California’s Great Central Valley once supported a diverse array of perennial bunchgrass ecosystems including prairies, oak-grass savannas, desert grasslands, as well as a mosaic of riparian woodlands, freshwater marshes, and vernal pools. In its original state, it comprised one of the most diverse, productive, and distinctive grasslands in temperate North America.
Perennial grasses that were adapted to cool-season growth dominated the habitats. Stipa pulchra was particularly important, although Stipa cerua, Elymus spp., Poa scabrella, Aristida spp., Koeleria cristata, Muhlenbergia rigins, and Melica imperfecta occurred in varying proportions (Sims 1988). Most growth occurred in the late spring after winter rains and the onset of warmer and sunnier days. Interspersed among the bunchgrasses were a rich array of annual and perennial grasses and forbs, the latter creating extraordinary flowering displays during certain years. Some extensive mass flowerings of the California poppy (Eschscholtzia californica), lupines (Lupinus spp.), and Purple Owl Clover (Orthocarpus purpurascens) still occasionally occur in several areas, and are now best known from Antelope Valley in the Tehachapi foothills (Schoenherr 1992).
Native grasslands supported several herbivores including pronghorn antelope (Antilocarpa americana), elk (including a valley subspecies, the Tule Elk, (Cervus elaphus nannodes), mule deer (Odocoileus hemionus), California ground squirrels, gophers, mice, hare, rabbits, and kangaroo rats. Several rodents are endemics or near-endemics to southern valley habitats including the Fresno kangaroo rat (Dipodomys nitratoides exilis), Tipton kangaroo rat (Dipodomys nitratoides nitratoides), San Joaquin pocket mouse (Perognathus inornatus), and giant kangaroo rat (Dipodomys ingens). Predators once included grizzly bear, gray wolf, coyote, mountain lion, ringtail, bobcat, and the San Joaquin Valley kit fox (Vulpes velox), another southern valley-foothill endemic.
The valley and deltas once supported enormous populations of wintering waterfowl in extensive freshwater marshes. Riparian woodlands acted as important migratory pathways and breeding areas for many neotropical migratory birds. Three species of bird are largely endemic to the valley, surrounding foothills, and portions of the southern coast ranges, namely, the Yellow-billed magpie (Pica nuttalli), the tri-colored blackbird (Agelaius tricolor), and Nuttall’s woodpecker (Picoides nuttalii).
The valley contains a number of reptile and amphibian species including several endemic or near-endemic species or subspecies such as the San Joaquin whipsnake (Masticophis flagellum ruddocki), the blunt-nosed leopard lizard (Gambelia sila), Gilbert’s skink (Eumeces gilberti) and the giant aquatic garter snake (Thamnophis couchii).
Although many endemic plant species are recognized, especially those associated with vernal pools (e.g., solano grass, Tuctoria mucronata). Various invertebrates are known to be restricted to valley habitats. These include the delta green ground beetle (Elaphrus viridis) known only from a single vernal pool site, and the valley elderberry longhorn beetle (Desmocerus californicus dimorphus) found only in riparian woodlands of three counties.
Vernal pool communities occur throughout the valley in seasonally flooded depressions. Several types are recognized including valley pools in basin areas which are typically alkaline or saline, terrace pools on ancient flood terraces of higher ground, and pools on volcanic soils. Vernal pool vegetation is ancient and unique with many habitat and local endemic species (Holland and Jain 1995). During wet springs, the rims of the pools are encircled by flowers that change in composition as the water recedes. Several aquatic invertebrates are restricted to these unique habitats including a species of fairy shrimp and tadpole shrimp.
Riparian forests once bordered many of the valley’s major rivers and their tributaries. Willows, Western Sycamore, Box elder, Fremont Cottonwood, and the Valley Oak (Quercus lobata) were dominant tree species. Some riparian forests and associated woodlands were up to 30 km wide along the lower reaches of the San Joaquin and Sacramento Rivers. These are unusual forests for California because the trees are deciduous in winter, perhaps because of the cool, foggy conditions in many parts of the valley (Barbour et al. 1993). The California Hibiscus (Hibiscus californicus) and the Valley Elderberry Longhorn Beetle (Desmocerus californicus dimorphus) are restricted to limited areas of remaining woodlands. Many neotropical migratory birds use these forests for dispersal pathways or breeding habitat. The western yellow-billed cuckoo (Coccyzus americanus) and the Yellow-breasted Chat (Icteria virens), as well as several other riparian specialists, have all declined significantly in California due to loss of riparian forests.
The extensive rivers and lakes of the Central Valley supported vast freshwater marshlands dominated by rushes, bulrushes (tules), sedges, cattails, willows, and floating plants. The Delta and several large lakes such as Tulare, Buena Vista, and Kern were bordered by vast marshes which, in turn, supported huge populations of fish, waterfowl, shorebirds, and blackbirds. Tulare Lake was the largest lake west of the Mississippi River (Schoenherr 1992). The native fish fauna was continentally distinctive with four separate Chinook salmon runs and a number of endemic species (and higher taxa) such as the Tule perch, delta smelt, Sacramento blackfish, Sacramento splittail, Sacramento perch, and the Sacramento sucker (P. Moyle, pers. comm.).
Habitat Loss and Degradation
Virtually all Central Valley habitats have been altered. Introduced annual grasses now dominate grassland habitats. In most habitats native taxa comprise less than 1 percent of the standing grassland crop. Agricultural development, urban expansion, alteration of hydrologic regimes and channelization, grazing by domestic livestock, fires, and introduced plants and animals have all contributed to the pervasive destruction of native habitats.
