Grasslands and prairies

Grasslands occupied vast areas of the world over ten thousand years ago, before the development of agriculture, industrialization, and the subsequent explosive growth of the human population. They are characterized by the presence of low plants, mostly grasses, and are distinguished from woodlands, deserts, and tundra. They experience sparse to moderate rainfall and are found in both temperate and tropical climatic zones. The main grasslands of the planet include the prairies of North America, the pampas and llanos of South America, the steppes of Eurasia, and the savannas and velds of Africa. Grasslands that form in the midst of forested areas are known as "meadows." Grasslands are intermediate between deserts and woodlands in terms of precipitation and biomass. The warmer tropical savannas average 60 to 150 centimeters (25 to 60 inches) of rain. The temperate grasslands range between 25 to 75 centimeters (10 to 30 inches) of precipitation, some of which may be in the form of snow. The biomass of grasslands, predominantly grasses, is quantitatively intermediate between that of deserts and woodlands, which produce 10 to 15 percent and 200 to 300 percent, respectively, of the amount of plant material. It should be recognized that the grassland biomes can be subdivided in terms of climate, plant species, and animal species. It should also be noted that grasslands do not always shift abruptly to deserts or woodlands, leading to gradations between them. In addition, grasslands do have scattered trees, often along streams or lakes, and low-lying brush.

Grasses have extensive root systems and the ability to become dormant. These permit them to survive low rainfall, including periodic droughts, or the winter cold typical of temperate regions. In fact, heavy snow can serve as an insulating layer. Furthermore, grasslands have always been subjected to periodic fires, but the deep root systems of grassland plants also permit them to regrow after fire, although tree and shrubs cannot regrow as easily. These grass root systems help prevent soil erosion and encourage decomposition of decaying matter as well.

Grasses coevolved over millions of years with the grazing animals that depend on them for food. Ten thousand years ago, wild ancestors of cattle and horses, as well as antelope and deer, were on the Eurasian steppes; bison and pronghorn prospered on the North American prairies; wildebeest, gazelle, zebra, and buffalo dominated African savannas; and the kangaroo was the predominant grazer in Australia. Grazing is a symbiotic relationship, whereby animals gain their nourishment from plants, which in turn benefit from the activity. It removes vegetative matter, which is necessary in order for grasses to grow, facilitates seed dispersal, and disrupts mature plants, permitting young plants to take hold. Urine and feces from grazing animals recycle nutrients to the soil and then to the plants. The grassland ecosystem also includes other animals, including worms, insects, birds, reptiles, rodents, some amphibians, and predators. The grasses, grazing animals, and grassland carnivores, such as wolves or large cat species, constitute a food chain.

Humans have been an increasing presence in grassland areas, where over 90 percent of contemporary crop production—grown for human consumption, animal feed, or biomass for biofuels—now occurs and where much urbanization and industrialization has taken place. Remaining grassland areas are not used for crops, habitation, or industry because of inadequate water supplies or unsuitable terrain but instead are used for grazing domesticated or wild herbivores. In addition, many woodland areas around the world have been cleared and converted to grasslands for crops, livestock, living, or working.

The Prairies of North America

Originally stretching east from the Rocky Mountains to Indiana and Ohio, and from Alberta, Canada, to Texas, the prairies were the major grassland of North America. The short-grass prairie extended about two hundred miles (three hundred kilometers) east of the mountains, and the long-grass prairie bordered the deciduous forest along the eastern edge, while the mixed-grass prairie was between the two. Going from west to east, the amount of precipitation increases, causing changes in plant populations. The short-grass prairie receives only about 25 centimeters (10 inches) of precipitation each year, mostly as summer rain, and, as its name suggests, has short grass, less than 60 centimeters (2 feet) tall. Today, it is used primarily for grazing because the soil is shallow and unsuited for farming without irrigation. The mixed-grass prairie receives moderate precipitation, ranging from 35 to 60 centimeters (14 to 24 inches) and has medium-height grasses, ranging from 60 to 120 centimeters (2 to 4 feet) tall. Much of it is now used for growing wheat. The tall-grass prairie receives more than 60 centimeters (24 inches) of precipitation, mostly in the summer, and had grasses that grow to over 150 centimeters (5 feet) tall. It has rich soil and has been mostly converted to very productive cropland, primarily for corn and soybeans. The prairies experience very cold winters (down to −45 degrees Celsius, −50 degrees Fahrenheit) and very hot summers (up to 45 degrees Celsius, 110 degrees Fahrenheit). They are often windy and experience severe storms, blizzards in winter, thunderstorms and tornadoes in summer.

Like other biomes, the prairies have a characteristic assortment of animals, herbivores that eat the plants and carnivores that prey on the herbivores. Before 1500 CE, two ruminants, the bison (commonly but inaccurately called buffalo) and the pronghorn (not a true antelope), were the major grazers on the prairies. The prairie dog, a herbivorous rodent that burrows, lived in large communities on the prairies. The major predators were the wolf and coyote for bison and pronghorn, and the black-footed ferret and fox for prairie dogs. A variety of birds, herbivorous and carnivorous, reptiles, and insects also made their home on the prairie.

