Grasslands are areas where the vegetation is dominated by grasses (Poaceae) and other herbaceous (non-woody) plants (forbs). Plants of the sedge (Cyperacae) and rush (Juncaceae) families can also be frequent in grasslands. Grasslands occur naturally on all continents except Antarctica, and in many other areas they have replaced the natural vegetation due to human influence. In temperate latitudes, such as north-west Europe, grasslands are dominated by perennial species, whereas in warmer climates annual species form a greater component of the vegetation.[1] Grasslands are quite varied in lifeforms, and can be found in most terrestrial climates. Grassland vegetation can vary in height from very short, as in chalk downland where the vegetation may be less than 30 cm high, to quite tall, as in the case of North American tallgrass prairie, South American grasslands and African savannah. Woody plants, shrubs or trees, may occur on some grasslands - forming wooded, scrubby or semi-wooded grassland, such as the African savannahs or the Iberian dehesa. Grasslands cover nearly fifty percent of the land surface of the continent of Africa.[2] While grasslands in general support diverse wildlife, given the lack of hiding places for predators, the African Savannah regions support a much greater diversity in wildlife than do temperate grasslands.[3] The appearance of mountains in the western United States during the Miocene and Pliocene epochs, a period of some 25 million years, created a continental climate favorable to the evolution of grasslands. Existing forest biomes declined, and grasslands became much more widespread. Following the Pleistocene Ice Ages, grasslands expanded in range in the hotter, drier climates, and began to become the dominant land feature worldwide.[4] As flowering plants, grasses grow in great concentrations in climates where annual rainfall ranges between 500 mm and 900 mm (approximately 20 to 35 inches).[5] The root systems of perennial grasses and forbs form complex mats that hold the soil in place. Mites, insect larvae, nematodes and earthworms inhabit deep soil, which can reach 20 feet underground in undisturbed grasslands on the richest soils of the world. These invertebrates, along with symbiotic fungi, extend the root systems, break apart hard soil, enrich it with urea and other natural fertilizers, trap minerals and water and promote growth (Chadwick 1995). Some types of fungi make the plants more resistant to insect and microbial attacks.
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Climate and grasslands
Grasslands receive about 380 to 900 mm (15 to 36 in) of rain per year, as compared with deserts, which receive less than 300 mm (12 in) and tropical rainforests, which receive more than 2,000 mm (80 in).[6] While extreme temperatures may occur in some grasslands, normally they range between -20 to 30 degrees Celsius. Tropical grasslands have dry and wet seasons, but remain warm all the time. Temperate grasslands have cold winters and warm summers with rain or some snow. Since some grasses die back above ground annually, the soil and the sod protect the roots and the new buds from the cold of winter or dry conditions.
Grassland biodiversity and conservation
The true grasslands are those where wild-plant communities naturally occur. This is where grasslands have remained undisturbed by agricultural improvement. In many parts of the world, examples that have escaped agricultural improvement (fertilising, weed killing, ploughing or re-seeding) contain many species of wild plants - grasses, sedges, rushes and herbaceous species - 25 or more species per square metre is not unusual. Chalk downlands in England can support over 40 species per square metre. However, such examples, such as original North American prairie or lowland meadows in the UK, are now rare and their associated wild flora equally threatened. Associated with the wild-plant diversity of the "unimproved" grasslands is usually a rich invertebrate fauna; also there are many species of birds that are grassland "specialists" - for example snipe, or the Great Bustard. Agriculturally improved grasslands, which dominate modern agricultural landscapes, are usually poor in wild plant species due to the original diversity of plants having been destroyed by cultivation, the original wild-plant communities having been replaced by sown monocultures of cultivated varieties of grasses and clovers, such as Perennial ryegrass and White Clover. In many parts of the world "unimproved" grasslands are one of the most threatened habitats, and a target for acquisition by wildlife conservation groups or for special grants to landowners who are encouraged to manage them appropriately.
Human impact and economic importance
Grasslands are of vital importance for raising livestock for human consumption and for milk and other dairy products. Grassland vegetation remains dominant in a particular area usually due to grazing, cutting (such as for hay), or natural or man-made fires, all discouraging colonisation by and survival of tree and shrub seedlings. Fire was used in North America by the Native Americans as a means of maintaining grassland. Some of the world's largest expanses of grassland are found in African savannah, and these are maintained by wild herbivores as well as by nomadic pastoralists and their cattle, sheep or goats. Grasslands may occur naturally or as the result of human activity. Grasslands created and maintained by human activity are called anthropogenic grasslands. Hunting peoples around the world often set regular fires to maintain and extend grasslands, and prevent fire-intolerant trees and shrubs from taking hold. The tallgrass prairies in the American Midwest may have been extended eastward into Illinois, Indiana, and Ohio by human agency. Much grassland in north-west Europe developed after the Neolithic Period, when people gradually cleared the forest to create areas for raising their livestock.
Types of grassland
Tropical and subtropical grasslands
These grasslands are classified with tropical and subtropical savannas and shrublands as the tropical and subtropical grasslands, savannas, and shrublands biome. Notable tropical and subtropical grasslands include the Llanos grasslands of northern South America.
Temperate grasslands
Mid-latitude grasslands, including the Prairie of North America, the Pampa of Argentina, calcareous downland, and the steppes of Europe. They are classified with temperate savannas and shrublands as the temperate grasslands, savannas, and shrublands biome. Temperate grasslands are the home to many large herbivores, such as bison, gazelles, zebras, rhinoceroses, and wild horses. Carnivores like lions, wolves and cheetahs are also found in temperate grasslands. Other animals of this region include: deer, prairie dogs, mice, jack rabbits, skunks, coyotes, snakes, fox, owls, badgers, blackbirds, grasshoppers, meadowlarks, sparrows, quails, and hawks.
Flooded grasslands
Grasslands that are flooded seasonally or year-round, like the Everglades of Florida or the Pantanal of Brazil, Bolivia and Paraguay. They are classified with flooded savannas as the flooded grasslands and savannas biome and occur mostly in the tropics and subtropics.
Montane grasslands
High-altitude grasslands located on high mountain ranges around the world, like the Páramo of the Andes Mountains. They are part of the montane grasslands and shrublands biome, and also constitute alpine tundra.
Polar grasslands
Similar to montane grasslands, arctic tundra can have grasses. However high soil moisture means that few tundras are grass-dominated today. However, during the Pleistocene ice ages, a polar grassland known as steppe-tundra occupied large areas of the Northern hemisphere.
Xeric grasslands
Also called desert grasslands, these are sparse grasslands located in deserts and xeric shrublands ecoregions.
See also
- Coastal plain
- Dambo
- Desert
- Field
- Flooded grasslands and savannas
- Flood-meadow
- Llanos
- Meadow
- Pampa
- Pasture
- Plain
- Plateau
- Prairie
- Savanna
- Steppe
- Taiga
- Tundra
- Veld
- Water-meadow
- Wet meadow
References
- ^ NASA Earth Observatory webpage
- ^ University of California Museum of Paleontology Grasslands website
- ^ University of California- Santa Barbara Temperate Grasslands website
- ^ University of California Museum of Paleontology Grasslands website
- ^ NASA Earth Observatory webpage
- ^ University of California Museum of Paleontology Grasslands website
