USGS Western Ecological Research Center

Home Who We Are Where We Are What We Do Products Search Outreach Jobs Contacts
Joshua tree burned (Nevada) --Photographer: Ben Young Landis, USGS

Bioregions of the Pacific Southwest

The Western Ecological Research Center has organized its research, monitoring, and information transfer programs in response to a growing biogeographical orientation in management activities among the Department of Interior and other natural resource management agencies and organizations. We have adopted a slightly modified version of the Forest Service's National Hierarchical Framework of Ecological Units (ECOMAP 1993), to define the ecoregions within the southwest United States where the majority of our work occurs.

Environmental problems are best addressed in the context of geographic areas defined by natural features rather than by political or administrative boundaries. More and more we recognize that the natural resources of an area do not exist in isolation. Instead, they interact so that use of one affects another. Ecological units provide basic information for natural resource planning and management. Ecological unit maps may be used for activities such as delineating ecosystems, assessing resources, conducting environmental analyses, establishing desired future conditions, and managing and monitoring natural resources. This geographic definition of ecological units will be used by federal agencies and others for a variety of broad planning and assessment efforts, including in the proposed National Interagency Ecoregion-Based Ecological Assessments.

The National Hierarchical Framework of Ecological Units is a regionalization, classification and mapping system for stratifying the earth into progressively smaller areas of increasingly uniform ecological potentials for use in ecosystem management. Ecological types are classified and ecological units are mapped based on associations of those biotic and environmental factors that directly affect or indirectly express energy, moisture, and nutrient gradients which regulate the structure and function of ecosystems. These factors include climate, physiography, water, soils, hydrology, and potential natural communities, and natural processes for cycling plant biomass and nutrients (e.g. succession, productivity, fire regimes).

There are 10 major "Bioregions", or what the National Framework calls "Provinces" within the areas that WERC's activities are focused. We have blended the classification of the National Framework with a classification adopted by the California Biodiversity Council to create this delineation of bioregions. Many of our future activities, such as symposia that address new research results and definition of new research initiatives, will be organized around the concept of bioregions.

  1. Southern California Coast
  2. Sonoran Desert
  3. Mojave Desert
  4. Central Valley
  5. Central California Coast
  6. Sierra Nevada
  7. Northern California Coast
  8. Southern Cascade Range / Modoc Plateau
  9. San Francisco Bay
  10. Intermountain Desert
 

Southern California Coast

The Southern California Coast Bioregion encompasses terrestrial and marine resources from Point Conception on the north to the border with Mexico. It extends from the outer edge of the continental shelf to the base of the Transverse and Peninsular Ranges. This bioregion coincides with the State of California's South Coast Bioregion. Forest Service (Avers and McNab 1994) ecological units in the bioregion include the Southern California Coast and Southern California Mountains and Valleys sections. These Sections comprise off-coast islands, narrow mountain ranges and broad fault blocks, as well as alluvial lowlands and coastal terraces. Elevation ranges from sea level to 3,500 meters. The aquatic resources include subtidal and intertidal marine and deep water habitats.

This region is affected by an enormous human population and its continued growth. In 1990, over 16 million people (56% of the State's population) lived in the region, mostly along the coast. The populations of many coastal cities have grown at rates ranging from of 20-50% over the last decade. The human population overlays a region of high natural biodiversity, which includes many unique natural community types. Coastal wetlands, vernal pools, riparian woodlands, grasslands, and coastal sage scrub all have been reduced to a small fraction of their former landcover, and remaining communities typically are fragmented and/or degraded. As a result, this region has the largest number of endangered and threatened species and species of special concern in the contiguous 48 states. Most of these species are associated with the habitats listed above.

