The Great Basin is the most northerly of the four American deserts. Unlike the other three, which have almost exclusive ties to warm-temperate and tropical/subtropical vegetation types, the Great Basin has affinities with cold-temperate vegetation. The Chihuahuan and Sonoran deserts may in fact be more closely related to the Argentine "Monte" than they are to the Great Basin (Turner 1994).
The Great Basin is composed of a series of uplifted mountain ranges and their associated intervening valleys. There are approximately 100 internally drained basin within this ecoregion. The Basin is bounded to the east by the Colorado Plateau and central Rocky Mountains, to the north by the Columbia Plateau, and to the west by the Cascade-Sierra Range. The southern boundary is generally placed at the confluence of the Colorado River drainage and the Mojave Desert of southern California and southernmost Nevada (Morris 1994).
Dominant species in the region include such distinctly cold-temperate species as sagebrushes (Artemisia), saltbrushes (Atriplex), and winterfat (Ceratoides lanata). These scrub species are much-branched, non-sprouting, aromatic semishrubs with soft wood and evergreen leaves. The Great Basin also contains species with evolutionary ties to warmer climates, such as rabbitbrush (Chrysothamnus), blackbrush (Coleogyne), hopsage (grayia) and horsebursh (Tetradymia). The region, however, contains few cacti, either in numbers of individuals or species, and also lacks characteristic desert plants in minor waterways (Turner 1994).
Sagebrush communities are often considered steppe or shrub steppe because of the role of grasses, and in parts of the Great Basin grasses are important understory elements in distinctly shrub-steppe communities. Towards the south of the ecoregion, however, sagebrush may grow to the virtual exclusion of grasses even in the absence of grazing. The near absence of grasses may be due to fact that the peak precipitation occurs in winter (Turner 1994).
Grazing and fire are the two most important forces in the sagebrush communities of the Great Basin. Neither domestic nor native ruminants eat sagebrush, resulting in reduction of more palatable grasses and forbs. Native annuals suffered under heavy grazing, and since approximately 1990 introduced annuals have become increasingly conspicuous throughout the region. These annuals, such as cheatgrass brome (Bromus tectorum) and Russian thistle (Salsola paulensii), have become important in arresting succession in many areas. The highly flammable cheatgrass brome also greatly increases the incidence of fire. After a fire, sprouting species of such genera as Chrysothamnus, Tetradymnia, and Gutierrezia may take on the role of the nonsporouting sagebrush.
The other dominant vegetation communities in the Great Basin are shadscale (Atriplex confertifolia) and blackbrush (Coleogyne ramosissima ). Shadscale is a wide ranging species, but it is found as a dominant only within and adjacent to the Great Basin. Blackbrush occurs primarily in southern Nevada, southeastern California, north-central Arizona, and southeastern Utah.
The Great Basin lies mostly to the north of the 36th parallel. Part of the ecoregion occurs to the south of that line along the Little Colorado River drainage in Arizona and New Mexico, where in some places it is referred to as the Painted Desert. The region generally receives less than 250 mm of precipitation per year. Mean monthly precipitation shows a strong winter-dominated pattern in the west, with a gradual shift eastward toward a stronger summer influence with wet and dry seasons less distinct than in the other deserts (Turner 1994).
The Great Basin is the largest arid area in the United States. It is a true basin and range, with completely self-contained drainage. The region supports numerous threatened and endangered species–Nevada is third in the nation in listed species–as well as an endemic species of greasewood (Larrea spp. ) and an endemic kangaroo mouse (Microdipodops pallidus ).
This ecoregion also contains significant evolutionary and ecological phenomena. The size and orientation of Goshute Mountain, for example, concentrates hawk migrations. The small lakes in the Basin contain endemic shrimp and other species.
Habitat Loss and Degradation
Virtually the entire basin has been grazed and browsed, and less than 10 percent remains as intact habitat. Exotic species have become established across the ecoregion. Irrigation for alfalfa has increased salinization in the region while mining has led to pollution by heavy metals. Urban areas in the region, particularly Las Vegas, Reno, and Salt Lake City, are growing rapidly.
Remaining Blocks of Intact Habitat
The Great Basin contains several large intact habitat blocks. The most important are:
•Black Rock Desert - northwestern Nevada
•Sheep Range - southern Nevada
•Desert National Wildlife Range - southern Nevada
•Nevada test site - southern Nevada
•Great Basin National Park - eastern Nevada
Degree of Fragmentation
High elevation, lush habitats in the Great Basin are badly fragmented, but low elevation habitat is relatively intact. Much of the fragmented area can be restored.
Degree of Protection
The most important protected areas in the Great Basin are:
•Desert National Wildlife Reserve - southern Nevada
•Dugway Proving Grounds - northwestern Utah
•National Electronic Warfare Center
•Hill Air Force Base; Wendover Range - northwestern Utah
•Arc Dome Wilderness Area
•Still water National Wildlife Refuge
•Important lakes of conservation significance: Pyramid, Walker, Mono, Topaz
Types and Severity of Threats
Grazing poses the greatest conversion threat to the Great Basin. While levels of grazing are unlikely to increase, grazing creates on-going opportunities for invasion by non-native species. The expansion of cities like Salt Lake City and Provo also may to lead to increased development and further conversion of habitat.
Invasive species and fires are degrading the habitats of the Great Basin, while hunting threatens populations of fur-bearing mammals.
Suite of Priority Activities to Enhance Biodiversity Conservation
•Protect the desert experimental range near Milford, UT. This range has the most detailed historical grazing records in the United States. These provide the opportunity for research to determine the impact of grazing, the return of native species, and so on.
•BLM should make wilderness designations in Utah and Nevada.
•The Department of Defense should increase stewardship activities in Dugway Proving Grounds and Hill Air Force Range, Utah.
•Restore riparian areas.
•Find and protect representative pinyon-juniper sites.
•Southern Utah Wilderness Alliance
Relationship to other classification schemes
The Great Basin corresponds to Omernik’s Northern Basin and Range ecoregion. The Great Basin falls within Bailey’s Intermountain Semi-Desert and Desert Province, and covers parts of several Bailey sections, some of which extend as far east as central Colorado. We believe that vegetation and climatic patterns distinguish the Great Basin from the Colorado Plateau and the intervening Wasatch and Uinta Montane forests.
Prepared by: B. Holland, G. Orians, J. Adams