Population Ecology of Sagebrush Steppe

Population Ecology provides information on populations of species, indicating the long term sustainability of that population and the degree to which a species utilizes it’s available habitat. Population can be defined as the individuals of a particular species that occupy a certain area.

Demography is the study of how a population of a species changes over time; including population size and density, distribution, and survivorship type. The dominant vegetation species of Sagebrush Steppe is Big Basin Sagebrush, while examples of some of the dominant Wildlife Species is mule deer (1) and Burrowing Owls (7).
Population Growth and Control describes that there a variety of factors that influence changes in population size and density; density independent factors and density dependent factors. Density dependent factors can be geometric (e.g. periodic reproduction such as when deer reproduce annually), or exponential (e.g. continuous reproduction such as when rabbits reproduce at any time); if resources are unlimited. Since resources are not unlimited, a system will only support a maximum number of individuals, and a population will reach Carrying Capacity and slow or decline. Birth and death rates must be balanced to avoid growth, until migration or habitat area expands producing more resources. Therefore, Density Dependent can be defined as the regulation of population size based on its relation to carrying capacity. Density independent factors are regulated by environmental factors such as drought/wildfire or community dynamics such as predation/allelopathy.

Population distribution and dynamics are influenced by both abiotic and biotic factors. The two main abiotic factors are physical geography and climate, while the biotic factors include species interactions: allelopathy, predation, competition, mutualism, etc.

Disturbance and Climate Change involve a variety of different factors. Disturbance may be related to invasive plants, fire, drought, grazing, agriculture, urbanization, recreation. Climate Change involves shifting of climatic factors such as precipitation, temperature, wind, and pressure fronts in its variation, intensity, or global distribution. Shifts in climatic factors can influence change in species population dynamics and distribution. Since Biogeography is dependent on climate and geology, shifts in climate will produce biogeographic shifts for a species range.
The following examples of demography, population growth and control, and disturbance and climate change in Sagebrush Steppe ecosystems focus on one dominant vegetation species, and one dominant animal species.


The dominant vegetation of Sagebrush Steppe is Big Basin Sagebrush (Artemisia tridentata var. (ssp. tridentata)):

Demography- Native to the Intermountain Western United States, Great Basin, the lifeform of this vegetation is shrub and it is resistant/tolerant to drought. It has a perennial life cycle, a fast growth rate, and fast regrowth rate. Capable of allelopathy, can grow 2-15 feet depending on environment. Adaptable to sand, loam, or clay soil with fine, medium, or coarse texture. Big Basin Sagebrush has a high tolerance for CaCO3 and grows in deep, seasonally dry, well drained soil with a minimum depth of 3 ft. It requires a minimum of 110 frost free days, and moderate moisture use. Grows in Soil pH 6.5 to 8.5, slightly acidic to slightly basic soil. Receives low precipitation, 10-19 inches annually. Flowers August to October and reproduces by seed in Fall and Winter. Seedling Vigor is Moderate and depends on precipitation, seeds sheltered by mature sagebrush are more likely to survive. This means that Big Basin Sagebrush populations survive in semi-arid environments with deep rooted soil.

Population Growth and Control- The abiotic factors that influence population dynamics include the geographical and climatic characteristics previously discussed in the Demography section of Big Basin Sagebrush. So this species may increase in population size/density where these environmental factors apply, and will be less abundant where they do not. One major biotic factor that influences population dynamics is allelopathy, a species interaction that helps Big Basin Sagebrush compete with other vegetation through inhibiting the other plant.

Disturbance and Climate Change- Big Basin Sagebrush has a long life span of 40-50 years but does not resprout after disturbance, such as fire (8). It is important to consider fire in land management of this species, because this disturbance will wipe a population from a community. Climate change results in higher temperatures and lower precipitation, influencing an increase in fire frequency and intensity; and possible reduction of Big Basin Sagebrush.


