Prairie

Hundreds of species of insects, birds, reptiles, amphibians and mammals make their home in prairie. Wildlife species living in prairie have adapted to a semi-arid, windy, open environment. Many animals, including elk, deer, coyote and bobcat, spend all or part of their lives in or near coulees and river valleys where they find shelter from summer’s heat and winter’s cold as well as cover for raising their young. Badgers, jackrabbits, ground squirrels, pocket gophers and other small mammals live in burrows to escape the extreme heat and cold. Migratory birds escape winter by flying south.  Reptiles (e.g. prairie rattlesnake and short-horned lizard) and amphibians (e.g. plains spadefoot and Great Plains toad) hibernate underground.

For millennia immense herds of bison roamed the grasslands, leaving large swaths of prairie grazed and trampled in their seasonal round. Grizzly bear and gray wolf, once abundant on the prairie, were the main predators of bison. Today, domestic livestock are the main grazers of prairie grasses. Some prairie wildlife (eg. Richardson’s ground squirrel, burrowing owl and McCowan’s longspur) are adapted to thrive in heavily grazed prairie, whereas other species (eg. Meadow vole, Sprague’s pipit, Baird’s sparrow) prefer lightly grazed prairie. Most wildlife frequents grasslands with moderate levels of grazing.

Riparian areas, or those areas adjacent to water sources, are an important component of prairie wildlife habitat. Not only do numerous species of waterfowl, shorebirds and other water birds rely on prairie wetlands for breeding habitat or temporary resting sites during migration, but many North American songbirds rely on riparian areas for all or part of their lifecycle. The same is true for prairie mammals and amphibians.

Historically, tall-grass prairie extended from southeastern Manitoba southward along the eastern fringe of the American Great Plains (Figure 1). Mixed-grass prairie extended from Canada’s Prairie Provinces south to central Texas. In the rain shadow of the southern Rocky Mountains native prairie vegetation was dominated by short grasses (e.g. buffalo grass and hairy grama) mixed with drought resistant shrubs (e.g. acacia, mesquite, live oak). The central grasslands of northern Mexico were dominated by even more drought resistant short grasses (e.g. grama species, three-awn species, ear muhly, burrograss) and narrow-leaved prickly shrubs and cactus. 

Mixed grasslands dominate the native vegetation of the drier south-central part of Canada’s prairie region.  This vegetation type is a mix of short grasses (e.g. blue grama grass and June grass) and mid-height grasses (e.g. wheat grasses, needle-and-thread, and porcupine grass). The composition of dominant grasses varies according to slope and soils. Northward and westward, as moisture increases, rough fescue grasslands dominate.  In still moister areas of southeastern Manitoba, native grasslands are characterized by tall grasses (big bluestem, Indian grass, prairie cordgrass, and switchgrass). At the prairie fringe, grasslands intermingle with aspen woodlands, or oak woodlands in Manitoba, to form parkland.   

Plants of the Canadian prairies are adapted to survive winter cold and severe summer moisture deficits with the resulting high frequency of fires.  They have co-evolved with grazers – large mammals such as bison, small mammals such as ground squirrels and insects such as grasshoppers. 

Grasses, the predominant cover in prairie vegetation, have the following adaptations:

Deep or extensive roots.  Bunch grasses have roots up to 4 metres deep to help secure soil moisture and nutrients. Rhizomatous grasses have fast-growing and wide-spreading horizontal root systems that allow capture of moisture and nutrients over a broad area at shallow depth.

Growing points at or near the soil surface that are tucked in the crown of the plant. Here they are protected from dessication by wind, from severe cold, from damage by the hooves and teeth of large herbivores and from all but the hottest fires. If leaves are eaten or trampled, new ones grow again from the crown.

Narrow leaves. Less leaf surface area results in less evaporation and drought stress. The leaves of grasses are often inrolled, curled or folded to help protect stomata (leaf pores) that are located on the adaxial (upper) surface and hence lower drought stress.

Small flowers in dense heads that are pollinated by wind.  Wind is a prevailing force in the prairies and easily can carry the abundance of pollen produced by grasses to fertilize related plants near or far.

Tough stems and leaves hardened with silica phytoliths (plant stones). Erect flower stalks enhance wind pollination. Erect leaves minimize surface area exposed to direct sunlight. Phytoliths in leaves may also be an adaptation to discourage grazing animals by wearing down tooth enamel. Herbivorous mammals such as horses, rabbits and rodents have evolved high-crowned molars that continue to grow through their lives to compensate for the abrasion from eating such gritty food – an example of co-evolution.

Strategies to optimize photosynthesis without losing moisture.  Cool-season grasses, such as needle-and-thread, use C3 photosynthesis that is efficient but requires leaf pores be open.  Cool-season grasses are dormant through the winter, renew growth in early spring, mature by early summer, become semi-dormant during hot summer months, and renew growth in autumn. Alternatively, warm-season grasses, such as blue grama, use C4 photosynthesis that is less efficient but allows plants to continue photosynthesis with leaf stomata closed due to structural differences in the leaf cells. Warm-season grasses renew growth late in the spring and continue to grow and flower during summer heat and drought reentering dormancy in the autumn.

