Identifying Regional Threats

Remnant big sagebrush

Wisdom and others (2005a) outlined a process for identifying threats to consider in regional assessment, using criteria such as spatial extent of the threat, available resources to address the threat, and cost-benefit analysis. The threats confronting sagebrush habitats in the Great Basin ecoregion typify those in most sagebrush-dominated ecoregions of Western North America. Two of these are pervasive and accelerating: the displacement of native vegetation by cheatgrass invasion and by encroaching pinyon-juniper woodlands (see figure on the right).

Invasion by exotic species, particularly cheatgrass, is consistently cited as one of the major challenges to maintenance of healthy sagebrush communities (Connelly and others 2004, Knick 1999, Pyke 2000). In addition to its displacement of native understory species, the autecology of cheatgrass leads to an increased risk of catastrophic wildfires that eliminate the sagebrush overstory (Billings 1994, Booth and others 2003). The presence of cheatgrass exacerbates fire hazard because of its high flammability. Cheatgrass can drastically shorten the fire recurrence interval in native sagebrush communities from 20 to 100 years to 3 to 5 years in some sites.

Pinyon-juniper woodlands have expanded greatly in the Great Basin when compared to their distribution more than 150 years ago (Miller and Tausch 2001). Tree density also has increased in established woodlands. These changes have been linked to a decrease in area burned by wildfire, a result of increased fire suppression and removal of fine fuels by livestock; climate change; historical patterns of livestock grazing; and increased atmospheric carbon dioxide (Polley and others 2002, Sakai and others 2001). Detrimental outcomes of pinyon-juniper woodland expansion include increased soil erosion, changes in soil fertility, losses in forage production, reductions in wildlife habitat for some species, and alteration of presettlement native plant communities (Miller and Tausch 2001). During its status review of the petition to list greater sage-grouse under the Endangered Species Act, the U.S. Fish and Wildlife Service explicitly cited habitat degradation from invasive species, such as cheatgrass, and encroachment of pinyon-juniper woodlands as key threats to greater sage-grouse (USDI Fish and Wildlife Service 2005a).

Although many other threats affect sagebrush habitats in the Great Basin such as overgrazing by domestic and wild ungulates (Nachlinger and others 2001), empirical data are often insufficient to model or estimate the risks posed by these threats at large scales. We chose these two threats—cheatgrass and pinyon-juniper woodlands—in the Great Basin assessment to exemplify the process of evaluating threats to sagebrush ecosystems at regional scales.