Although 60 percent of the region is privately owned, there are major federal and state holdings in the region. National forests comprise 29 percent of the region, military reservations 2 percent, and Native American reservations another 2 percent. There are several DOI land holdings in the region as well. Three percent of the region is managed by the Bureau of Land Management (BLM) in scattered parcels, primarily in the inland valleys. The Fish and Wildlife Service (FWS) operates the Sweetwater Marsh, Tijuana Slough, Seal Beach, and San Diego National Wildlife Refuges (NWRs). These coastal wetland refuges provide important waterfowl and shorebird habitat and protect several endangered species. San Diego NWR presently includes about 3,000 acres of coastal sage scrub habitat, the first installment of a planned 30,000 acre refuge. In addition, the FWS manages the Bitter Creek and Hopper Mountain NWRs as traditional feeding and roosting habitat for the California condor. Channel Islands National Park, Santa Monica National Recreation Area, and Cabrillo National Monument are administered by the National Park Service (NPS). These park units protect important natural and cultural resources.  

Sonoran Desert

The Sonoran Desert Bioregion includes the Colorado Desert and Upper Sonoran Desert sections of California and Arizona, and a portion of the Chihuahuan Basin and Range Section in Arizona and New Mexico (Forest Service 1994). In California, this region coincides with the Sonoran Desert Bioregion. The region is bounded by the Salton Sea on the west, the Mojave Desert and Colorado Plateau on the north, the Mexican border on the south, and plains grasslands on the east. The region lies within the southern portion of the Basin and Range Physiographic Province. Its primary landforms include small mountain ranges, alluvial slopes known as bajadas, and intermountain basins. Dunes and desert plains are also present. Elevation ranges from below sea level at the Salton Sea to over 3,250 meters in the Pinaleño Mountains.

Most of this area is under federal government management. Few roads exist and much of the land is relatively inaccessible. Agencies with the greatest amount of land to manage include the Department of Defense, Bureau of Land Management (BLM), U. S. Fish and Wildlife Service, and National Park Service. In addition, a large portion of this region is under the management of several Native American tribes. DOI parcels range in size from a portion of the multimillion-hectare BLM California Conservation Area to the 6-hectare Tumacacori National Monument. National Park Service holdings include Saguaro National Park, Chiricahua, Casa Grande, Organ Pipe Cactus, and Tonto National Monuments, Fort Bowie National Historic Sites, Coronado National Memorial, and parts of Lake Mead National Recreation Area. In California, the Fish and Wildlife Service administers the Coachella Valley Preserve (which is co-administered with BLM and The Nature Conservancy), and the Salton Sea, Havasu and Cibola NWRs. In Arizona, besides Kofa and Cabeza Prieta, are Imperial, Buenos Aires, Bill Williams River, Leslie Canyon and San Bernardino NWRs.

A basic need in the region is the development of thorough biological inventories on federal lands. These inventories will provide the foundation for development of monitoring programs employing sound data management systems including GIS. Other areas of concern are: (1) the effect of invasive plants on native biomes, especially in relation to changing fire regimes on the Arizona Upland subdivision of the Sonoran Desert, (2) effects of introduced aquatic predators on native vertebrate species, and (3) the decline of native amphibians.  

Mojave Desert

This desert occurs in southern California, southern Nevada, northeastern Arizona, and southwestern Utah. The area comprises widely separated mountain ranges and broad desert plains and ranges in elevation from 100 meters below sea level in Death Valley National Park to over 3,000 meters in the mountains. Vegetation is predominately creosote bush, pinyon-juniper, and desert saltbush with occasional Joshua Tree woodlands. Higher elevations in Death Valley Desert National Wildlife Refuge and other mountain ranges within the Mojave support ponderosa pine, bristlecone pine, and white fir. A wide diversity of endemic plant species is found in the Mojave Desert. Some plant species that are substrate specialists are found exclusively on gypsum soils, seleniferous soils, and limestone outcroppings.

Mule deer and desert bighorn sheep are the dominant native ungulates in most of the desert ranges. Desert bighorn sheep have declined in number through recent historic times. Market hunting and poaching, habitat loss from mining and construction, loss of water sources, forage competition, and the introduction of diseases from domestic livestock have been the main sources of disruption. Transplantation programs have been largely successful in Arizona and Nevada, but less so in California.

