Physiographic Region 87: Colorado Plateau
Description and Ecology: The semidesert shrubland, or cold desert shrub, habitat is typical of arid
continental interiors and is widespread in the vast rain shadows of western North America and
central Asia, where annual precipitation is usually below 25 cm (10 in). Generally the lowest
upland vegetation type in elevation, it consists of expanses of low widely-spaced shrubs growing
in regions with cold harsh winters. North America's best expressions of this type lie in the Great
Basin, which begins almost half the state of Utah away from Colorado. However, the rainshadow
effect of several mountain chains preserve the arid conditions of the Great Basin onto the
Colorado Plateau and into the state of Colorado. Colorado's semidesert shrubland habitats lie in
the basins along the western border, in the ascending valleys of the major rivers and tributaries
(Platte River excepted), and on the floor of the San Luis Valley.
Semidesert shrubland is a structurally simple habitat. Typical shrub species include budsage,
fourwing saltbush, Nuttall's saltbush, mat saltbush, spiny horsebrush (cottonthorn), greasewood,
and shadscale. Mormon tea, winterfat, green molly, spiny hopsage, blackbrush, and other shrubs
may occur. Native grasses include galleta, blue grama, squirreltail, Indian ricegrass, Sandberg
bluegrass, and Salina wildrye. Crested wheatgrass, an exotic species, has been planted widely.
Native flowering forbs are numerous, but evening primroses, sego lilies, Indian paintbrushes, and
globemallows are among the best known. Annual plants are more abundant in semidesert
shrubland than in any other habitat type. Unfortunately, most of these are aggressive non-natives.
Cheatgrass tops this list. It, along with filaree, provide the green in the desert winter and early
spring. In the fall, Russian thistle becomes the well known tumbling tumbleweed. Cryptobiotic
soils are common over these arid soils. Mosses, lichens, and blue-green algae occur, likely in that
order of prevalence.
Importance and Conservation Status: Post-Columbian use of this habitat has been primarily
livestock grazing, and to a lesser extent hunting and agriculture. With irrigation, cropland and
dense settlement has developed in scattered locations. In a few areas, coal mining and petroleum
development indicate that some of these lands were not always desert.
Throughout its range, semidesert shrubland is largely intact. However, livestock over-grazing,
especially between 1880 and 1934, altered the amount and composition of the vegetal
components across much of this zone. Patches of native grasses died out, cheatgrass and
halogeton among other undesirable species took their place. Wildfires became more devastating,
killing off large expanses of shrub cover (Whisenant 1990).
Bird diversity and density are typically low in semidesert shrubland (Wiens and Rotenberry 1981).
This is due to its structural and floristic simplicity (MacArthur and MacArthur 1961, Willson
1974, Rotenberry 1985). However, all semidesert shrubland is not equal. In the washes where
tall greasewood becomes dense, the density and diversity of birds matches that of the mountain
shrub zone. Nesting species typical of semidesert shrubland habitats include Horned Lark (the
dominant and sometimes only species in low semidesert shrubland country), Mourning Dove,
Western Meadowlark, Northern Mockingbird, Loggerhead Shrike, Black-throated Sparrow, and
Lark Sparrow. Sage Thrasher, Sage Sparrow, and Brewer's Sparrow occur in the best developed
greasewood habitat, especially where big sagebrush is in the mix. Because prairie dogs are typical
inhabitants of semidesert shrubland, especially on deeper soils that have a reduced shrub density,
this habitat is of considerable importance to Golden Eagles, Ferruginous Hawks, and Prairie
Falcons. Lark Buntings are irruptive breeders in semidesert shrubland outside of their usual range
(Andrews and Righter 1992, AOU 1998). In a rare year these buntings suddenly appear on the
desert, becoming locally common.
Priority Species Accounts: Three species are identified as high priority in semidesert shrubland habitats
in Physiographic Area 87: Burrowing Owl, Loggerhead Shrike, and Horned Lark.