There are many more species of forbs than grasses in prairie but they seldom dominate plant communities because they are not as well adapted to the prairie environment.  A small proportion of prairie forbs and grasses are annuals; that is, they complete their life-cycle in one season.  Annual plants evade freezing and drought by persisting as seed through the winter, germinating in spring and completing growth and seed set by early summer before the soil dries out.  

Most prairie forbs are perennials; that is, they persist for more than two growing seasons.  Perennial plants are usually better competitors than annual plants, especially under stable, resource-poor conditions. Some of their adaptations include root systems that increase access to water and nutrients (such as deep taproots, extensive rhizomes, and underground storage structures such as tubers, corms or bulbs), leaf adaptations that reduce water loss and herbivory (specialized hairs and thick leathery epidermis minimize water loss, narrow leaves and compound leaves reduce surface area exposed to evaporation), and unpalatable or toxic substances to dissuade grazers.

Most prairie shrubs and trees occur in river valleys, coulees, sand hills and in depressions where fresh water is close to the surface and there is shelter from winds that cause evaporative stress. Cottonwood, the predominant tree in prairie river valleys, has a deep taproot that draws on floodplain aquifers and also has the ability to shed outer branches during times of drought stress.  Silver sagebrush grows on sites where episodic runoff events provide more water and sediment than typical for well drained uplands. Aspen and many upland prairie shrubs have extensive underground stems (rhizomes) that not only help to secure water but also produce vegetative sprouts following fire or soil disturbance.  Some prairie shrubs have thorns or prickles to dissuade browsers.  As with prairie grasses and forbs, many shrubs have leaf adaptations to reduce water loss such as reduced surface area, hairs and thick epidermis.

Canada’s prairie is underlain by flat-lying sedimentary bedrock of Cretaceous age, scoured by the continental glaciers and over which was deposited till and sorted fluvial and lacustrine sediments as the ice receded 12,000 years ago. Landforms include glacial lake beds, morainal plains and hills, sand dunes, meltwater channels, and river valleys.

Soils developed under grassland vegetation are mainly Chernozems, characterized by a relatively thick, dark-coloured surface (A) horizon resulting from the accumulation and decomposition of organic matter derived from prairie plants. Some of the highest levels of carbon in the world are sequestered in intact prairie chernozems. Weakly developed soils known as Regosols are associated with sand dunes, active floodplains and steep slopes. Solonetzic soils that are hard when dry and sticky when wet develop in prairie areas where sodium salts occur close to the surface. Nutrient-poor soils known as Gleysols are associated with wetlands where decomposition is hindered by flooding. Luvisols develop under woodlands in parkland and are characterized by accumulation of clay in moist lower horizons.

Prairie Ecosystem Processes

While prairie is often characterized by its individual components, it is also influenced by natural processes. The prairie is a dynamic landscape modified over time by: Fire, Precipitation (i.e., flood, drought), Grazing, Wind

The range of variation of these processes is a critical component of prairie bio-diversity.

Fire: Prior to western colonization, periodic lightning fires ignited the dry prairie vegetation. Wildfires typically burned surface growth, but usually did not kill off the deep root systems of prairie grasses. Fire also helped impede the colonization of new sites by invader plant species, altered soil structure, and reduced insect populations. When natural fires didn’t occur, aboriginal people set fires, to stimulate vegetative growth and bring back animals that depended on early successional vegetation. In the last century prairie grass fires have been routinely suppressed.

Precipitation (i.e. Flood & Drought)
Drought: The influence of drought on prairie ecosystems can be complex. An 11-year study in Minnesota found that diverse plant communities are more resistant to, and recover more fully from, major drought. Such findings raise concerns about resilience and maintenance of productivity on prairie lands that have been converted to less diverse crop land and tame pasture.
Flooding: Periodic floods deposit seeds and rich silts on river valley floors, and create ideal seedbed conditions for the rejuvenation of riparian cottonwood forests.

Grazing: Grazing helps keep a prairie in grass. Grass flourishes under the onslaught of ungulate teeth and prairie forbs respond by growing into bushier plants. Grazing prevents trees from overtaking the grasses, because tree seedlings are destroyed when their tips are eaten. Recent evidence using analysis of pollen from prairie lake sediments suggests grazing by bison checked aspen encroachment onto grasslands. The variability of grazing activity created by different animals at different times is a vital part of the ecosystem. While most birds and animals flourish under moderate grazing regimes, species such as ground squirrels and mountain plovers need heavily grazed areas to exist. Meadow voles and Le Conte’s sparrows are two species that thrive only in lightly grazed areas.

Wind: Wind erosion keeps some sand dunes active, providing a home for kangaroo rats and western spiderworts.

¹ Alberta Environmental Protection, The Grasslands Natural Region of Alberta, (March 1997), p. 5.
² Ibid
³ Ibid, p. 22