Cities in the Mojave Desert such as Las Vegas, Palmdale, California and St. George, Utah are some of the fastest growing municipalities in the country. The desert provides for urban growth and recreational needs for a burgeoning human population, but is also critical habitat for a number of sensitive species. Scientists at the Las Vegas, Box Springs, Canyon Crest, and St. George field stations are active in providing scientific information in support of the development of Habitat Conservation Plans designed to balance economic growth and recreation with the need for protection of desert habitats. BRD scientists have provided technical information about the ecology of organisms and their habitats to decision makers. Habitat Conservation Plans are developed with the cooperation of several entities including county supervisors, the U.S. Fish and Wildlife Service, state wildlife agencies, the Bureau of Land Management, Stockmen's Associations, recreational organizations, universities, city governments, and others.

The largest single DOI landholder is the Bureau of Land Management, which administers over 5.3 million hectares, primarily in the California Desert Conservation Area. Death Valley and Joshua Tree National Parks, Mojave National Preserve, and Lake Mead National Recreation Area are also large, comprising a total of more than 5 million hectares. Several refuges are administered by the Fish and Wildlife Service in this region as well. These include the Ash Meadows, Moapa Valley, and Pahranagat National Wildlife Refuges and the 600,000-hectare Desert National Wildlife Range in Nevada. Numerous military installations are found in the area including the Chocolate Mountains Aerial Gunnery Range, the Naval Weapons Center at China Lake, Fort Irwin Army Training Center, the Marine Corps Air Ground Combat Center at Twenty-nine Palms, Edwards Air Force Base, and Nellis Air Force Base. The area also contains a significant portion of the Nevada Test Site.  

Central Valley

The Central Valley Bioregion generally coincides with the Sacramento and San Joaquin valleys and portions of the Bay Area Bioregions as defined by the California Resources Agency (1995), and with the California Great Valley Section on the Forest Service (1994) map of the ecological units of California. The Central Valley extends 640 km northwest and southeast through the heartland of California, averaging 64 km wide and encompassing 41,500 square km. Bounded on the east by the Sierra Nevada foothills and on the west by the Coast Ranges, the landscape is relatively flat, consisting of basins, plains, terraces, alluvial fans, and scattered hills or buttes. The area as defined here also includes the Delta region, encompassing the confluence of the Sacramento and San Joaquin rivers and several lesser tributaries.

The predominant landscape feature of the Central Valley Bioregion is a wide variety of agricultural croplands. The Valley is intensively farmed and produces over 250 crops shipped to worldwide markets. The region is the mainstay of the multi billion dollar agricultural economy in California. The productivity of the Valley is made possible through irrigation water supplied by a network of delivery canals and reservoirs. A major component of this water delivery system is the Central Valley Project (CVP) managed by the Bureau of Reclamation. The CVP and other water delivery systems managed by the state have had a major impact on the development of the Central Valley. In recent years the Valley has experienced tremendous urban growth, which has created additional pressures on dwindling habitat resources. Despite its urban and agricultural development, the Central Valley serves as a major migration corridor and wintering ground for millions of migratory birds in the Pacific Flyway. The Valley also supports numerous endemic species and unique biotic communities.

In spite of the small area comprised by Interior lands, habitat within this bioregion is critically important for the protection of trust species. From north to south, significant Fish and Wildlife Service holdings include the Sacramento National Wildlife Refuge complex. (Sacramento, Delevan, Colusa, Sutter, Butte Sink and Sacramento River NWRs), Stone Lakes NWR, and Antioch Dunes NWR, a unit managed by the San Francisco Bay NWR complex. The San Joaquin Valley forms the southern portion of the Central Valley Bioregion, the and includes the following Service lands: San Luis NWR complex (San Luis, Kesterson, Merced, and San Joaquin River NWRs), the Grasslands Wildlife Management Area, and the Kern and Pixley NWRs, which comprise a portion of the Kern NWR complex. Small parcels of BLM land are scattered in the southern and northern ends of the Central Valley.  

