entangle his jacket, a soup of leaf mold and sphagnum moss sucks his boots deeper into the mud.

“I kind of like this spot,” he says. “It tells us a lot about the land.”

Brookside Farm, on Middle Road, offers one of the Vineyard’s most beloved vistas: lichen-dappled stone-walls, oxen grazing the fields that run down to the Tiasquam River and woodlands rising beyond to an uninterrupted, undulant horizon.

Through the generosity of the owners, the writer Wendy Gimbel and her husband, lawyer Doug Leibhafsky, that vista is protected by a conservation restriction, which is an owner’s voluntary agreement with a preservation organization to limit development of private land. Brookside’s 36 acres could have been subdivided for several dwellings. Instead, through a contract with the Vineyard Conservation Society and Sheriff’s Meadow Foundation, its important natural attributes and numerous habitats — pasture, meadow, wetlands and wooded uplands — will be spared from development. VCS monitors the restricted property and suggests ways that the conservation values of the property might be enhanced over time to better support rare species or achieve other goals.

This winter, VCS hired Lloyd Raleigh, who has a master’s degree in forest science from the Yale School of Forestry and Environmental Studies, and who worked for eight years on the Vineyard as ecologist for The Trustees of Reservations, to do a detailed environmental assessment of each of the 17 properties on which it holds conservation restrictions. That will establish a baseline, says Brendan O’Neill executive director of VCS. “We’ll be able to identify and track any changes, year by year, and make sure the intentions (of the owners) to protect the natural assets and conservation values of the land are met in perpetuity.”

While efforts over the last hundred years have resulted in protection from development for just over a third of the Vineyard’s almost 60,000 acres, only a quarter of the Island is actually built out to the limits of current zoning regulations. Almost 31 per cent is available for development — or conservation. Subdivision of now-vacant land could mean 10,000 more dwellings, “and each new structure contributes to fragmentation of habitat, more people, more cars, more septic systems adding nitrogen to our fragile ponds,” says Mr. O’Neill.

Conservation restrictions are a boon to preservation, he says, because they allow large areas to be protected at little cost. Few conservation groups have the ability to raise sufficient funds to buy threatened land outright, especially at inflated Vineyard prices, but voluntary agreements with preservation-minded property owners can achieve the same results. The owners also are usually able to realize a tax benefit by the gift of a conservation restriction.

On a bitterly cold morning in early March, when the lion hadn’t even begun to consider making way for the lamb, Mr. Raleigh set out across a crunchy layer of snow toward Davis Pond, which was still three-quarters iced-over, making a convenient basking platform for a resident otter.

Equipped with rugged hiking gear and a Garmin handheld GPS, he starts each survey by walking the property’s edges and locating any boundary markers. That can be easier said than done. On Brookside, things start straightforwardly enough, following the line of old stone wall that runs down from Middle Road. The only potential obstacles are the oxen, a formidable pair, of massive girth and impressively long, pointed horns.

Fortunately, they’re docile, and distracted; one grazing, the other rubbing his thick, russet coat on an upended car wash brush, considerately placed in the paddock as an ox-sized back scratcher.

But once Mr. Raleigh climbs the far wall of the paddock, dense wetland vegetation reclaims the land, and boots sink deep into mossy bogs. When he can find his footing under a clump of red maples, he selects a monitoring point where he takes pictures and makes notes of the diverse vegetation: sweet pepperbush, swamp azalea, and the first red and yellow curved sprout of skunk cabbage pushing through the ice, powered by its own internal chemical reaction to generate heat.

He is able to read the story of the forest by the kinds of trees he finds. A large tree with wide-spreading boughs signals that the area must have been cleared in the past, to give the lone “wolf tree” room to spread out unencumbered. “If there are no wolf trees,” he says, “that means it was probably always forest.” He explains that it can be difficult to date trees on the Island: harsh conditions mean even an unimpressively sized oak can be over 100 years old. Core samples, he says, reveal growth rings from some years that are less than a millimeter thick.

A little further along, Mr. Raleigh peers intently at a sample of excrement. “It’s a large scat,” he says. “Beetles. Berries. But no fish bones, so it’s not otter . . . Fascinating.”

Not all of VCS’s restrictions are on large, high-profile estates such as Brookside.

The Pollan family has given a restriction on a small but significant beach lot on Moshup Trail. On a warmer day in mid-March, Mr. Raleigh surveyed it in painstaking detail. VCS has worked hard to protect land along Moshup Trail. Its sea-level fens and stranded dunes, subtle topography and tiny plant communities are a habitat that is globally rare, making it “the most important unprotected habitat in New England,” according to Dr. Peter Dunwiddie, a moorland specialist with The Nature Conservancy. The area is one of the few where VCS has bought land outright to protect it, and for several years has been in a protracted legal fight with developers who want to carve roads through the fragile dunes to reach areas with disputed access. “At Moshup, we have a chance to conserve an unspoiled heathland habitat that has been almost completely eradicated globally because of road building and house development,” Mr. O’Neill says.

The Pollan lot is a small piece in the mosaic of protection. Without a conservation restriction, it could potentially have become a parking lot for beach access, but now, aside from a tiny area set aside for family use, it will remain unspoiled dunes.