Endangered Oasis In South Texas. Encroaching agriculture and developers threaten Lower Rio Grande Valley Refuge. CONSERVATION: WILDLIFE

THE lower Rio Grande Valley of Texas was once covered with a tangle of subtropical mesquite, grasses, vines, cattails, and Spanish moss. The lush zone was a haven for millions of migrating birds, as well as for other birds, snakes, coyotes, ocelots, and beavers that inhabited the area year-round. Today 95 percent of the valley's original habitat is gone, replaced by vast fields of citrus, sugar cane, and various vegetables, and increasingly by subdivisions, mobile-home parks, and commercial centers.

But amid the man-tailored environment stand 2,000 acres of original brushland on the banks of the Rio Grande. Set aside in 1943, the Santa Ana Wildlife Refuge is an oasis for the plants and creatures that make it home, but it is a threatened oasis.

Recently the Wilderness Society placed Santa Ana and the Lower Rio Grande Valley National Wildlife Refuge, the larger refuge Santa Ana is now part of, on a list of the nation's 10 most endangered wildlife refuges. In issuing its report, the Washington-based environmental organization said, ``Many of our 445 national wildlife refuges are refuges in name only.''

Stepped-up commercial activity on refuge land during the Reagan administration is one cause of the serious degradation of the nation's refuges, the report said. It also listed pesticide contamination from agricultural runoff and loss of adjacent habitat as contributing factors. Noting last year's reauthorization by Congress of the Endangered Species Act, the report stated, ``No matter how many species are put on the list and how many recovery plans are devised for them, they have no hope of surviving unless there are places for them to feed, breed, and rest.''

George T. Frampton Jr., president of the Wilderness Society, says the 10 refuges singled out as the most imperiled ``are just the tip of the iceberg.''

In naming the Lower Rio Grande Valley National Wildlife Refuge - a string of 60 parcels covering 36,000 acres over a 220-mile-long stretch of the valley - to its 10-most-endangered list, the society cited shrinking habitat, water contamination, and land development as the major threats to the refuge.

The society also recognized its importance at the convergence of North America's two major flyways.

On a sun-dappled afternoon at Santa Ana recently, least grebes, coots, and other waterfowl ply a reed-ringed pond while coyotes screech and howl like a field of schoolchildren on recess. But once the racket dies down, another noise breaks the calm, indicating the threat this refuge faces: A tractor can be heard working a nearby field of sugar cane.

``This is a 2,000-acre island surrounded by other uses,'' Nita Fuller, manager of the Santa Ana refuge, says. ``If you're serious about maintaining wildlife, you know you just can't do it on this amount of land.''

It was that understanding that led to establishment of the Lower Rio Grande Refuge in 1978. Plans call for the refuge to comprise 120,000 acres, but to date only one-quarter of that has been purchased.

``The key will be how quickly it's done,'' Ms. Fuller says. ``We can't make [completing the Lower Rio Grande refuge] a 20-year project.'' She notes that many birds, including the rose-throated becard and the Altamira oriole, are much less common sights now, while as few as four ocelots still inhabit Santa Ana.

Fuller agrees with a Wilderness Society criticism that more studies must be done on the impact of agricultural runoff and other contaminants on the refuge's wildlife, but she says that adding habitat is the top priority. One piece of agricultural land next to Santa Ana was recently purchased and will soon be planted with trees and grasses to yield the same brushy habitat.

Cooperation is growing between refuge supporters and farmers, Fuller says. Circle-burns of sugar-cane fields have been largely replaced by two-sided burns, she notes, to allow animals an escape route.

But the pressures wrought by the valley's expanding human population appear more difficult to mitigate. A dark cloud hovering over the Lower Rio Grande Valley Refuge is the proposed Playa del Rio development, a massive resort and commercial project at the mouth of the Rio Grande that would be the largest private wetland development ever undertaken in the United States.

The Fish and Wildlife Service was emphatic in its recommendation that the project be denied, saying it would ``have an unacceptable adverse impact on wetlands, shellfish beds, fishery areas, wildlife habitat, and recreation areas.'' The proposal is now before the Army Corps of Engineers.