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Wyoming Basin shrub steppe

The Wyoming Basin Shrub Steppe is an expansive ecoregion of high, open, arid country. The ecoregion is nearly surrounded by mountain ecoregions. It is drained by three major river systems: the Green River to the south, the Wind-Bighorn River to the north, and the North Platte to the east. A fourth watershed, the Great Divide Basin, has no surficial connection to any river system; it is also the driest area of the ecoregion.

The ecoregion is generally arid or semiarid. It is in the rain shadow of the Rocky Mountains and thus receives little precipitation. Latitude and physiography are influential factors in distinguishing this ecoregion from other similar ecoregions, such as the Snake/Columbia and Great Basin Shrub Steppes. The Snake/Columbia Shrub Steppe is lower in elevation than the Wyoming Basin, and the two are segregated by mountainous areas. Unlike the other two, the Wyoming Basin receives at its eastern limit some summer moisture from the south. This precipitation does not penetrate into either the Bighorn Basin or the Great Divide Basin, the most desert-like portions of the ecoregion (Knight 1994).

  • Scientific Code
    (NA1313)
  • Ecoregion Category
    Nearctic
  • Size
    51,100 square miles
  • Status
    Vulnerable
  • Habitats

Description
Biological Distinctiveness
The dominant vegetation in the ecoregion is sagebrush (Artemisia spp.), often associated with various wheatgrasses (Agropyron spp.) or fescue (Festuca spp.). Knight (1994) describes the great variability of the sagebrush-steppe mosaic in the ecoregion. Elevation, aridity, snow accumulation, prevailing winds, and other factors all affect the species composition, morphology, and density of sagebrush communities in the ecoregion. Ecotones between sagebrush steppe and adjacent mountain ecoregions may appear at elevations as high as 3000 m. The sagebrush steppe is also interspersed with desert shrublands, dunes, and barren areas in more arid regions (e.g., Red Desert); and with mixed-grass prairie at the eastern limit of the ecoregion (Knight 1994).

Fire, wind, grazing, and variations in precipitation and temperature are the major disturbances in the ecoregion. Burning may encourage grass growth and impede sagebrush. Sagebrush, on the other hand, adapts to drought conditions that kill grasses (West 1988). Excessive moisture along riparian areas or in places where snow accumulates can also encourage grass growth, which displaces many sagebrush species (Knight 1994). Seasonal browsing and grazing by native herbivores including elk (Cervus elaphus), mule deer (Odocoileus hemionus), and pronghorn (Antilocapra americana a.) is another influential disturbance. Native herbivory is a somewhat distinctive factor in this shrub steppe ecoregion, since most of the ecoregion is in close proximity to extensive mountain ecoregions supporting large herds of migratory ungulates.

A particularly interesting aspect of the ecoregion is the presence of remnant prairie dog ecosystems, developed in this case by white-tailed prairie dogs (Cynomys leucurus). Knight (1994:87-88) states that prairie dogs maintain biological diversity by creating habitat for other organisms through their small-scale disturbances . . . [their influence] far exceeds the biomass they constitute in the ecosystem. Raptors, mesopredators (e.g., coyote [Canis latrans] and swift fox [Vulpes velox]), and ungulates disproportionately use prairie dog towns when available. Presumably, ungulates like pronghorn and bison (Bison bison) key on these sites because forage quality is higher, due to greater concentrations of nutrients from animal waste. Prairie dog towns (near Meteetse) in this ecoregion maintained the last known blackfooted ferrets (Mustela nigripes) when they were thought to have gone totally extinct, and have served as the first reintroduction site (near Medicine Bow) for captive-reared ferrets. The persistence of these ecosystems may be due to the harsh climate and remote location of many areas of the ecoregion, impeding agricultural conversion and persecution.

Conservation Status

Habitat Loss and Degradation
Habitat loss, degradation, and outright conversion are severe in some locales. Conversion of sagebrush habitat types to grasslands for domestic grazing has taken place in areas where the climate will support grass production. Large-scale energy and mineral developments have had major long-term impacts in certain areas.

Native ungulates that use the shrub steppe either year-round or as seasonal range are often intolerant of industrial activities and are thus displaced from otherwise productive habitat. Displacement tends to concentrate ungulates, leading to over-utilization of winter range and rapid transmission of pathogens. Additionally, the road networks associated with resource extraction have allowed hunters greater access to ungulates, resulting in dramatically reduced herd sizes in some areas (Debevoise and Rawlins 1996).

Land reclamation has had a mixed success rate at restoring native vegetation (Knight 1994, Debevoise and Rawlins 1996). Reclamation in some cases has relied on non-native grass species, in addition to unintentional introduction of noxious weeds. Sagebrush and other important ungulate forage plants have, on the whole, been difficult to restore on reclamation sites (Debevoise and Rawlins 1996).

