The Western Short Grasslands is the second largest grassland ecoregion of North America, covering slightly more than 435,000 km2. This unit ranges over portions of western Nebraska and southeastern Wyoming, across much of eastern Colorado, southwestern Kansas, west Texas, the Oklahoma panhandle and into eastern New Mexico. This grassland ecoregion is distinguished from other grassland units by low rainfall, relatively long growing seasons, and warm temperatures. From a structural standpoint, the short stature of the dominant sod-forming grasses, grama (Bouteloua gracilis) and buffalo grass (Buchloe dactyloides), separate the Western Short Grasslands from other units.
Küchler (1964) classified this ecoregion as grama-buffalo grass prairie, bluestem-grama prairie, sandsage-bluestem prairie, and wheatgrass-bluestem-needlegrass prairie. Under more natural conditions, major sources of ecosystem disturbance were drought and grazing by wildlife, rather than fire as in some of the other Great Plains grassland ecoregions to the east. Unlike in the more mesic grassland ecoregions, fire is thought to be detrimental to shortgrass prairie plants (Wright and Bailey 1980, 1982). Today, livestock grazing is the major form of disturbance, but much of this ecoregion is in better condition as far as grazing impact than other ecoregions. Part of the explanation is that many of the sod-forming perennial grassland plants, the dominant species in this ecoregion, evolved under intense grazing and trampling by migratory herds of bison (Bison bison).
The Western Short Grasslands is among the richest ecoregions in the United States and Canada for species of butterflies, birds, and mammals. Part of this pattern can be explained by the closer proximity of this ecoregion to the subtropics. This ecoregion once supported one of the most impressive migrations of a large ungulate species anywhere in the world–the American bison migration. Today, bison no longer migrate, but bison ranching is becoming increasingly popular. The Western Short Grasslands also contains the fastest declining bird populations on the continent–the endemic birds of the short grasslands of the Great Plains. These species are declining faster than many neotropical migratory birds whose plight receives much more attention.
Habitat Loss and Degradation
Much of this area was severely affected by largely unsuccessful efforts to develop dryland cultivation. In western Kansas and eastern Colorado this damaging activity still continues. The dustbowl of the 1930s was centered in this ecoregion, and stands as proof of the unsuitability of this area for farming, unless heavily irrigated. Areas in the southern part of this ecoregion in Texas have been invaded by mesquite (Prosopis glandulosa) and thorny shrubs (e.g. Opuntia spp., Zizyphus obtusifolia, Berberis trifoliata), forming a savanna or shrubland with a shortgrass prairie understory (Sims 1988).
Nearly all of this ecoregion is in farms and ranches. Cropland cover varies between 30-60 percent across the ecoregion (even higher in western Kansas and western Nebraska), with grazing lands occupying the remainder. The amount of irrigated farmland varies across the ecoregion. In the northern section in Colorado, almost all of the land is in farms or ranches with a build-up of urban areas along the eastern edge of the Rockies. About 68 percent is used for grazing domestic livestock, with about 15 percent of the area planted in dry crops.
In the extreme southern section of the ecoregion, grazing covers more than 75 percent of the area. Overgrazing has allowed the spread of woody shrubs and trees and the near permanent conversion of plains grasslands to desert scrublands (Dick-Peddie 1993). Approximately 40 percent of the remaining habitat in this ecoregion is considered to be intact, one of the highest percentages among North American grasslands, and the highest among grassland ecoregions greater than 70,000 km2. In the western section of this ecoregion, rangelands are moderate to heavily grazed.
Remaining Blocks of Intact Habitat
Six of the national grasslands in this ecoregion contain intact blocks of habitat. They include:
•Black Kettle - western Oklahoma,
•Commanche - southeastern Colorado
•Pawnee - northeastern Colorado
•Cimarron - southeastern Kansas. This is one of the strongholds of the Lesser Prairie Chicken
•Rita Blanca - along the borders of New Mexico, Texas, and Oklahoma
•Kiowa - panhandle of Texas
Other significant areas of intact habitat include:
•the Arkansas River Sandsage - southwestern Kansas
•Tule/Palo Duro Canyon areas - northwestern Texas
Degree of Fragmentation
The species that occur in the Western Short Grasslands are relatively widespread and good dispersers. Aside from areas that have been plowed, the native fauna is relatively unaffected by the levels of fragmentation found in this ecoregion.
Degree of Protection
Although there is considerable rangeland and grassland worthy of conservation, few sites are formally protected. National Grasslands have considerable potential for biodiversity conservation but grazing on these units by domestic livestock must be modified.
Types and Severity of Threats
The main threat to this ecoregion is conversion to agriculture. New technologies, including: four-wheel drive tractors, precision farming, herbicides, and irrigation make farming more productive in areas that were previously difficult to cultivate. The Conservation Reserve Program (CRP) in the eastern and southern part of this unit in the Oklahoma and Texas panhandles has kept some areas under good conservation management. If the CRP program were legislatively modified or discontinued, some of these areas would be under great threat of conversion. Overgrazing is less of a threat in most parts of the ecoregion because of the abundance of grazing-tolerant plant species. The invasion of tree species is a problem in some areas.
Suite of Priority Activities to Enhance Biodiversity Conservation
The most effective steps to conserve and restore representative examples of this grassland ecoregion include:
•making National Grassland management more sensitive to biodiversity
•working with conservation associations to maintain rangelands
•increasing the use of rotational grazing to mimic natural grazing patterns
•improving relationships with private landowners
•improving management of irrigation systems
•restoring bison populations
•Kansas Natural Heritage Inventory
•National Cattlemen's Association
•Natural Resources Conservation Service
•Nebraska Natural Heritage Program
•Society for Range Management
•U.S. Forest Service
Relationship to other classification schemes
The Western Short Grasslands are derived from Omernik (1995) ecoregions 25 (Western High Plains) and 26 (Southwestern Tablelands). It corresponds to Küchler (1985) unit no. 58 (Grama-Buffalo grass). Bailey (1994) classifies this as parts of six different sections (331H, 331C, 331I, 331B, 315A, 315B).
Prepared by: T. Cook, E. Dinerstein, P. Simms, S. Chaplin, S. Smith, K. Carney