One dominant animal species of Sagebrush Steppe is Mule Deer (Odocoileus hemionus):

Demography- The rate of movement shows that mule deer have crepuscular activity patterns (3). Deer densities are high along the Columbia River, and where trees may serve as a food source, visual and thermal cover for protection (3). Natural factors that affect homerange include topography, season, food availability, mating activity, population density, and cover density (3). This species has lower and more variable fawn survival, but is compensated by high fecundity rates (4). They will move to higher elevation during hot periods, and lower elevation during colder periods (1)

Population Growth and Control- Mule deer are affected by both Density Dependent and Density Independent factors. Predation (density independent) is the highest cause of mortality, but balances out the high deer densities (4). Mule deer are limited by forage availability (density dependent) and climate (density independent). They also reproduce annually (density dependent). Population Dynamics of mule deer include populations are limited by both forage availability and climate, adult females are limited by forage availability, while fawns are limited by both forage availability and predation, and population growth is constrained by fecundity and fawn predation; and overall can be destabilized if large changes in the abundance of predators or alternative prey change predation risk (4).

Disturbance and Climate Change- In response to human disturbance mule deer will exhibit behavior to avoid risk, resulting in less food availability near anthropological activity; and indirect habitat loss 4.6 times greater than direct habitat loss (2) This is important for management in energy development, as indirect habitat loss due to human avoidance reduces mule deer populations, as mule deer carrying capacity is limited by forage availability (2). Whether directly or indirectly; mule deer populations will decline or slow in response to urbanization and human activity. Immediate response to the disturbance of fire is to flee, but this species may seek out recently burned sites that have more fertile and productive shrubland vegetation. In response to climate, mule deer will migrate due to deep snow and cold temperatures (6). Climate change results in changes in biogeography and vegetation dynamics, resulting in a changing environment and range for the mule deer. Less forage will result in reduced habitat.


Arizona-Sonora Desert Museum. “Animal Fact Sheet- Mule Deer”. Arizona-Sonora Desert Museum. 2008. https://www.desertmuseum.org/kids/oz/long-fact-sheets/Mule%20Deer.php
Dwinell, Samantha, Kevin Monteith, Hall Sawyer, Gary Fralick. “Effects of human disturbance on the Nutrition ecology of mule deer”. Wyoming Game and Fish Department. 2017. https://wyocoopunit.org/projects/effects-of-human-disturbance-on-the-nutritional-ecology-of-mule-deer
Eberhardt, Lester E., Eric E. Hanson, Larry L. Cadwell. “Movement and Activity Patterns of Mule Deer in the Sagebrush-Steppe Region”. The American Society of Mammalogists. Journal of Mammalogy. 1984. https://academic.oup.com/jmammal/article-abstract/65/3/404/850660
Forrester, Tavis and Wittmer, Heiko. “A review of the population dynamics of mule deer and black‐tailed deer Odocoileus hemionus in North America”. Wiley Online Library. 2013. https://onlinelibrary.wiley.com/doi/abs/10.1111/mam.12002
Gibson, Yvette. “Ch 3: Population Ecology”. Oregon State University. 2018.
Monteith, Kevin L, Vernon C. Bleich, Thomas R. Stephenson, Becky M. Pierce, Mary M. Conner. “Timing of seasonal migration in mule deer: effects of climate, plant phenology, and life‐history characteristics”. Ecosphere. Ecological Society of America. 2011. https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/ES10-00096.1
Rich, Terrell. “Habitat and Nest-Site Selection by Burrowing Owls in the Sagebrush Steppe of Idaho”. U.S. Bureau of Land Management. The Journal of Wildlife Management. Vol. 50, pp. 548-555. Oct., 1986. http://www.jstor.org/stable/3800962?seq=1#page_scan_tab_contents
Young, James and Evans, Raymond. “Population dynamics after Wildfire in Sagebrush grasslands”. Journal of Range Management. Vol. 31, No. 4 pp. 283-289. 1978. http://www.jstor.org/stable/3897603?seq=1#page_scan_tab_contents

Species Biology of Sagebrush Steppe

Species Biology can be described by a Species’ life strategies. Life strategies are how an organism allocates energy and materials to be able to compete in an environment, to survive and reproduce. Evolving through natural selection, developing tradeoffs of growth/survival/reproduction; life strategies are a sum of a species’ morphology, physiology, environmental responses, resource requirements, energy acquisition, storage and allocation, reproduction strategy, and life cycle. The main life strategies of Sagebrush Steppe Species evolved to be adaptations to heat and aridity (drought).