An estimated 11,310 km2 (2.8 million acres) of vernal pools have been destroyed, over 66 percent, with the most intact pools left on the higher terraces. Agriculture, conversion to pastureland, water diversion and channelization, and draining have all taken their toll on these unique habitats. The USFWS has just begun an ecosystem recovery plan for over 40 species dependent upon vernal pools (B. Holland, pers. comm. 1996).
Out of 416 km2 of remaining riparian woodland, only about 40 km2 or 1 percent of original riparian woodlands can be considered intact, down from an estimated 4,000 km2 (Schoenherr 1992). Channelization, dams, clearing for pasture, flood control, alien plants, overgrazing by domestic livestock, fires, and logging have all taken their toll.
Intensive agricultural development has left few freshwater marshlands (less than 6 percent of their original extent), and those that are left are generally degraded and heavily managed for duck production, water impoundments, or runoff and effluent storage. Dams, channelization of rivers, and pollution continue to threaten the unusual and productive freshwater biodiversity of the region.
Remaining Blocks of Intact Habitat
Scattered large vernal pool sites are functionally intact including vine plains and Jepson Prairie. The largest blocks of desert grassland and scrub are owned by oil companies in the southern portion of the valley. Small patches of riparian forests, woodlands, and marshlands are scattered throughout the valley, especially along the Sacramento River and some of its tributaries. The Kaweah Oaks Preserve near Visalia and the Creighton Ranch near Corcoran conserve remnant oak savanna (Johnston 1994). The Nature Conservancy’s Cosumnes River Preserve between Stockton and Sacramento conserves one of the best remaining examples of valley oak riparian woodlands. A few notable remnant freshwater marsh sites include the Creighton Ranch Reserve, a relict of Tulare Lake, Gray Lodge, and Butte Sink in the northern valley, portions of the delta, and various managed National Wildlife Refuges scattered throughout the Valley. The huge Carrizo Plain natural area (3,180,000 acres) located just west of the valley proper, but representative of the San Joaquin Valley, embraces extensive stands of saltbush scrub, desert grassland, alkali scrub, and wetlands.
Degree of Fragmentation
Remaining patches of relatively undisturbed native habitats are all severely fragmented and isolated. Loss of habitat linkages are likely to be most significant for species that rely on contiguous riparian woodlands for dispersal corridors, such as migratory warblers, cuckoos, and reptiles and amphibians. Populations of threatened mammals and reptiles in the southern desert grasslands are also effected by fragmentation. The foothill areas are more intact than the valley floor, but fragmentation has altered them as well.
Degree of Protection
A number of important vernal pool sites have been protected within small reserves, such as the Nature Conservancy’s Pixley Reserve in Tulare County, Jepson Prairie Reserve near the Delta, and Vine Plains Preserve near Chico. Several of the larger blocks of desert grasslands in the southern portion of the valley are privately owned by oil companies who work with Federal and State agencies and conservation organizations to manage the areas for biodiversity conservation. Consumnes River Preserve protects some of the last blocks of riparian woodland, marshland, and vernal pools in the valley. A system of managed wetlands owned by private landholders and State and Federal agencies protects large populations of wintering waterfowl of the Pacific Flyway, as well as associated marshland species such as the Giant garter snake.
Types and Severity of Threats
Remaining native habitats are threatened by continuing habitat clearance for agriculture, alteration of hydrologic regimes, dams, channelization, fires, grazing by domestic livestock, and alien species. Over 526 species of introduced plant species are known from California, and many of these occur in the annual grasslands of the Central Valley (Heady 1995). Several Federally endangered species occur in the valley including the San Joaquin kit fox, the Blunt-nosed lizard, the Delta Green Ground Beetle (Elaphrus viridis), the Valley Elderberry Longhorn Beetle (Desmocerus californicus dimorphus), the Delta smelt, and a number of vernal pool plant species. Populations of the Tri-colored blackbird are also declining precipitously in recent years, due to loss of breeding habitat and changes in agricultural practices. Many amphibian species show dramatic declines associated with habitat loss and introduced predators (Fisher and Shaffer 1996). Extensive riparian forests and woodlands have been destroyed by decades of tree-cutting, channelization, and flood-control projects. Salinization, toxic runoff, and erosion from ecologically-unsound agricultural practices increasingly degrade habitats.
Suite of Priority Activities to Enhance Biodiversity Conservation
Strict protection and restoration of the last remnants of native communities is needed for all valley habitats.
•Marshlands and riparian woodlands need extensive restoration. Introduced annual grasses dominate grassland and savanna communities, intensive management is required for possible restoration of the original bunchgrass prairies.
•Linkages between lowland valley habitats and foothill habitats needs to be protected and restored. The Nature Conservancy has identified 385 priority sites in the northern half of the valley.
•Bureau of Land Management
•California Department of Fish and Game
•California Native Grass Association
•The California Native Plant Society
•California Oak Foundation
•The Nature Conservancy
•The Nature Conservancy
•The Nature Conservancy - California Region
•Sacramento River Preservation Trust
•The Sierra Club
Relationship to other classification schemes
The Central Valley ecoregion was originally based on Omernik’s Central California Valley ecoregion (7), although boundaries in some areas have been revised by Bob Holland, an expert on vernal pools and valley vegetation. This ecoregion generally corresponds to Bailey’s Great Valley Section (262A), and Küchler’s California prairie, Blue Oak-Digger Pine, and riparian forest vegetation classes in the region.
Prepared by: David Olson and Robin Cox.