Central California Coast

The Central California Coast Bioregion includes marine, freshwater, and terrestrial resources from the Santa Cruz Mountains on the north to Point Conception on the south. The edge of the continental shelf forms the western boundary; on the east the region borders the Central Valley Bioregion.

The coastal zone of western North America is a highly valued region with unique environmental problems. Humans have chosen this area as a preferred place to live, thus greatly enhancing land values but compromising the limited remaining habitat for biological resources. These conflicting values create difficult challenges and a broad range of issues in policy, planning, and natural resource management. The coastal marine environment is extremely productive, supporting valuable fisheries as well as a diversity and abundance of other marine life. The coastal zone also contains the major north-south shipping lanes in western North America and associated environmental problems, notably the potential for petroleum spills. The continental shelf of western North America also yields valuable mineral resources, the extraction of which often conflicts with the needs of biological resources. Finally, the coastal marine environment is a receptacle for numerous contaminants from the nearby land, including fertilizers, toxic metals, and organochlorines. The sustainability of this coastal ecosystem, so productive on the one hand and so intensively utilized on the other, is a matter of genuine importance and concern for human welfare.

The most important threats to native freshwater and terrestrial ecosystems on the Central California Coast are water projects that dam, drain, divert, pump, pollute and otherwise use the water that sustains these systems. Increasing urbanization of the relatively sparsely populated area also poses potential problems. The destruction of aquatic habitats has fragmented already scarce resources, and pressures to continue the decimation are mounting. Another pressing issue is the degradation of habitats by introduced plants and animals, including eucalyptus, German ivy, ice-plant, bullfrogs and sunfish.

DOI-managed lands include 220,000 hectares of BLM's Hollister District, the National Park Service's Pinnacles National Monument, and the Elliot Slough and Salinas River NWRs.  

Sierra Nevada

The Sierra Nevada Bioregion extends from the Feather River in the north to Tejon Pass in the south. It is bounded on the west by the Central Valley Bioregion and on the east by the Intermountain and Mojave Desert Bioregions. Included in the region are the headwaters of 24 river basins extending to the foothills on the west side and the base of the high Sierra Nevada escarpment on the east side. The State of California (1995) divides the bioregion into Northern and Southern Sierra Nevada Bioregions. On the Forest Service (1994) map of the ecological units of California, the Bioregion is represented by the Sierra Nevada and Sierra Nevada Foothills sections.

The Sierra Nevada Bioregion is over 8,000,000 hectares in size; elevations range from 150 meters in the foothills to nearly 4,400 meters at the crest. Vegetation ranges from valley grasslands and woodlands through chaparral-covered slopes to montane coniferous forests and alpine meadows. Isolated groves of giant sequoias are interspersed along the length of the range. In the past, the primary economic activities have been grazing, mining, and logging. As the human population has increased, these activities have shifted.

Major concerns in the Sierra Nevada are the threat of catastrophic fire as a result of the buildup of fuel hazards and the loss of biodiversity caused by various human activities. Critical issues for DOI managers stem from these concerns. The history of fire in the Sierra Nevada, the effects of fire on watersheds, and the nature of fuel build-up are issues specific to the threat of fire. To determine the effects of anthropogenic change, inventory level information on existing natural resources needs to be collected, and long-term monitoring and demography studies need to be initiated. The status of many mammal and amphibian species is undocumented; causes of declines or increases in other species are unknown. Visitors to the Sierra Nevada also impact the resources, and the nature of that impact needs to be determined and mitigating measures developed.

Forty-one percent of the Sierra Nevada Bioregion is managed by the Forest Service as national forests. The Bureau of Land Management administers eleven percent of the region. Yosemite, Sequoia, and Kings Canyon national parks, Devils Postpile National Monument, and Manzanar National Historic Site comprise eight percent of the region and are managed by the National Park Service. The 360-hectare Blue Ridge National Wildlife Refuge, a summer roosting area for California condors, is the only Fish and Wildlife Service unit in the Sierra Nevada Bioregion.  

