Known as the lower Rio Grande Valley, the lower four counties of Texas contain a documented 1,200 plants, 300 butterflies, and approximately 700 vertebrates, of which at least 520 are birds. The Lower Rio Grande Valley National Wildlife Refuge was established to protect this important biodiversity.
A species of great concern to the U.S. Fish and Wildlife Service is the ocelot, an endangered cat whose numbers have dwindled to fewer than 50 in the United States. The Mid-Delta Thorn Forest, an ecosystem that once covered much of the delta, is a hunting ground for this nocturnal species. Texas ebony, Granjeno and colima are but a few of the trees and shrubs that house an array of prey for the ocelot like small mammals and birds. Represented in remnant strips along fence rows, canals and ditch banks, the diminished thorn forest habitat forces the solitary ocelot to cross open fields and risk the dangers of vehicular traffic and predators.
Before dams and water control structures significantly reduced the flow of the Rio Grande, periodic floods cut shifting channels into the delta creating crescent-shaped oxbows, referred to in the Valley as “resacas.” Resacas, complemented by dense bottomland hardwood forest, are characteristic of the Mid-Valley Riparian Woodlands biotic community. This habitat is particularly favored by birds such as chachalacas and green jays, as well as another endangered and elusive cat, the jaguarundi. Draping Spanish moss and the rare Bailey’s ball moss, cling to cedar elm and Texas ebony. Found throughout the delta, brush-bordered resacas typical of this community attract many of the neotropical migrants and waterfowl that funnel through the Valley on their way to and from Central and South America.
On the edge of thewoodlands is a unique and severely diminished community of sabal palms. Originally spanning more than 40,000 acres, the remaining palms are restricted to about 50 acres. They make up one of the few strongholds for one of the rarest snakes in the U.S., the speckled racer. The southern yellow bat, a rare, year-round resident, roosts within the fronds of the remaining sabal palms that grow along the southernmost bend of the Rio Grande. The more than 900 species of beetles found within the small grove represents only a fraction of the insect community that aids plant pollination and other essential ecological functions.
The woodland potholes and basins ecosystem contains numerous freshwater playa lakes and three hypersaline lakes. Deep ruts, remnant tracks of oxcarts used by Spanish colonists to haul mined salt, can still be seen at the 530-acre salt lake, La Sal del Rey or “the King’s salt.” Black-necked stilts, black skimmers, and least and gull-billed terns can be found nesting along the shorelines of the salt lakes. The salty waters support brine shrimp and a few species of salt-tolerant water insects. Set in a low woodlands of honey mesquite, prickly pear, and lote bush, the freshwater potholes and playa lakes serve as favorite roosting and feeding areas for migrating geese, waterfowl, shorebirds, and sandhill cranes. Vegetated corridors will eventually connect this tract to the river.