Grassland
A grassland is an area where the vegetation is dominated by grasses. Sedges and rushes can also be found along with some of legumes such as clover, and other herbs. Grasslands occur naturally on all continents except Antarctica and are found in most ecoregions of the Earth. Furthermore, grasslands are one of the largest biomes on Earth, covering 31–69% of the Earth's land area. There are different types of grasslands: natural grasslands, semi-natural grasslands, and agricultural grasslands.
Definitions
Included among the variety of definitions for grasslands are:- "...any plant community, including harvested forages, in which grasses and/or legumes make up the dominant vegetation."
- "...terrestrial ecosystems dominated by herbaceous and shrub vegetation, and maintained by fire, grazing, drought and/or freezing temperatures."
- "A region with sufficient average annual precipitation to support grass..."
- Grassland existing as a result of human activity, where environmental conditions and the species pool are maintained by natural processes.
- "Semi-natural grasslands are one of the world's most biodiverse habitats on a small spatial scales."
- "Semi-natural grasslands belong to the most species rich ecosystems in the world."
- "...have been formed over the course of centuries through extensive grazing and mowing."
- "...without the use of pesticides or fertilisers in modern times."
Evolutionary history
The graminoids are among the most versatile life forms. They became widespread toward the end of the Cretaceous period, and coprolites of fossilized dinosaur feces have been found containing phytoliths of a variety of grasses that include grasses that are related to modern rice and bamboo.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 favourable to the evolution of grasslands.
Around 5 million years ago during the Late Miocene in the New World and the Pliocene in the Old World, the first true grasslands occurred. Existing forest biomes declined, and grasslands became much more widespread. It is known that grasslands have existed in Europe throughout the Pleistocene. Following the Pleistocene ice ages, grasslands expanded in the hotter, drier climates, and began to become the dominant land feature worldwide. Since the grasslands have existed for over 1.8 million years, there is high variability. For example steppe-tundra dominated in Northern and Central Europe whereas a higher amount of xerothermic grasslands occurred in the Mediterranean area. Within temperate Europe, the range of types is quite wide and also became unique due to the exchange of species and genetic material between different biomes.
The semi-natural grasslands first appeared when humans started farming. So for the use of agriculture, forests got cleared in Europe. Ancient meadows and pastures were the parts that were suitable for cultivation. The semi-natural grasslands were formed from these areas. However, there's also evidence for the local persistence of natural grasslands in Europe, originally maintained by wild herbivores, throughout the pre-Neolithic Holocene. The removal of the plants by the grazing animals and later the mowing farmers led to co-existence of other plant species around. In the following, the biodiversity of the plants evolve. Also, the species that already lived there adapted to the new conditions.
Most of the grassland areas have been turned to arable fields and disappeared again. The grasslands permanently became arable cropping fields due to the steady decrease in organic matter. Nowadays, semi-natural grasslands are rather located in areas that are unsuitable for agricultural farming.
Ecology
Biodiversity
Grasslands dominated by unsown wild-plant communities can be called either natural or "semi-natural" habitat. Although their plant communities are natural, their maintenance depends upon anthropogenic activities such as grazing and cutting regimes. The semi-natural grasslands contain many species of wild plants, including grasses, sedges, rushes, and herbs; 25 plant-species per 100 square centimeters can be found. A European record that was found on a meadow in Estonia described 76 species of plants in one square meter. Chalk downlands in England can support over 40 species per square meter.In many parts of the world, few examples have escaped agricultural improvement. For example, original North American prairie grasslands or lowland wildflower 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; there are also many species of birds that are grassland "specialists", such as the snipe and the little bustard. Owing to semi-natural grasslands being referred to as one of the most-species rich ecosystems in the world and essential habitat for many specialists, also including pollinators, there are many approaches to conservation activities lately.
Agriculturally improved grasslands, which dominate modern intensive agricultural landscapes, are usually poor in wild plant species due to the original diversity of plants having been destroyed by cultivation and by the use of fertilizers.
Almost 90% of the European semi-natural grasslands do not exist anymore due to political and economic reasons. This loss took place during the 20th century. The ones in Western and Central Europe have almost disappeared completely. There are a few left in Northern Europe.