Photosynthesis is the foundation of the food-chain, providing energy for all trophic levels. Solar radiation is used to convert H20 and C20 into carbohydrates that produce energy for plants and animals. There are three photosynthetic pathways that evolved/adapted and thrive in different environments: C3, C4, and CAM. Plants are Primary producers, in that they produce energy by using sunlight to synthesize water and carbon dioxide into carbohydrate, for all upper trophic levels of the food chain.
C3 pathway produces 3-Carbonic acid. There is a one step carbon fixation process in which CO2 is fixed by Rubisco directly in the chloroplasts of a plant. C3 plants have the most ancient pathway because they evolved first, during a time period of high CO2 concentration and low O2. Therefore C3 plants can be inhibited by high levels of O2, an issue called photorespiration: where O2 binds to Rubisco instead of CO2. They are cool season plants, sensitive to warm and dry climates (thriving in temperatures 65-75 degrees F).
C4 pathway produces 4-carbonic acid. It can perform the one step function of the C3 pathway; or it can use ATP as energy for a two step process that reduces photorespiration. This two step process involves PEPcase acting as the initial receptor of CO2, not Rubisco. PEPcase has high affinity for CO2 and none for oxygen. Temperature ranges from 90-95 degrees F, so they are warm season plants. C4 plants evolved after C3, during a period with high O2 concentration.
CAM plants have evolved adaptations that conserve water in hot and arid environments, with high evapotranspiration. Stomata open in the nighttime (dark) instead of daytime (light), when CO2 enters the plant. CAM plants start photorespiration with PEPcase without solar radiation, and continue in the daytime when light is available. CAM plants are most closely related to C4 pathway, the most recently evolved pathway.
The dominant types of plants in a Sagebrush Steppe ecosystem are shrubs and grasses including Basin Big Sagebrush, Antelope Bitterbrush, Idaho Fescue, Bluebunch Wheatgrass, Rubber Rabbitbrush, Green Rabbitbrush, Cheatgrass, Ventenata, Sandberg Bluegrass, and Basin Wildrye. The general adaptations are to drought (aridity) and heat, with abundant vegetation in areas with enough precipitation to support shrubs and grasses, but not trees. They survive in the system by lasting through snowy winters and hot, dry summers. The dominant vegetation is plants that can survive in a semi-arid environment. The adaptations to heat and drought include mechanisms to survive the low precipitation, low temperature, heavy winds, and high salinity of semi-arid environments. Sagebrush Steppe ecosystem include plant species adapted for wind-dispersed seed pollination. Soil quality involves clusters of bacteria, algae, moss, and lichen growth. These soil features are heat and arid resistant, as well as fix their own nitrogen. This influences soil stability and erosion control, water infiltration, nitrogen fixation, facilitate seed germination, and nutrient cycling. Whether adaptations of Avoidance (dependent on precipitation) or Tolerance (leaf polymorphism, stem photosynthesis, and phreatophytes to reduce transpiration/photosynthesis) or Resistance (many CAM plants resistant to heat and aridity), plants have evolved to survive in a variety of different environments of heat and drought.

The Species Biology of Animals in Sagebrush Steppe Ecosystems involves behavioral adaptations to heat and drought. The main habit of animals as an adaptation to heat and aridity is Avoidance. Animals can be: nocturnal, where they are active at night (e.g. javelina); crepuscular, where they are active at dawn and dusk (e.g. coyote). To avoid heat; animals may burrow, seek shade of plants, or hide between rocks. Behavioral adaptations have evolved to seek cool micro-climates. Thermal Inertia is an advantage of larger mammals, whose bodies take longer to heat up. The dominant types of animals in Sagebrush Steppe ecosystems include Pygmy Rabbits, Coyotes, Sagebrush voles, Sagebrush lizard, golden eagles, Pronghorn, mule deer, elk, Kangaroo Rat, owls, livestock (cattle and sheep), wild horses, jackrabbits, and Sage Grouse (2). General adaptations include heat and aridity. These animals also depend on Sagebrush ecosystems for energy and nutrition. For example, the Pygmy rabbit is 99% dependent on a diet of Sagebrush in the winter to survive.
Other adaptations to heat and drought include morphological and physiological characteristics. The three categories include heat dissipation, evaporative cooling, and alternate water acquisition. Heat dissipation can involve shedding or in cases like the Jackrabbit, long/tall ears have dilating blood vessels that dissipate body heat to air. Evaporative cooling is when an animal cools itself through evaporating water from it’s surface; such as when an animal pants, or through it’s nasal passages. Alternate water acquisition involves a physiological process that regulates and balances internal water availability of an organism in the face of heat and drought. For example; the Pronghorn eats cholla fruit to obtain water and nutrients when there is limited water. A Kangaroo rat may utilize oxidized water from seeds, or retain water by use of concentrated urea and dry feces.


