The Central and Southern Mixed Grasslands ecoregion has a north-south orientation, spanning central Nebraska, central Kansas, western Oklahoma, and north-central Texas. It separates the Tall-Grass Prairie and the Central Forests/Grassland Transition Zone from the Western Short Grasslands. Essentially, this region is a broad ecotone that covers slightly more than 282,000 km2. It is distinguished from the Northern Mixed Grasslands by warmer temperatures and a much longer growing season, and from the adjacent tallgrass and short grasslands by the intermediate stature of the grassland layer. It is distinguished from the Central Forests/Grassland Transition Zone to the east by the relative scarcity of trees and shrubs. The major disturbance regimes are drought, the degree and frequency of grazing by domestic animals and wild ungulates, and fire.
The mixed grass prairie contains the floristic elements of the tall and short grass prairies and, combined with a rich forb flora, contains the highest floral complexity of any North American grassland ecoregion (Barbour et al. 1980). Typical grasses include little bluestem (Schyzachrium scoparium), western wheatgrass, (Agropyron smithii), and grama grasses (Bouteloua cartipendala). These species mix with taller grasses in the wetter areas and give way to shorter grasses in the drier areas (e.g., Bouteloua, Buchloe, Muhlenbergia, and Aristida) (Sims 1988). The effects of drought cycles and grazing intensity shift floristic composition to favor drought-tolerant species during dry periods and more shallow-rooted mesic loving plants during wetter periods.
The Central and Southern Mixed Grasslands is among the top ten ecoregions in the number of reptile species and is an important breeding area for endemic Great Plains bird species. It also contains very important stopover sites for migratory birds, particularly on wetland sites scattered throughout this ecoregion. The Platte River Valley in Nebraska is a prominent area for sandhill cranes and the Cheyenne Bottoms in Kansas for populations of shorebirds during spring migration.
Habitat Loss and Degradation
Overall, only about 5 percent of the remaining habitat is considered to be intact. During the dustbowl of the 1930s, basal cover of grasses on even moderately grazed and heavily grazed grasslands declined from 80 percent or more to less than 10 percent in a period of 3-5 years (Sims 1988), but has since mostly regained its cover. Natural vegetation has been converted to cropland or pasture on about 90 percent of this ecoregion in Oklahoma and Texas. In Kansas and Nebraska, about 60 percent is in cropland and about 35 percent is grazed (USDA 1994).
Remaining Blocks of Intact Habitat
Most of the remaining blocks of intact habitat are quite small. Some of the most prominent include:
•Wichita Mountains Wildlife Refuge, a 90 km2 site for re-establishment of bison and an important area for conservation of black-capped vireos - southeastern Oklahoma
•Platte River Valley, a 45 km2 important restoration site, a major stopover for migratory sandhill cranes - southern Nebraska
•Rainwater Basins, a 45 km2 area managed in part by USFWS, which includes a series of clay bottom wetlands that are highly fragmented - southern Nebraska
•Central Kansas wetlands, including Cheyenne Bottoms and Quivira National Wildlife Refuge - central Kansas
•Great Salt Plains - north central Oklahoma
•Red Hills - Oklahoma and Kansas
•Smokey Hills River Breaks - west central Kansas - 45 km2
Degree of Fragmentation
Much of the ecoregion that occurs in Kansas and Oklahoma still contains fragments of grassland associated with farms (i.e., unbroken sod). The remaining habitat in the rest of the ecoregion is fragmented.
Degree of Protection
With the exception of a high level of protection in the Wichita Mountains and Salt Plains, remaining habitat in the ecoregion is essentially unprotected.
Types and Severity of Threats
The major threat is conversion to agriculture. High wheat prices in the mid 1990s encouraged land conversion in western portions of the ecoregion. Center pivot irrigation has also caused conversion. Water flow into streams due to diversions is another problem. Fire apparently increases forage production in the eastern portion of this ecoregion, makes grasses more palatable, eliminates undesirable annuals, and suppresses the invasion of mesquite, juniper, and cacti (Wright and Bailey 1980). Thus, fire suppression constitutes another threat. Overgrazing by domestic stock, particularly in riparian areas, is a localized serious threat.
Suite of Priority Activities to Enhance Biodiversity Conservation
•management and preservation of wetlands, primarily by seeking to maintain water flows to wetland areas
•improvement of grazing management to make it more compatible with biodiversity conservation, such as by encouraging rotation grazing
•restoration and enlargement of the best representative areas for biodiversity
•Kansas Natural Heritage Inventory
•Natural Resources Conservation Service
•The Nature Conservancy of Nebraska
•U.S. Fish and Wildlife Service- wildlife refuges
•U.S. Forest Service- National Grasslands border this ecoregion
Relationship to other classification schemes
The Central and Southern Mixed Grasslands corresponds to Omernik (1995) ecoregion 27 (Central Plains Grasslands) and most of Küchler (1985) units 62 (Bluestem-grama prairie) and 76 (Mesquite-buffalograss). It also corresponds to Bailey (1994) sections 332E (South Central Great Plains), 311A (Redbed Plains), and 315C (Rolling Plains).
Prepared by: Steve Chaplin, Terry Cook, Eric Dinerstein, Phillip Simms, and Karen Carney