Physiographic Region 36: Central Shortgrass Prairie
Description and Ecology: Shortgrass is dominated by two low-growing warm-season grasses,
blue grama and buffalo grass; western wheatgrass is also present, along with taller vegetation,
including widespread prickly-pear cactus and yucca, and cholla in the south. Sandsage prairie is
found where sandy soils occur, and is dominated by sand sagebrush and the grasses sand bluestem
and prairie sand-reed. Mixed grass (needle-and-thread, side-oats grama) and tallgrass (big
bluestem, little bluestem, switchgrass) communities occur locally.
Ecological forces that shape the shortgrass prairie landscape include fire, grazing, and climate.
Little is known about the ecological role of fire in shortgrass, although even before the advent of
fire suppression by humans, fire was probably less frequent than in either mixed grass or tallgrass
prairie (Weaver et al. 1996). Prescribed burns have been used in shortgrass to remove woody
vegetation, cacti, and accumulated litter and to improve grazing conditions for livestock, but the
grasses recover slowly, requiring 2-3 years with normal precipitation (Wright and Bailey 1980).
Before widespread settlement by European-Americans, grazing regimes consisted of native
ungulates wandering widely across the shortgrass prairie--spreading over the landscape the
impact of their grazing and trampling--and prairie dog colonies expanding, contracting, and
moving in response to climatic influences on vegetation so that, at any given time, they grazed
some areas intensively and others not at all (Knopf 1996b). The severity of the semi-arid climate
and sharp differences in precipitation over relatively short distances produced contrasts in
vegetation and advanced the formation of a variegated landscape. Grassland birds thus evolved in
a shifting landscape mosaic, with access to patches of vegetation in a variety of successional
stages and conditions.
Importance and Conservation Status: The driving conservation issues in the Central Shortgrass
Prairie are habitat loss and habitat alteration. Colorado's rapid population growth and
accompanying land development are responsible for much of the habitat conversion and
degradation. Within the shortgrass area much of that development is concentrated along the
Front Range in Denver, Boulder, Jefferson, Arapahoe, Larimer, and Douglas counties, where
population densities are as high as 1,180 people/kmÓ (3,050/miÓ). Human population densities in
counties within the physiographic area but away from the Front Range corridor range from as
high as 6.6 people/kmÓ (17.1/miÓ) in Morgan County to as low as 0.4 people/kmÓ (1.0/miÓ) in
Kiowa County (U.S. Department of Commerce 1991).
Because much of the shortgrass prairie is too dry to farm without irrigation, the proportion of
plowed land is not high and much of the region is still grassland (Weaver et al. 1996). In
Colorado, approximately 67% of the historical shortgrass prairie still exists (Knopf 1994),
although some sources suggest that only 20% of the original shortgrass prairie exists in an
unaltered state--the rest having been converted to cropland and urban development or degraded
by overgrazing. Conversion to agriculture results in an absolute loss of grassland habitat, but
much of the area is managed for grazing, which maintains grassland habitat but often with changes
in plant height, vigor, and community composition.
Ideally, modern management would replicate the timing, intensity, and landscape distribution of
the natural disturbances that shaped the shortgrass prairie (unfortunately, detailed information
about presettlement conditions is lacking). In practice, however, modern grazing tends to spread
grazing intensity evenly, producing a comparatively homogeneous landscape. Shortgrass birds are
left with few options if grazed prairie does not meet their habitat needs. "There is no shortage of
grazed and hayed lands for those species that benefit from these activities. By comparison, habitat
for species with breeding requirements that are not compatible with grazing and haying is
exceedingly rare and continues to diminish" (Dobkin 1994).
Habitat loss and alteration have contributed to population declines among shortgrass bird species.
These declines have been largely overlooked by the conservation community until recently, due at
least in part to widespread concern about population declines in Neotropical migrant bird species
of the eastern deciduous forest. The result of newly redirected focus is that grassland habitat is
now arguably the highest conservation priority in the U.S. "As a group, grassland species have
shown steeper, more consistent, and more geographically widespread declines than any other
behavioral or ecological grouping of North American species," including Neotropical migrants
Priority Species Accounts: This habitat is represented by 14 priority species--more than any
other habitat in Colorado. These species are Swainson's Hawk, Ferruginous Hawk, Prairie
Falcon, Plains Sharp-tailed Grouse, Greater Prairie-Chicken, Lesser Prairie-Chicken, Mountain
Plover, Upland Sandpiper, Long-billed Curlew, Burrowing Owl, Cassin's Sparrow, Lark Bunting,
Grasshopper Sparrow, and McCown's Longspur.