Results of Model Application in the Great Basin

Outputs from both the pinyon-juniper and cheatgrass models were summarized by landcover type and by species, (i.e., habitats at risk). Maps and associated spatial data in tabular form are in Wisdom and others (2005a); we present a brief summary of results below.

Vegetation at Risk of Displacement by Pinyon-Juniper

Of the 4.8 million ha of sagebrush in the three provinces of the eastern Great Basin, 35 percent (1.7 million ha) was estimated to be at high risk of displacement by pinyon-juniper woodlands, whereas 60 percent (2.9 million ha) was at low risk. Very little sagebrush was in the moderate risk category (6 percent), which represents a transitional phase of encroachment. Mountain big sagebrush (A. tridentata vaseyana) appeared to be the most susceptible sagebrush taxon (42 percent at high risk). However, Wyoming-basin big sagebrush (A. t. wyomingensis-A. t. tridentata) was the dominant sagebrush cover type in the modeled area and comprised 55 percent of all the high-risk sagebrush, exceeding 930,000 ha.

Fig. 1 - Mapped risk of displacement
by pinyon-juniper

The spatial pattern of areas estimated to be at high risk of displacement by pinyon-juniper closely followed the distribution of pinyon-juniper woodlands throughout the three provinces (Figure 1). Areas of moderate and high risk tended to occur on the side slopes, with areas of low risk in the valley bottoms.

Vegetation at Risk of Displacement by Cheatgrass

Fig. 2 - Mapped risk of displacement
by cheatgrass

Nearly 80 percent of the landcover in the Great Basin was estimated to be susceptible to displacement by cheatgrass, (i.e., low or greater risk; Figure 2). Of the susceptible area, 26 percent was estimated to be at moderate risk and 40 percent at high risk. Among vegetation types, salt desert scrub was most at risk, with 96 percent (7.1 million ha) estimated to be at moderate or high risk of displacement by cheatgrass. Sagebrush was also at risk from cheatgrass, with 38 percent at moderate risk and 20 percent at high risk, a total of 4.8 million ha. The overwhelming majority (88 percent) of the sagebrush at high risk was Wyoming-basin big sagebrush.

Spatial patterns of risk of displacement by cheatgrass followed the typical north-south alignment of topographic features in the Great Basin. Areas of high risk generally occurred at lower elevations and on valley bottoms, whereas areas of low risk were typically in mountain ranges and higher elevation valleys.

Species’ Habitats at Risk

For 35 of the 40 species of concern considered in our assessment, more than 30 percent of their sagebrush habitat was at high risk of displacement by pinyon-juniper woodlands in the 3 provinces in which this risk was modeled. Twelve species each had about 1.6 million ha of sagebrush habitats at high risk; these species used all eight sagebrush cover types as habitat and were widely distributed across the three provinces.

Within the Great Basin, the dominant category for species’ habitat at risk of displacement by cheatgrass was high risk (mean=36 percent, n=40). For 33 species (88 percent), more than half of their habitat in the study area was at moderate or greater risk of displacement by cheatgrass. Ten species, including one amphibian, three reptiles, five raptors, and one small mammal, had more than 8 million ha of habitat at high risk, an area equivalent to about one-fourth of the ecoregion. Relative to other species in our assessment, habitat for greater sage-grouse and pygmy rabbit was at lower risk of displacement by cheatgrass.