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

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