Wildlife & Habitat
The Refuge is host to significant assemblages of birds, mammals, reptiles, amphibians, fish, insects, and plants; some of which, such as the California tiger salamander, the long-horned fairy shrimp, and San Joaquin kit fox, are endangered species. The Complex contains all the representative habitats found in the
northern San Joaquin Valley. All of the refuge units are located
adjacent to the San Joaquin River and its tributaries, and habitats are
highly variable, ranging from hydric (or wet) conditions to xeric (or
dry) conditions. The Complex and neighboring lands feature the largest
network of freshwater wetlands remaining in California.
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.