Grassland

A grassland is a biome, an area of land that mostly has grasses. There are wild grasses, and there may be some trees.
Several parts of the world have grasslands. Grasslands are found in Africa, North America, Central Asia, South America, and near the coasts of Australia. The largest grasslands are in East Africa. Grasslands with a few scattered trees are called savannas. Others are called prairies or steppes.
Evolutionary history
The graminoids became widespread toward the end of the Cretaceous period. The appearance of mountains in the western United States during the Miocene and Pliocene epochs created a continental climate which was favourable to the evolution of grasslands.
About five million years ago, in the Miocene in the New World and the Pliocene in the Old World, the first true grasslands grew. Existing forest biomes declined, and grasslands became much more widespread.
Vegetation
Grassland vegetation can vary greatly in height. The chalk grassland of southern England is quite short, often with small, delicate flowers.
Grass is quite tall in the Mississippi "tallgrass prairie", the South American grasslands and the African savannas. This provides a degree of cover for the smaller animals, and so increases the number of species which can live there.
Woody plants, shrubs or trees, may occur on some grasslands. The root systems of perennial grasses and forbs (herbaceous flowering plants) form complex mats that hold the soil in place.
Degradation
When shrubs and bushes can grow more and replace grass, this is called woody plant encroachment. This change may be harmful. It can make the number of plant species fewer. It allows less water to go into the ground and animals have less to eat. For that reason woody encroachment is often called a form of land degradation.
But this is not always the case. Encroachment can lead to good changes in nature.
North America
The tallgrass prairie is a native ecosystem of the drainage basin of the Mississippi. Retreating glaciers dropped moraine material about 10,000 years ago. Wind-dropped loess and organic matter accumulated. This made the soil. The prairie had the deepest level of topsoil recorded anywhere. For 5,000 to 8,000 years, more than of prairie grasslands were a major feature of the landscape.
Animals such as buffalo, elk, deer, and rabbits lived there. The animals added nitrogen to the soil with their urine and faeces. Prairie dogs dug tunnels that "aerated the soil and channeled water several feet below the surface".
Between 1800 and 1930, most of this natural environment was destroyed. Settlers transformed what they called "the Great American Desert" or "The Inland Sea" into farmland. They replaced the old grasses with new ones, mostly wheat and maize, grasses that are ecologically different from the original cereals. They replaced the bison with cattle, another kind of bovine. About 40% of the world's maize grows in the United States, mostly on land that formerly grew grass. The grazing pattern of European cattle, the near-extermination of prairie dogs, and the plowing and cultivation of the land did the damage. Plowing cut tallgrass root systems and interrupted reproduction. Drainage changed the soil's water content, and soil erosion lost soil.
Estimates differ of how much original tallgrass prairie survives. Perhaps less than 1% to 4%, mostly in "scattered remnants found in pioneer cemeteries, restoration projects, along highways and railroad rights-of-way, and on steep bluffs high above rivers".
Plants and animals
In many other parts of the world, the grasslands have little rain or long dry periods. There are few trees because of the low rainfall. Thorny trees called acacias are one of a handful of trees that are able to grow in some African savannas. Wildfires are common and destroy trees. The animals also prevent trees from growing as they eat the tender sprouts before they can develop into grown trees. Elephants are known to tear down grown-up trees and feed on their leaves, making trees even more scarce.
Grass can still grow because it survives the aridity and the trampling of the animals. This is why grasslands can feed vast numbers of animals. Animals include mice, rats that eat mostly seeds. Snakes, hawks and eagles eat mostly small animals. Big animals, mostly grazers, include zebras, ostriches, giraffes, rhinos, elephants, horses, and antelopes.