The combined effects of heavy livestock grazing and fire suppression have also altered the structure and composition of some areas of the ecoregion. Heavy grazing removes potential grass fuels, thus minimizing the likelihood of periodic fires. Woody plant species like sagebrush are quite robust under such conditions and can attain heights of up to 1.5 m. At ecotones with the surrounding montane ecoregions, conifer invasions occur over time, especially along relatively mesic riparian zones.

Remaining Blocks of Intact Habitat
Many areas of the ecoregion are remote from human population centers and have remained relatively intact. Scattered resource extraction sites and dispersed livestock grazing have occurred over most areas, but have left little long-term impact on many habitat blocks (Knight 1994; Debevoise and Rawlins 1996).

Degree of Fragmentation
Outside of major resource extraction sites, fragmentation is relatively limited. Since most of the ecoregion is far too arid for cultivation, there has been little effort at irrigation and row cropping. The exception to this is the Bighorn Basin, which contains several extensive cultivated areas. Fragmentation through localized conversion or degradation may affect specific vegetation types and smaller fauna.

With respect to megafauna, fragmentation is locally severe for species like elk and mule deer, where topography, climate, and development-related displacement severely restrict access to crucial winter range. Large predators like grizzly bears (Ursus arctos), which existed at low densities and probably used the habitat only seasonally, have been eliminated from the ecoregion. This extirpation probably has more to do with the loss of bears in more productive source habitats in the surrounding mountains than with any particular change in the shrub steppe itself.

Eliminating individual bears that frequented the basins was accomplished easily, since the open country afforded little visual cover; and since poisons were freely available for predator control. Recolonization or sporadic use of the shrub steppe by grizzly bears may occur in the near future as the Greater Yellowstone population shifts and expands its range; this phenomenon could be suppressed by widespread use of M-44 cyanide traps set for coyote control, and by uncontrolled human access.

Degree of Protection
Overall, protected areas in the shrub steppe are lacking. The largest designated area is the Flaming Gorge National Recreation Area, which consists of a dammed reservoir on the Green River. Arguably, the shoreline areas and surrounding terrain are protected, but a large impoundment clearly amounts to wholesale habitat conversion that may even alter non-innundated habitats nearby in southwestern Wyoming

There are many wilderness study areas (WSA) that may soon receive protection. A lower level of protection is afforded at some sites, administratively designated by the Bureau of Land Management as Areas of Critical Environmental Concern (ACEC). These include:

•Steamboat Mountain ACEC - south central Wyoming
•the Greater Sand Dunes ACEC. This area, along with the Sand Dunes WSA, could potentially comprise the core of a large reserve network in the Great Divide Basin - south central Wyoming
Types and Severity of Threats
Overgrazing by domestic livestock, fire suppression, wholesale replacement of sagebrush with grasses, and spread of exotic grasses are the major anthropogenic threats to the ecoregion. Oil and gas exploration, along with mining (strip mining for coal and hard-rock mining for trona and other industrial minerals), have the potential to cause considerable disturbance, wildlife displacement, and environmental pollution. Debevoise and Rawlins (1996) report that 6,000 to 11,000 new oil and gas wells could be sunk in southwestern Wyoming in the next 20 years. Unbridled industrial expansion could have severe long-term impacts on the ecoregion.

 Suite of Priority Activities to Enhance Biodiversity Conservation
Two major conservation activities should be pursued:

•The oil and gas leasing process should be carefully reviewed, undertaking new exploration only after thorough assessment of possible impacts and development of effective mitigation measures.
•Conservationists should make a concerted effort to develop and enact a wilderness bill that would provide full protection to existing WSA's and ACEC's.
•To help focus both activities, conservationists should undertake the following
•A rapid assessment of the ecoregion should be conducted.
•Rare plant communities and assemblages should be identified and documented. Dune communities and other geologic-vegetative associations should be cataloged and prioritized.
•Prairie dog ecosystems, ungulate habitat, and potential large predator recolonization areas should also be carefully evaluated. Much of this information is already available for southwestern Wyoming (Debevoise and Rawlins 1996).
•This information could be used to identify areas in which it may be acceptable to concentrate impacts, as well as areas that are at the threshold of systemic collapse (such as the elimination of elk herds or plant communities). Large undisturbed areas should receive high priority for protection.
Conservation Partners

•Greater Yellowstone Coalition
•Sierra Club - Northern Plains Field Office
•Wyoming Outdoor Council
•Wyoming Wildlife Federation
Relationship to other classification schemes
The ecoregion boundary is taken from Omernik (1995) and closely matches Bailey units 342 A, E, F, and G; it amalgamates Küchler units 50, 34, 31, 57, and part of 49.

Prepared by: S. Primm