Modeling sage-grouse habitat using a state-and-transition model

Graduation date: 2011Habitat for wildlife species that depend on sagebrush ecosystems is of great management concern. Evaluating how management activities and climate change may affect the abundance of moderate and high-quality habitat necessitates the development of models that examine vegetation dynamics, but modeling tools for rangeland systems are limited. I developed state-and-transition models using a combination of scientific literature and data for climate, soils, and wildfire to examine how different types of natural events, management activities, changing climate, and potential future vegetation dynamics may interact and affect the abundance of habitat for the greater sage-grouse (Centrocercus urophasianus). Specific periods examined include the era prior to 1850, the current era, and late in the 21st century in southeastern Oregon. A primary purpose of this study was to evaluate the use of climate data to define most event probabilities and, subsequently, the relative mix of ecological states, community phases, and sage-grouse habitat with an eye towards a modeling approach that was objective, repeatable, and transferrable to other locations.\ud \ud Contrary to expectations, model results of the conditions prior to 1850 indicated fire may not have been the most important disturbance factor influencing sage-grouse habitat abundance, merely the most visible. Other, more subtle disturbances that thinned sagebrush density, such as drought, herbivory, and weather-related mortality, may have been equally or more important in shaping sage-grouse habitat. Sage-grouse breeding habitat may have been slightly more abundant than levels currently recommended by sage-grouse biologists, brood-rearing habitat may have been as or more abundant, but wintering habitat may have been less abundant.\ud \ud Under the current conditions, livestock grazing during severe drought, postfire seeding success, juniper expansion probabilities, and the frequency of vegetation treatments were the most important determinants of sage-grouse habitat abundance. The current vegetation trajectory would lead to considerably less nesting, brood-rearing, and wintering habitat than sage-grouse biologists recommend. Model results suggested reducing or eliminating livestock grazing during severe drought, increasing postfire seeding success, and treating at least 10% of the so-called expansion juniper each year was necessary to maintain higher levels of sage-grouse habitat, although nesting and brood-rearing habitat remained in short supply.\ud \ud I examined three potential future climates based on long-term climate trends in southeastern Oregon and modeled climate and ecosystem projections for the Pacific Northwest generally. The first scenario produced warmer and drier conditions than present, the second scenario warmer and wetter conditions in winter, and the third scenario warmer and wetter conditions in summer. The implications for sage-grouse habitat abundance were very different between these three scenarios, but all would likely result in the loss or near complete loss of cooler, moister sagebrush communities important for nesting and brood-rearing. Salt desert shrub and warmer, drier sagebrush communities could expand under the first scenario but would have a high risk of displacement by cheatgrass. Juniper woodlands could increase in density and salt desert shrub may expand slightly under the second scenario. The remaining sagebrush communities would remain at high risk of displacement by cheatgrass. Pinyon-juniper woodland could largely displace sagebrush in the third scenario. Sage-grouse habitat quality likely would decline in all three scenarios and the abundance decrease significantly in the second and third scenario