Northern California Coast

Several Forest Service (1994) ecological units are grouped into the Northern California Coast Bioregion. These include the northern California Coast, Klamath Mountains, Northern California Coast Ranges, and Northern California Interior Coast Ranges sections. The State of California (1995) names this area the Klamath Bioregion. The region extends from the Oregon border to Point Arena, and from the continental shelf to the Central Valley. The region is one of rugged relief, with severely sheared, faulted and folded mountains forming parallel ridges and river valleys. It also has coastal terraces, lagoons, and populated flood plains, as well as off-shore islands, estuaries and subtidal deep water habitats. Elevation ranges from sea level to 2,700 meters. Vegetation types include redwood, mixed evergreen and montane forests, oak woodlands, bald hills prairies, and coastal prairie scrub. Port Orford cedar forests on serpentine soils and coastal dune communities are not common, but are important biologically for the number of rare species they support.

This region is affected by extensive timber harvesting, dam construction and operation, grazing and mining. Only a small percentage of old-growth forest remains in the North California Coast Bioregion, and as a consequence populations of old-growth dependent species such as the marbled murrelet and northern spotted owl have declined. Loss of habitat and habitat fragmentation are major concerns for terrestrial species. A decline of anadromous fish species throughout the region is also of concern. Factors contributing to the decline of salmon are fishing pressures, ocean conditions, diversion of river flows to the Central Valley, and degradation of habitat by timber harvest, road construction, and mining activities. Accelerated erosion and sedimentation damages the quality of spawning gravels, decreases the amount of available rearing habitat, increases water temperatures, and damages riparian corridors. Modification of flow regimes by dams has resulted in the accumulation of fine sediment in gravels and altered channel and valley morphology. Coho salmon, steelhead, and cutthroat trout are species of concern, and the coho salmon was recently federally listed as threatened. Exotic fauna, such as squawfish, have invaded some local rivers and compete with native species. A severe loss of wetlands has occurred along the coast due to sedimentation and draining of lands for agriculture. Declines in amphibians and migratory neotropical birds in this region have also been documented. Issues involving vegetation include invasion and competition by exotic species, and attempts to reestablish old-growth characteristics in second-growth forests under differing silvicultural techniques.

DOI lands include 2 national Park Service areas, two Fish and Wildlife Service refuges, and several Bureau of Land Management areas. The parks are Redwood National and State Parks and Whiskeytown National Recreation Area. Humboldt Bay and Castle Rock National Wildlife Refuges are important staging areas for migrating waterfowl. The King Range Conservation Area is recognized as a superlative section of undeveloped California coast. Six U.S. Forest Service units, numerous state parks and forests, and Native American lands in this region face similar resource challenges as the DOI lands.  

Southern Cascade Range/ Modoc Plateau

This bioregion generally coincides with the Modoc Bioregion as defined by the California Resources Agency (1995). It includes the Modoc Plateau Section and the Southern Cascade Section as defined by the Forest Service (1994). Physiographically, the region includes flats, basins, valleys, lava flows, and mountains. High desert and forests are the dominant vegetation communities. The region contains several major lakes (Goose, Eagle, and Tule), and Mount Shasta (4,317 m) and Mount Lassen (3,187 m) are dominant physical features. The bioregion shares many similarities with the Great Basin region that forms much of its eastern boundary. The Klamath Basin, which forms a portion of this bioregion, is shared by both California and Oregon and is well known as an important migration corridor for migratory birds and as a breeding area for waterfowl. The region contains endemic biota, including endangered suckers, and important raptor wintering grounds.

Major issues in this region are water quality concerns related to agricultural runoff, maintaining the viability of the agricultural economy, timber and range management, and protecting the biotic diversity of the Klamath Basin, particularly its importance as a breeding, staging, and wintering area for migratory birds.