Gibson, Yvette. “Chapter 2: Species Biology”. Oregon State University. 2018.
U.S. Fish and Wildlife Service. “Why care about America’s Sagebrush”. USFWS. 2014. https://www.fws.gov/mountain-prairie/factsheets/Sage-steppe_022814.pdf

Biogeography of Sagebrush Steppe

Solar Radiation And Geographic Location

Sagebrush Steppe Ecosystems are geographically located in the Northern Mid-Latitude(30 to 45 degrees North) region of approximately 40 million hectare of the Western UnitedStates (3) (Oregon, California, Idaho, Wyoming, Utah, Nevada), providing habitat for 350 vertebrae species (1). Solar radiation reaches Earth as insolation, and due to distance and angle of Earth from the Sun, insolation intensity decreases as you move farther from the equator.Declination is the point where insolation is at a maximum, and moves from 23.45 degrees North to 23.45 degrees South, which results in seasonal change (1). This also results in more variation between day/night length and maximum/minimum daily temperatures as you move away from the Equator, due to Earth’s rotation around the sun and declination (position of sun with respect to the equator). Northern Mid-Latitude regions have the warmest temperatures from June-August and the coldest temperatures from December to February, with a variation below 0 degreesCelsius to above 30 degrees celsius. Sagebrush Steppe ecosystems occur at elevations from 150to 2000 meters (500-6550 ft), with an average elevation of 1235 meters (4052 ft) (1). Vegetation is abundant Sagebrush with other shrubs, grasses, and flowering plants. Terrain is typically flat valleys and plains or gently rolling hills located in the Sierra-Nevada Mountains, CascadeMountains, and Rocky Mountains. There are 3 physiographic regions in the Intermountain West(2) including the Colorado Plateau, Great Basin, and Columbia-Snake River Plateau. Sagebrush steppe does not make up the Colorado Plateau (1).

Weather can be defined as the day to day state of the atmosphere (1). Climate can be defined as the patterns of precipitation, temperature, humidity, barometric pressure and wind over time (1). The main inputs influencing climate are solar radiation, Earth’s atmosphere, andTopography. Solar radiation heats the Earth’s surface producing wind patterns and the water cycle, influencing the distribution of weather patterns. Precipitation is the main determinant of vegetation production (1). In Sagebrush Steppe Ecosystems, precipitation falls in low amounts at9.84 inches or 200-500 mm per year, with an average of 250 mm per year (1). Due to the northern latitude location, elevation, and topography; precipitation falls mainly in the form of snow. A Mid-latitude location results in uneven distribution of solar radiation; influencing variation in photoperiod, which effects growing season of plants, daily duration of
photosynthesis, produces semi-arid plant communities, and an Orographic effect due to theCascades and Sierra-Nevada Mountains to the West and Rockys to the East. Due to the*Orographic effect, Precipitation is driven by global wind patterns and warm air flows inland off of the Ocean and crashes into a mountain. The warm air rises, cools, and the moisture condenses to fall as rain. As the air mass moves over the mountain and loses moisture as rain, the air becomes dry on the other side. Therefore, as warm air moves inland off of the Pacific Ocean, it hits the westward side of the Cascades and Sierra Nevada Mountains, rises and cools and falls as rain on the Westward side of the Mountains, then becomes dry as it rests on the Eastward side of the Mountains. Therefore, the dry air produced by the Mountains settles and produces a semi-arid environment that supports a Sagebrush Steppe Ecosystem.The Sagebrush Steppe Ecosystem land uses include forage for livestock, habitat for wildlife, aesthetic value of open space, cultural heritage of the West, energy development like wind and solar energy, and a high quality water source (4). Landscape considerations include habitat degradation such as fragmentation, erosion, decreased water quality, reduced forage and habitat, fire (suppression), converting land to agriculture, and drought (3).


