Integration of Multiple Stressors

The combined risk of displacement by cheatgrass and by pinyon-juniper woodlands was evaluated for the 4.8 million ha of sagebrush in the three ecological provinces in which the pinyon-juniper model was applied. Of the five combined risk classes, the two dominant classes were high cheatgrass (high risk from cheatgrass and low or moderate from pinyon-juniper; 35 percent) and low-moderate (moderate risk from both stressors; 33 percent). Only a trace amount (less than 1 percent) of sagebrush in this region was at high risk to both stressors (see Wisdom and others 2005a for details). Other stressors not considered in our prototype assessment, such as climate change, may be critical in future modeling of risks in the Great Basin from both cheatgrass and pinyon-juniper expansion. Under one modeling approach, direct and indirect effects of global warming may result in the elimination of up to 80 percent of existing sagebrush in the United States (Neilson and others 2005).