An 8,800-acre swath of fragile desert wilderness in the Coachella Valley — which helps to sustain palm oases, sand dunes and the endangered desert tortoise — will be preserved under a $26-million land deal finalized last week by the Nature Conservancy.

The acquisition of the Joshua Hills increases the Coachella Valley Preserve by nearly 50%. It also ensures a sand source for the dunes below and forms a link for bighorn sheep, bobcats and kit foxes traveling from Joshua Tree National Park to the valley.

"It's probably the most important Nature Conservancy has made during the past few years," said Julie Benson, spokesperson for the conservancy, based in Arlington, Va. "The Joshua Hills property has been a missing piece."

Six state, local and private agencies pieced together funding for the land, which stretches across the Coachella Valley north of Interstate 10, near Desert Hot Springs, to the Indio Hills. The California Department of Fish and Game, California State Parks and two local agencies will manage the land, which includes dramatic desert vistas, rocky hillsides and sandy plains.

The Nature Conservancy established the Coachella Valley Preserve in 1984 and soon added more parcels, bringing its total to 20,000 acres. But it did not buy Joshua Hills, assuming that the stark, remote alluvial fan that slopes down from the Little San Bernardino Mountains would never be developed.

In 1996, however, local developers proposed a city with 7,000 residences, 12 golf courses, a university, hotels, shops and restaurants. Their vision of a high-tech business park and luxury community clashed with the aims of environmentalists who saw the parcel as a crucial corridor in the shrinking desert wilderness.

In summer 2003, developer Richard Oliphant shelved plans for the city in the face of intense opposition from environmental groups and county officials.

The Nature Conservancy stepped in to buy the land from Cathton Holdings Ltd. of Canada, but the deal stalled last fall when the state Department of General Services rejected a $26-million appraisal. That decision blocked a contribution of $5.1 million from the state Wildlife Conservation Board, leaving the Nature Conservancy short of funds for the purchase.

The Wildlife Conservation Board challenged General Services' decision and ordered a more detailed appraisal, which found that the land was actually worth $28.4 million, slightly more than the sale price, said John Donnelly, assistant executive director of the Wildlife Conservation Board. However, bound by their earlier agreement, the owners sold it for $26 million.

General Services approved the second appraisal, clearing the way for the sale. The Nature Conservancy closed escrow on the final pieces of land last week, filling in what had been a patchwork of parkland and nature preserves.

"Years ago it was enough to buy large reserves," said Dave Van Cleve, regional director for the Nature Conservancy. "Now we recognize that we need connections between them."

California State Parks is planning a primitive campground in the crook of these rocky hills, which afford a view of the Santa Rosa Mountains and hint at the desert's historic solitude. To the east, palm oases in the Coachella Valley Preserve dot the San Andreas fault, providing shade and water to desert pupfish and Neotropical songbirds. Eventually, the area also will offer shelter to campers at a full-service campground planned by the state park service.

Cameron Barrows, director of the Coachella Valley Preserve, toured the area last week. A 12-foot-deep channel, winding down the plain, testifies to the force of flash floods that rip through the mountains, washing sand into five square miles of rippled dunes below.

The dunes are home to various rare and endangered plants and creatures, including the tiny Palm Springs pocket mouse, the Coachella Valley milk vetch and the Coachella Valley fringe-toed lizard, whose tracks crisscross the sand.

As he stood in the dunes, Barrows examined the tracks and prodded the sand with his fingers. To his delight, a lizard emerged from its burrow and skittered out of sight.

Without new sand sweeping down from the Joshua Hills, the dunes and their inhabitants would not survive.

"Sand dunes are such a difficult place to live that, once you've evolved to live there, you can't live anywhere else," he said.