Climate influences vegetation communities, water cycling, and Sagebrush Steppe ecosystem function because of temperature and the amount of precipitation. Temperature affects the rate of evaporation, degree growing days, and frost period; influencing the type, species, and abundance of a plant community. Sagebrush Steppe Ecosystems have temperatures from 34.6 to100.4 degrees F (1), and are therefore warm and dry in the summer, and colder with frost in the winter. Since precipitation is the driving factor of vegetation production (5); the low amount of precipitation in Sagebrush Steppe Ecosystems results in enough water to support grasses and shrubs, but not enough to support large amounts of trees. In Sagebrush Steppe Ecosystems; water flows through shallow creeks and contributes to the water cycle through evaporation, and is absorbed by plants and contributes to the water cycle through transpiration. A warming climate means more evaporation and less precipitation; which results in less vegetation, increased runoff,and erosion.


Topography influences vegetation communities, water cycling, and Sagebrush Steppe ecosystem function because it influences climate and geology. For example, temperature decreases as elevation increases, precipitation falls as snow (1). The main topography of Sagebrush Steppe is the formation of basins (valleys) and range (Mountains) (1). SagebrushSteppe topography includes flat valley, rolling hills, or foothills. The Orographic Effect (*refer toSolar Radiation and Geographic location) is the main reason that the Topography (Mountains
and Valleys) of the region influences the semi-arid environment of the Sagebrush SteppeEcosystem.


Soil is the primary determinant of the type of plant species in any given area. It’s influence is determined by parent material, soil organic matter, texture and structure, pH and salinity, and physical/biological crusts. Sagebrush Steppe Ecosystems have parent material that is volcanically derived with sand and clay particles. The Soil Orders and Suborders in this ecosystem include Aridisols ((Arigids and Cambids)) but also Mollisol (Ustolls and Xerolls) and Andisol (Xerands) (1). Aridisols occur in arid regions, Mollisols are dark in color because ofOrganic matter and rich in nutrients, and Andisols are derived from volcanic ash. The ColumbiaRiver region contains Loess Soils, while the Great Basin has deep alluvial soil (1). Soil is formed through primary succession of parent material such as Sedimentary, Igneous, and MetamorphicRock. Soil horizons build up as parent material mixes with Organic matter. Soil Organic Matter(SOM) reflects the fertility of a system and influences nutrient cycling, carbon sequestration, soil structure, plant rooting, water infiltration, water holding capacity, and microbe habitat. SOM amount and rate is affected by precipitation and temperature. Sagebrush Steppe has moderate levels of SOM. Soil texture and structure of a Sagebrush Steppe ecosystem is greatly affected by degradation. For example, human activities, wind, and water can put pressure on soil aggregate sand contribute to degradation. However, aggregate stability is important for erosion control,water infiltration, nutrient availability, and facilitating root growth.

(1)Stewart, Kandy. “Sagebrush Steppe Module”. Oregon State University. 2017.
(2)National Park Service. “Sagebrush Steppe”. NPS.gov. 2017.https://www.nps.gov/crmo/learn/nature/sagebrush-steppe.htm
(3)McIver, J.D.; Brunson, M.; Bunting, S.C., and others. 2010. “The Sagebrush SteppeTreatment Evaluation Project (SageSTEP): a test of state-and transition theory.” Gen.Tech. Rep. RMRS-GTR-237. Fort Collins, CO: U.S. Department of Agriculture, ForestService, Rocky Mountain Research Station.https://www.fs.fed.us/rm/pubs/rmrs_gtr237.pdf
(4)U.S Fish and Wildlife Service. “Why Care About America’s Sagebrush”. USFWS. 2014.https://www.fws.gov/mountain-prairie/factsheets/Sage-steppe_022814.pdf
(5)J.D. Bates, T. Svejcar, R.F. Miller, R.A. Angell. “The effects of precipitation timing on sagebrush steppe vegetation”. Journal of Arid Environments. 2005.