DOI lands in this bioregion include the Klamath Basin NWR complex (Lower Klamath, Tule Lake and Clear Lake NWRs), Modoc NWR, Lava Beds National Monument, and Lassen Volcanic National Park. The Bureau of Land Management administers over 386,000 hectares in this bioregion.  

San Francisco Bay

The San Francisco Bay Bioregion includes the area from Point Arena to the Santa Cruz Mountains and extends from the continental shelf to the delta of the Sacramento and San Joaquin Rivers. The region coincides with California's (1995) Bay Area Bioregion, although the eastern boundary does not extend into the Central Valley. Ecological units from the Forest Service (1994) include portions of the Central California Coast, Northern California Coast, Central California Coast Ranges, and Northern California Coast Ranges sections. The region has coastal terraces, parallel ridges, coastal islands, estuaries and marine habitats. San Francisco Bay is one of the largest estuaries on the Pacific coast. Vegetation types include redwood forests, Douglas- fir mixed evergreen forests, bishop pine forests, oak woodlands, and coastal prairie-scrub.

Habitat loss and fragmentation through human development are perhaps the most important reasons for the loss of biological diversity in the San Francisco Estuary, but they are not the only causes. Other factors such as the reduced flows of fresh water from the Sierra and water diversions for agriculture have seriously affected fisheries and waterfowl habitat. Pollution from waste water treatment plans, oil refineries, and agriculture continue to have marked influences on the biological resources of the Estuary. The persistence of DDT in the Estuary continues to cause eggshell thinning in reintroduced peregrine falcons of the San Francisco Bay Area. Human population growth and urban development continue to cause pressure on remaining wildlands, with the introduction and spread of exotic species invading native communities. The interruption of natural fire regimes poses serious threats of wild fire that not only affect life and property but can have disastrous effects on native systems. San Francisco Bay continues to be a rich resource that requires constant care if it is to remain ecologically viable.

The National Park Service manages the largest portion of the DOI lands in this region. Included are Golden Gate National Recreation Area, Muir Woods National Monument, Point Reyes National Seashore, San Francisco Maritime National Historic Park, and Fort Point, John Muir, and Eugene O'Neill National Historic Sites. The Fish and Wildlife Service manages the San Francisco Bay, San Pablo Bay, and Farallon National Wildlife Refuges. There are only a few BLM parcels scattered in the coast range portions of the region.  

Intermountain Desert

The Intermountain desert bioregion includes a large portion of the western United States, including most of Nevada and parts of eastern California, southeastern Oregon, southern Idaho, and western Utah. The region is characterized as a high desert, with harsh winters and dry, hot summers. This bioregion also has numerous large mountain ranges that extend throughout the area. The basin floor is dominated by big sagebrush with a variety of other drought-deciduous perennial shrubs and grasses. The alluvial fans and the lower elevations of the mountains are dominated by pinyon pine and western junipers. The higher elevations in the mountains are dominated by Jeffrey pines, white fir, and other coniferous species.

Human populations are relatively small in this region. Predominant human land uses include livestock grazing and mining. Water is the primary limiting factor. The limited availability of water shapes much of the land use and also has a profound influence on the biota of the region. A notable biological feature of the Intermountain desert is the distribution and diversity of rare desert fishes, relicts of recent eras of wetter climates when large lakes and rivers were prominent on this landscape.

The vast majority of the lands in the region are managed by federal agencies. The Bureau of Land Management manages the largest portion of the DOI lands in this region. The Fish and Wildlife Service has several large National Wildlife Refuges and the National Park Service manages the Great Basin National Park. The balance of federal land in the region is U.S. Forest Service and various military installations.




Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
Page Contact Information: webmaster@werc.usgs.gov

References to non-U.S. Department of Interior (DOI) products do not constitute an endorsement by the DOI.

* DOI and USGS link policies apply.