Northwestern mixed grasslands
Northwestern mixed grasslands are a semiarid ecosystem located in the central Great Plains of North America, encompassing regions of southern Alberta, Saskatchewan, Montana, and the Dakotas. This area features a temperate climate with significant seasonal temperature variations, receiving between 12 to 18 inches of precipitation annually, mostly in the form of snow during the winter months. The grasslands are characterized by a diverse range of drought-resistant vegetation, primarily short- and mid-height grasses like blue grama and needle-and-thread, alongside flowering plants and shrubs that enhance biodiversity.
Historically, these grasslands supported vast populations of bison, which played a crucial role in shaping the ecosystem. However, human activities have drastically altered the landscape, with only about 15% of the original grasslands remaining intact due to agriculture and development. Key species such as the black-tailed prairie dog serve as ecological keystones, supporting other wildlife like the endangered black-footed ferret. Conservation efforts are ongoing to manage livestock grazing and restore native habitats, as these grasslands are vital for carbon sequestration and maintaining ecological balance amid concerns about climate change. Understanding the dynamics of this ecosystem is essential, as it reflects the intersection of natural history and human impact in the American West.
Northwestern mixed grasslands
- Category: Grassland, Tundra, and Human Biomes.
- Geographic Location: North America.
- Summary: These extensively altered semiarid grasslands are characterized by drought-resistant plants and animals; the vegetation dynamics are largely shaped by grazing and burrowing rodents.
The Northwestern Mixed Grasslands ecosystem is located in the central Great Plains of North America. It is part of the temperate grasslands biome, covering the southern portions of Alberta and Saskatchewan in the north, and extending southward into Montana, North and South Dakota, Wyoming, and Nebraska. These grasslands gradually increase in elevation from east to west, reaching their maximum height at the base of the majestic Rocky Mountains. In the few areas still in natural vegetation, the grasslands provide expansive views of mildly rolling topography that eventually changes into forested habitat at the base of the Rockies.
The northwestern mixed grasslands are part of the iconic history of the American West, where bison used to number in the millions and early settlers traveled in covered wagons to homestead Western lands. Today, little of the northwestern mixed grasslands remain untouched by human activity. While they are an important economic resource because of their utility for agriculture, the associated loss of native biodiversity makes them of conservation concern.
The northwestern mixed grasslands have a semiarid climate, with precipitation ranging from 12 to 18 inches (30 to 45 centimeters) annually. Precipitation decreases along an east-to-west gradient, with the westernmost portion of the grasslands located in the rain shadow of the Rocky Mountains. The climate is mid-continental, with pronounced seasons characterized by warm summers and extremely cold winters. Marked variations in temperature, coupled with low water availability, make the climate harsh. Heavy snow blankets the region in winter and provides much of the annual precipitation.
These grasslands are also windy and subject to severe summer storms. However, summer rains are often short-lived and may provide only enough precipitation to infiltrate the shallow topsoil layers. As a result, plants are often water-limited because they lose more water to the atmosphere through evaporation than enters the system through precipitation. For this reason, many of the plants as well as animals have physiological or behavioral adaptations to tolerate low water availability. Grasses have several adaptations for surviving drought, including dense, fibrous root systems that effectively capture moisture, and narrow leaf blades that lose less water to the atmosphere than broader leaves do. Small mammals, including some species of rodent, can extract water effectively from food items such as seeds.
Vegetation
The dominant natural vegetation of the northwestern mixed grasslands is composed of both short- and mid-height grasses. Representative species include blue grama (Bouteloua gracilis), needle-and-thread (Stipa comata), and western wheat grass (Agropyron smithii). Although grasses dominate in terms of their abundance, forbs (flowering plants that are not grasses) make the main contribution to plant-species diversity. It is common to see colorful explosions of the showy flowers of asters and legumes (members of the pea family) during the spring rains.
Several shrub species, including sage (Artemisia tridentata), contribute to the structural diversity of the grasslands by providing safe havens from predators and inclement weather to the rodents and rabbits.
Fauna
Historically, bison (Bison bison americana) fed on the grasses that dominate the northwestern mixed grasslands. As a result, some of the grasses, especially those that are of smaller stature, are tolerant to grazing by ungulates. Unfortunately, the bison were nearly exterminated by early European settlers, despite having numbered an estimated 50 million head in the early 1800s. They now exist predominantly in managed populations. In fact, pronghorns (Antilocapra americana) are the only large native ungulates that are still common here.
The black-tailed prairie dog (Cynomys ludovicianus) is also an important herbivore in the northwestern mixed grasslands system. It is considered to be a keystone species because it has marked effects on soil and vegetation characteristics, which influence the distribution and abundance of associated plant and animal species.
The endangered black-footed ferret (Mustela nigripes) is entirely dependent on prairie dogs, both as prey and because the ferret lives in the extensive burrow systems constructed by the prairie dogs. As prairie-dog populations declined because of extermination campaigns (they were viewed as pests by livestock owners), loss of habitat, and sylvatic plague, the ferret populations declined along with them. Today, captive breeding programs have brought the black-footed ferret back from the brink of extinction.
Effects of Human Activity and Threats
Grassland ecosystems worldwide have been important in shaping patterns of human settlement, because the fertile soils that underlie grasslands support many crop species, and the native grasses provide good forage for cattle and other livestock. Today, only 15 percent of the northwestern mixed grasslands remains in its native state, with much of it having been converted for agriculture or having been degraded due to various human activities. In Canada, the losses are particularly pronounced, with only 2 percent of the native grasslands remaining.
In areas used for livestock grazing, there is the potential to maintain a healthy, functioning ecosystem by using appropriate stocking rates. In fact, rangelands are considered to be conservation resources, because grazing, when managed properly, simply mimics the historic disturbance regime imposed by native grazers such as bison.
A conservation issue more difficult to address is the total conversion of these grasslands for other types of land use. In the Canadian mixed grasslands, disturbances such as road building are associated with increasing oil and gas development. These are fragmenting the remaining native habitat, while many rangelands are being converted for hay crops. Long-term effects from climate change are unknown, but the robust and rugged mixed grasslands may be resilient enough to withstand climate change. The biome has survived many drought periods, including super-droughts, in the past. Preserving the grasslands, which sequester billions of tons of carbon, is of concern due to global climate change and the need to reduce emissions worldwide.
Bibliography
Coupland, Robert T., ed. Natural Grasslands: Introduction and Northwestern Hemisphere. Amsterdam, Netherlands: Elsevier, 1992.
“Northwestern Great Plains Mixedgrass Prairie.” Nature Serve Explorer, 31 May. 2024, explorer.natureserve.org/Taxon/ELEMENT‗GLOBAL.2.722984/Northwestern‗Great‗Plains‗Mixedgrass‗Prairie. Accessed 4 Dec. 2024.
Ricketts, Taylor H., et al. Terrestrial Ecoregions of North America: A Conservation Assessment. Washington, DC: Island Press, 1999.
Thompson, et al. Grasslands, Savannas, Shrublands—Northern Mixed Grassland. U.S. Geological Survey Professional Papers, 2007, . Accessed 4 Dec. 2024.
Wood, Stephanie. "Meet the People Saving Canada's Native Grasslands." The Narwhal, 31 July 2020, thenarwhal.ca/carbon-cache-grasslands/. Accessed 4 Dec. 2024.