Unfortunately, a large amount of red-listed species are specialists of semi-natural grasslands and are affected by the landscape change due to agriculture of the last century.
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 types of habitat, and a target for acquisition by wildlife conservation groups or for special grants to landowners who are encouraged to manage them appropriately.
Vegetation
Grassland vegetation can vary considerably depending on the grassland type and on how strong it is affected by human impact. Dominant trees for the semi-natural grassland are Quercus robur, Betula pendula, Corylus avellana, Crataegus and many kinds of herbs.In chalk grassland, the plants can vary from very tall to very short. Quite tall grasses can be found in North American tallgrass prairie, South American grasslands, and African savanna. Woody plants, shrubs or trees may occur on some grasslands—forming savannas, scrubby grassland or semi-wooded grassland, such as the African savannas or the dehesa and montado, in Spain and Portugal respectively.
As flowering plants and trees, grasses grow in great concentrations in climates where annual rainfall ranges between. The root systems of perennial grasses and forbs form complex mats that hold the soil in place.
Fauna
Grasslands support the greatest aggregations of large animals on Earth, including jaguars, African wild dogs, pronghorn, black-footed ferret, plains bison, mountain plover, African elephant, Sunda tiger, black rhino, white rhino, savanna elephant, greater one-horned rhino, Indian elephant and swift fox. Grazing animals, herd animals, and predators in grasslands, like lions and cheetahs live in the grasslands of the African savanna. Mites, insect larvae, nematodes, and earthworms inhabit deep soil, which can reach 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. Some types of fungi make the plants more resistant to insect and microbial attacks.Grassland in all its form supports a vast variety of mammals, reptiles, birds, and insects. Typical large mammals include the blue wildebeest, American bison, giant anteater, and Przewalski's horse.
The plants and animals that live in grasslands are connected through an unlimited web of interactions. But the removal of key species—such as buffalo and prairie dogs within the American West—and introduction of invasive species, like cane toads in northern Australia, have disrupted the balance in these ecosystems and damaged a number of other species. Grasslands are home to a number of the foremost magnificent animals on the planet—elephants, bison, lions—and hunters have found them to be enticing prey. But when hunting is not controlled or is conducted illegally, species can become extinct.
Ecosystem services
Grasslands provide a range of marketed and non-marketed ecosystem services that are fundamental to the livelihoods of an estimated one billion people globally.Carbon sequestration
Grasslands hold about twenty percent of global soil carbon stocks. Herbaceous vegetation dominates grasslands and carbon is stored in the roots and soil underground. Above-ground biomass carbon is relatively short-lived due to grazing, fire, and senescence. Grassland species have an extensive fibrous root system, with grasses often accounting for 60–80% of the biomass carbon in this ecosystem. This underground biomass can extend several meters below the surface and store abundant carbon into the soil, resulting in deep, fertile soils with high organic matter content. For this reason, soil carbon accounts for about 81% of the total ecosystem carbon in grasslands. The close link between soil carbon and underground biomass leads to similar responses of these carbon pools to fluctuations in annual precipitation and temperature on a broad spatial scale. Because plant productivity is limited by grassland precipitation, carbon stocks are highest in regions where precipitation is heaviest, such as the high grass prairie in the humid temperate region of the United States. Similarly, as annual temperatures rise, grassland carbon stocks decrease due to increased evapotranspiration.Grasslands have suffered large losses of organic carbon due to soil disturbances, vegetation degradation, fires, erosion, nutrient deficiencies, and water shortages. The type, frequency and intensity of the disturbance can play a key role in the soil organic carbon balance of grasslands. Bedrock, irrigation practices, soil acidification, liming, and pasture management can all have potential impacts on grassland organic carbon stocks. Good grassland management can reverse historical soil carbon losses. The relationship of improved biodiversity with carbon storage is subject of research.
There is a lack of agreement on the amount of carbon that can be stored in grassland ecosystem. This is partly caused by different methodologies applied to measure soil organic carbon and limited respective datasets. Further, carbon accumulation in soils changes significantly over time and point in time measurements produce an insufficient evidence base.