Overgrazing Grasslands

While grazing is of mutual benefit to plant and animal, overgrazing is ultimately detrimental to both the plant and animal populations, as well as the environment. Continued heavy grazing leads to deleterious consequences. Removal of leaf tips, even repeated, will not affect regeneration of grasses provided that the basal zone of the plant remains intact. While the upper half of the grass shoot can generally be eaten without deleterious consequences, ingesting the lower half, which sustains the roots and fuels regrowth, will eventually kill the plants. Overgrazing leads to denuding the land, to invasion by less nutritious plant species, to erosion due to decreased absorption of rainwater, and to starvation of the animal species. Because the loss of plant cover changes the reflectance of the land, climate changes can follow and make it virtually impossible for plants to return, with desertification an ultimate consequence. It is not just the number of animals, but the timing of the grazing that can be detrimental. Grasses require time to regenerate, and continuous grazing will inevitably kill them. Consumption too early in the spring can stunt their development.

Semiarid regions are particularly prone to overgrazing because of low and often unpredictable rainfall; regrettably, these are the areas of the world where much grazing has been relegated, because the moister grassland areas have been converted to cropland. Overgrazing has contributed to environmental devastation worldwide. Excessive grazing by cattle, sheep, goats, and camels is partly responsible for the desert of the Middle East, ironically the site of domestication for many animals and plants. Uncontrolled livestock grazing in the late 1800’s and early 1900’s negatively affected many areas of the American West, where sagebrush and juniper trees have invaded the grasslands. Livestock overgrazing has similarly devastated areas of Africa and Asia. In the early twenty-first century, feral horses in the American West and the Australian outback are damaging those environments. Overgrazing by wildlife can also be deleterious. The 1924 Kaibab Plateau deer disaster in the Grand Canyon National Park and Game Preserve is one such example, where removal of natural predators led to overpopulation, overgrazing, starvation, and large die-offs.

Riparian zones, the strips of land on either side of a river or stream, are particularly susceptible to overgrazing. Because animals naturally congregate in these areas with water, lush vegetation, and shade, they can seriously damage them by preventing grasses from regrowing and young trees from taking root, as well as trampling and compacting the soil and fouling the water course. The ecosystem can be devastated, threatening survival of plant and animal species and leading to serious erosion. While herding and fencing can be used to control animals in these areas, a less expensive method is to disperse the location of water supplies and salt blocks to encourage movement away from rivers or streams. If deprived of salt, grassland animals crave it and will seek it out.

Grassland Management

Grassland areas need not deteriorate if properly managed, whether for livestock, wildlife, or both. Managing grasslands involves controlling the number of animals and enhancing their habitat. Carrying capacity, which is the number of healthy animals that can be grazed indefinitely on a given unit of land, must not be exceeded. Because of year-to-year changes in weather conditions and hence food availability, determining carrying capacity is not simple; worst-case estimates are preferred in order to minimize the chances of exceeding it. The goal should be a healthy grassland achieved by optimizing, not maximizing, the number of animals. For private land, optimizing livestock numbers is in the long-term interest of the landowner, although not always seen as such. For land that is publicly held, managed in common, or with unclear or disputed ownership, restricting animals to the optimum level is particularly difficult to achieve. Personal short-term benefit often leads to long-term disaster, described as the “tragedy of the commons” by biologist Garrett Hardin.

Appropriate management of grasslands involves controlling animal numbers and enhancing grassland plants. Restricting cattle and sheep is physically easy through herding and fencing, although it can be politically difficult and expensive. Much more problematic is controlling charismatic feral animals, such as horses, or wildlife, when natural predators have been eliminated and hunting is severely restricted. As for habitat improvement, the use of chemical, fire, mechanical, and biological approaches can increase carrying capacity for either domesticated or wild herbivores. Removing woody vegetation by burning or mechanical means will increase grass cover, fertilizing can stimulate grass growth, and reseeding with desirable species can enhance the habitat. Plants native to a particular region can be best for preserving that environment. Effective grassland management requires matching animals with the grasses on which they graze.

Climatic changes in precipitation levels and frequency, as well as temperature fluctuations, introduce or intensify such pressures as drought, flooding, wildfires, encroachment of nonnative species, including insects, and diseases. Water conservation measures, removal of nonnative species, and intentional planting of diverse native species are some of the mitigation efforts used in managing grasslands and prairies.

Principal Terms

biomass: the dry weight of the living material in an organism, population, or community

biome: region with a particular climate and a characteristic community of living organisms

carnivore: animal dependent on animal material in its diet

carrying capacity: the maximum number of animals that a given area can support indefinitely

denude: strip the covering from, as vegetation from overgrazed grassland

feral: once domesticated but no longer under the control of humans

herbivore: animal dependent on plant material in its diet

ruminant: animal with four stomach compartments that regurgitates its food and rechews it

Bibliography

Bittner, Stephanie. "Grasping Grasslands." ASU–Ask a Biologist, Arizona State U, 1 Mar 2014, askabiologist.asu.edu/plants-grassland. Accessed 7 Oct. 2016.

Collinson, Alan. Grasslands. New York: Dillon, 1992.

Demarais, Stephen, and Paul R. Krausman, eds. Ecology and Management of Large Mammals in North America. Upper Saddle River: Prentice Hall, 2000.

Humphreys, L. R. The Evolving Science of Grassland Improvement. New York: Cambridge UP, 1997.

"Manage Grasslands and Prairie Habitats for Climate Change." Conservation in a Changing Climate, Land Trust Alliance, 2015, climatechange.lta.org/manage-grasslands. Accessed 7 Oct. 2016.

Owen, Oliver S., Daniel D. Chiras, and John P. Reganold. Natural Resource Conservation. 7th ed. Upper Saddle River: Prentice Hall, 1998.

Pearson, C. J., and R. L. Ison. Agronomy of Grassland Systems. 2nd ed. New York: Cambridge UP, 1997.

Steele, Philip. Grasslands. Minneapolis: Carolrhoda, 1997.