EE-Unit-I Grassland ecosystem

A biological community that contains few trees or shrubs, is characterized by mixedherbaceous (nonwoody) vegetation cover, and is dominated by grasses or grasslike plants.Mixtures of trees and grasslands occur as savannas at transition zones with forests or whererainfall is marginal for trees. About 1.2 × 10⁸ mi² (4.6 × 10⁷ km²) of the Earth’s surface iscovered with grasslands, which make up about 32% of the plant cover of the world. In NorthAmerica, grasslands include the Great Plains, which extend from southern Texas into Canada.The European meadows cross the subcontinent, and the Eurasian steppe ranges from Hungaryeastward through Russia to Mongolia; the pampas cover much of the interior of Argentina andUruguay. 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 tosupport a closed herbaceous plant canopy that is lacking in deserts. Thus, temperategrasslands 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 primarilyon plains or rolling topography in the interiors of great land masses, and from sea level toelevations of nearly 16,400 ft (5000 m) in the Andes. Because of their continental location theyexperience large differences in seasonal climate and wide ranges in diurnal conditions. Ingeneral, there is at least one dry season during the year, and drought conditions occurperiodically.

Significant portions of the world’s grasslands have been modified by grazing or tillage or havebeen converted to other uses. The most fertile and productive soils in the world have developedunder grassland, and in many cases the natural species have been replaced by cultivatedgrasses (cereals).

Different kinds of grasslands develop within continents, and their classification is based onsimilarity of dominant vegetation, presence or absence of specific dominant species, orprevailing climate conditions.

The climate of grasslands is one of daily and seasonal extremes. Deep winter cold does notpreclude grasslands since they occur in some of the coldest regions of the world. However, thesuccess of grasslands in the Mediterranean climate shows that marked summer drought is notprohibitive either. In North America, the rainfall gradient decreases from an annual precipitationof about 40 in. (100 cm) along the eastern border of the tallgrass prairie at the deciduous forestto only about 8 in. (20 cm) in the shortgrass prairies at the foothills of the Rocky Mountains. Asimilar pattern exists in Europe. Growing-season length is determined by temperature in thenorth latitudes and by available soil moisture in many regions, especially those adjacent todeserts. Plants are frequently subjected to hot and dry weather conditions, which are oftenexacerbated by windy conditions that increase transpirational 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 arehighly fertile, with well-developed profiles. The soils are rich because rainfall is inadequate forexcessive leaching of minerals and because plant roots produce large amounts of organicmaterial. With less rainfall, grassland soils are shallow, contain less organic matter, frequentlyare 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 leachingfrom higher rainfall, and are frequently red to yellow.

Grassland soils are dry throughout the profile for a portion of the year. Because of their densefibrous root system in the upper layers of the soil, grasses are better adapted than trees tomake 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, andflowers 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 becomessmaller.

There are many more invertebrate species than any other taxonomic group in the grasslandecosystem. Invertebrates play several roles in the ecosystem. For example, many areherbivorous, 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 non arthropod 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 birdspecies inhabit the grassland ecosystem, although many more species are found in theflooding pampas of Argentina than in the dry grasslands of the western United States. Theirrole 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, groundsquirrels, 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 andelk. The most characteristic large mammal species of the North American grassland is thebison, although many of these animals were eliminated in the late 1800s. Mammals includeboth ruminant (pronghorns) and nonruminant (prairie dogs) herbivores, omnivores (opossum),and predators (wolves).

Except for large mammals and birds, the animals found in the grassland ecosystem undergorelatively large population variations from year to year. These variations, some of which arecyclical and others more episodic, are not entirely understood and may extend over severalyears. Many depend upon predator–prey relationships, parasite or disease dynamics, orweather 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, includingbacteria, 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 bylightning. Human inhabitants have also routinely started fires intentionally to remove predatorsand undesirable insects, to improve the condition of the rangeland, and to reduce cover forpredators 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.