Wednesday, May 11, 2011

Grassland ecosystem

Grassland ecosystem

            A biological community that contains few trees or shrubs, is characterized by mixed herbaceous (non woody) vegetation cover, and is dominated by grasses or grass like plants. Mixtures of trees and grasslands occur as savannas at transition zones with forests or where rainfall is marginal for trees. About 1.2 × 108 mi2 (4.6 × 107 km2) of the Earth's surface is covered with grasslands, which make up about 32% of the plant cover of the world. In North America, grasslands include the Great Plains, which extend from southern Texas into Canada. The European meadows cross the subcontinent, and the Eurasian steppe ranges from Hungary eastward through Russia to Mongolia; the pampas cover much of the interior of Argentina and Uruguay. Vast and varied savannas and velds can be found in central and southern Africa and throughout much of Australia.

             Grasslands occur in regions that are too dry for forests but that have sufficient soil water to support a closed herbaceous plant canopy that is lacking in deserts. Thus, temperate grasslands usually develop in areas with 10–40 in. (25–100 cm) of annual precipitation, although tropical grasslands may receive up to 60 in. (150 cm). Grasslands are found primarily on plains or rolling topography in the interiors of great land masses, and from sea level to elevations of nearly 16,400 ft (5000 m) in the Andes. Because of their continental location they experience large differences in seasonal climate and wide ranges in diurnal conditions. In general, there is at least one dry season during the year, and drought conditions occur periodically.

            Significant portions of the world's grasslands have been modified by grazing or tillage or have been converted to other uses. The most fertile and productive soils in the world have developed under grassland, and in many cases the natural species have been replaced by cultivated grasses (cereals).
           Different kinds of grasslands develop within continents, and their classification is based on similarity of dominant vegetation, presence or absence of specific dominant species, or prevailing climate conditions.

           The climate of grasslands is one of daily and seasonal extremes. Deep winter cold does not preclude grasslands since they occur in some of the coldest regions of the world. However, the success of grasslands in the Mediterranean climate shows that marked summer drought is not prohibitive either. In North America, the rainfall gradient decreases from an annual precipitation of about 40 in. (100 cm) along the eastern border of the tall grass prairie at the deciduous forest to only about 8 in. (20 cm) in the shortgrass prairies at the foothills of the Rocky Mountains. A similar pattern exists in Europe. Growing-season length is determined by temperature in the north latitudes and by available soil moisture in many regions, especially those adjacent to deserts. Plants are frequently subjected to hot and dry weather conditions, which are often exacerbated by windy conditions that increase inspirational water loss from the plant leaves.
        Soils of mesic temperate grasslands are usually deep, about 3 ft (1 m), are neutral to basic, have high amounts of organic matter, contain large amounts of exchangeable bases, and are highly fertile, with well-developed profiles. The soils are rich because rainfall is inadequate for excessive leaching of minerals and because plant roots produce large amounts of organic material. With less rainfall, grassland soils are shallow, contain less organic matter, frequently are lighter colored, and may be more basic. Tropical and subtropical soils are highly leached, have lower amounts of organic material because of rapid decomposition and more leaching from higher rainfall, and are frequently red to yellow.

        Grassland soils are dry throughout the profile for a portion of the year. Because of their dense fibrous root system in the upper layers of the soil, grasses are better adapted than trees to make use of light rainfall showers during the growing season. When compared with forest soils, grassland soils are generally subjected to higher temperatures, greater evaporation, periodic drought, and more transpiration per unit of total plant biomass.

         Throughout the year, flowering plants bloom in the grasslands with moderate precipitation, and flowers bloom after rainfall in the drier grasslands. With increasing aridity and temperature, grasslands tend to become less diverse in the number of species; they support more warm-season species; the complexity of the vegetation decreases; the total above-ground and below-ground production decreases; but the ratio of above-ground to below-ground biomass becomes smaller.

        There are many more invertebrate species than any other taxonomic group in the grassland ecosystem. Invertebrates play several roles in the ecosystem. For example, many are herbivorous, and eat leaves and stems, whereas others feed on the roots of plants. Earthworms process organic matter into small fragments that decompose rapidly, scarab beetles process animal dung on the soil surface, flies feed on plants and are pests to cattle, and many species of invertebrates are predaceous and feed on other invertebrates. Soil nematodes, small nonarthropod invertebrates, include forms that are herbivorous, predaceous, or saprophagous, feeding on decaying organic matter.

         Most of the reptiles and amphibians in grassland ecosystems are predators. Relatively few bird species inhabit the grassland ecosystem, although many more species are found in the flooding pampas of Argentina than in the dry grasslands of the western United States. Their role in the grassland ecosystem involves consumption of seeds, invertebrates, and vertebrates; seed dispersal; and scavenging of dead animals.

         Small mammals of the North American grassland include moles, shrews, gophers, ground squirrels, and various species of mice. Among intermediate-size animals are the opossum, fox, coyote, badger, skunk, rabbit, and prairie dog; large animals include various types of deer and elk. The most characteristic large mammal species of the North American grassland is the bison, although many of these animals were eliminated in the late 1800s. Mammals include both ruminant (pronghorns) and non ruminant (prairie dogs) herbivores, omnivores (opossum), and predators (wolves).

         Except for large mammals and birds, the animals found in the grassland ecosystem undergo relatively large population variations from year to year. These variations, some of which are cyclical and others more episodic, are not entirely understood and may extend over several years. Many depend upon predator–prey relationships, parasite or disease dynamics, or weather conditions that influence the organisms themselves or the availability of food, water, and shelter.
        Within the grassland ecosystem are enormous numbers of very small organisms, including bacteria, fungi, algae, and viruses. From a systems perspective, the hundreds of species of bacteria and fungi are particularly important because they decompose organic material, releasing carbon dioxide and other gases into the atmosphere and making nutrients available for recycling. Bacteria and some algae also capture nitrogen from the atmosphere and fix it into forms available to plants.

         Much of the grassland ecosystem has been burned naturally, probably from fires sparked by lightning. Human inhabitants have also routinely started fires intentionally to remove predators and undesirable insects, to improve the condition of the rangeland, and to reduce cover for predators and enemies; or unintentionally. Thus, grasslands have evolved under the influences of grazing and periodic burning, and the species have adapted to withstand these conditions. If burning or grazing is coupled with drought, however, the grassland will sustain damage that may require long periods of time for recovery by successional processes.

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