Wildlife & Habitat
Riparian habitats – along rivers and sloughs – largely consist of woodlands dominated by willows, cottonwoods, and oaks. These habitats with their multi-layered structure provide nesting habitat for colonial-nesting waterbirds such as egrets, herons, and cormorants, as well as many species of raptors. The largest density of breeding songbirds are found in these areas. These habitats also serve as movement corridors for many wildlife in the Central Valley.
Wetlands comprise a quarter of all Complex lands. Wetlands include permanent marshes which contain water year-round and are dominated by robust water-loving vegetation such as cattail and bulrush. These dense stands of reed provide ideal nesting habitat for coots, grebes, blackbirds, bitterns, ibis, and marsh wrens. Seasonal marshes are the prevalent wetland type throughout the Complex. These seasonal wetlands contain water from early-autumn through spring. Dominant vegetation includes swamp timothy, smartweed, millet, dock, and sedges – all of which provide a valuable food source for wildlife. These seasonal wetlands attract large numbers of ducks, geese, shorebirds, and other waterbirds.
Uplands at the Complex comprise three-quarters of the landbase and include grasslands and croplands. Grasslands are the most common and are dominated by annual and perennial grass species. Many of the dominant grass species in the San Joaquin Valley are exotic and difficult to eliminate. Two common native perennial grasses, which the Complex encourages, are creeping wild rye and alkali sacaton. These uplands provide habitat for a host of herbivorous wildlife including elk, black-tailed deer, desert cottontail rabbits, black-tailed jackrabbits, and voles. Common songbirds include western meadowlarks, savannah sparrows, and horned larks.
Vernal pools are seasonally-flooded depressions in impermeable soils that hold rainwater until evaporation occurs. The pools are home to specialized plants and animals adapted to the alternating wet/dry regime. Aquatic insects and crustaceans that live in the pools are a rich food source for waterbirds. As the pools dry during late spring, concentric rings of colorful flowers grow in halos around pool edges. At this time many of the invertebrates retreat deep into the mud and wait for the next rainy season. In late fall and early winter, as the pools fill, the invertebrates emerge to serve an important role in the food chain. Most vernal pools on the Complex are found at the Kesterson and West Bear Creek Units of the San Luis NWR and the Arena Plains and Snobird Units of the Merced NWR.
The Merced NWR contains approximately 300 acres of cultivated corn and winter wheat crops and more than 500 acres of irrigated pasture for wildlife. Not only do these managed agricultural areas provide important sources of carbohydrates for the tens of thousands of arctic-nesting geese and Sandhill cranes that make Merced County their winter home, they also help ensure that the birds will have adequate nutrient stores to make the long migration north to their breeding grounds. Local farmers, under agreements with the Refuge, oversee the ground preparation, seeding, and irrigation of these croplands.