Grasslands, Shrublands, Deserts
Introduction Providing splendid scenic vistas, grassland, shrubland and desert environments cover two-thirds of the world's land surface. The greatest diversity of grazing animals on the planet calls these open habitats their home, and they are preyed on by a variety of wild cats, dogs, wolves and hyenas. Grassland habitats appeared on Earth some 70 or 80 million years ago, following the extinctions of the dinosaurs. As flowering plants, grasses grow in great concentrations in semi-arid climates ‒ where, as a general rule, annual rainfall ranges between 10 and 39 inches (Brown 1985). 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. 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.
Among the various grasses grow many kinds of wildflowers, succulent plants and other non-woody species. On savannahs, trees and small woodlands are scattered in the grasslands. Flowers brighten grasslands in annual splashes of dazzling colors. An array of gentians, poppies, wild tulips, irises, orchids, bluebells, daisies, asters and myriad other flowers bursts into bloom each year, creating a brilliant montage against the green of new grasses.
A great diversity of insects, including colorful butterflies and tiny bees, pollinate these plants. Their seeds are dispersed by the wind, as well as by birds and mammals. Natural grasslands are the ideal habitat for herds of hoofed animals who fertilize the soil with their dung, which is later broken into nutrients by a host of invertebrates, fungi and microbes. Rodents and other burrowing animals excavate holes and chambers that feature complex side passages and multiple entrances, providing escape routes, birth chambers and sleeping areas. Other animals, including snakes, tortoises and ferrets, use the burrows as their habitat. Natural grasslands are intricate webs of animal, plant and fungal communities, and much remains to be learned about their functions.
Ecologists divide grasslands into two major types: tropical and temperate. Temperate grasslands tend to grow in the rain shadow of mountain ranges (Simon 1995). The Central Asia steppe ‒ a treeless plain ‒ has a low rainfall because the Himalayas and other mountain ranges block humid winds. The Rocky Mountains impede Pacific winds that bring rainfall from the North American prairies. Winters in temperate grasslands are usually severe, with freezing temperatures and howling winds. While temperate perennial grasses regenerate from dormancy each spring by sprouting from their roots, grasses in the tropics do not enter a dormancy period and instead grow year-round. Tropical grasslands tend to be located between forests and deserts (Simon 1995). Climate in the latter type of grassland is generally mild, and most have seasonal dry and wet seasons, each lasting about half the year. In some environments, especially those bordering on shrub or desert, temperatures can rise to searing heights in the dry season, drying up natural ponds and streams.
Some grassland is in areas of high rainfall, but other conditions prevail that inhibit the establishment of trees and forest, such as shallow soil that prevents trees from taking root, or stony, nutrient-poor soil. The presence of large numbers of ungulates who crop the grasses also prevents trees from taking over the land. The Serengeti and other East African grasslands would soon become acacia woodland if not for the presence of millions of wildebeests and other ungulates. Fires, either ignited by natural lightning or set by people, and mowing also maintain grassland by removing young trees and shrubs. Grasslands and related habitats tend to grow in areas where forests have been cut and rich top soil has washed away; the moors of the British Isles, for example, are composed of heath, grasses and ground plants that replaced the forests cut centuries ago, following erosion of the topsoil (Simon 1995).
During the Pleistocene, 100,000 to 10,000 years ago, rich grasslands with thick topsoil covered millions of square miles in Africa, Australia, Asia, South and North America. They supported an unparalleled diversity of large mammals. Sabre-toothed and huge lion-toothed cats, dire wolves, cheetahs, hyenas and bears preyed on herds of bison, deer, mastodon, mammoth, camel, rhinoceros, antelope, wild horses and gazelles on North America's rich grasslands. In Australia, giant marsupials were the predators of large kangaroos and wallabies. Outside Africa, almost all these wild species disappeared by the end of the Pleistocene, concurrent with heavy hunting by native peoples. Some large mammals survived, however, and in many areas great herds grazed the grasslands in untold millions until recent times. In these natural grasslands, each large mammal consumed a slightly different type of vegetation, whether new grass, forbs, mature grass or other plants, allowing many species to coexist without damaging or overgrazing the environment. Today, grasslands, shrub and mixtures of these habitats cover about one-fourth of the Earth's surface.
Shrubland ecosystems, characterized by bushes, shrubs and scattered trees, require very little water. This habitat is often degenerated woodland or grassland, as seen in many parts of coastal Greece and the Near East where topsoil has eroded. Desert and shrub habitats in Mediterranean countries are the product of thousands of years of abuse of the land, beginning with forest clearance for grazing and agriculture. Shrubland quickly turns to desert when desecrated by brush clearance or heavy livestock grazing. In some parts of the world, shrubland is a natural habitat; many species of birds, mammals, reptiles and invertebrates have evolved to adapt to live in such an environment.
Deserts, which cover almost 40 percent of the Earth's surface (Allan and Warren 1993), receive less than 10 inches of rain a year (MacMahan 1985). Within these ecosystems, the annual rainfall limits the types of vegetation able to survive. Arid zones receive less than 8 inches of rain per year, while in hyper-arid zones, less than an inch of rain falls per year (Allan and Warren 1993). Rainfall in deserts tends to be unpredictable. In some deserts, no rain falls for several years, followed by a series of torrential storms in one season. Desert life has evolved remarkable adaptations to these conditions. Many plants have leathery, water-retentive leaves or spongy trunks for storing water, and seeds that can remain dormant in the soil for many decades. A few species of desert animals never drink, only obtaining water from plants they eat. Desert toads can remain in a torpor underground for years until a rainstorm awakens them.
The Sonoran Desert of the southwestern United States and northern Mexico is the lushest and most botanically rich of all deserts, with 2,500 species of plants, 300 of which are cactuses. A fairly mild climate provides habitat for deer, peccaries, pronghorn antelopes (Antilocapra antilocapra), bighorn sheep (Ovis canadensis), various cat and canine predators, tortoises and many types of birds, snakes and lizards. In contrast, very little vegetation grows in the Sahara, where thousands of miles of sand stretch to the horizon. The Atacama Desert of Chile has the lowest rainfall of any desert in the world, and relatively few wildlife species are able to survive. Its temperature falls rapidly from searing hot during the day to freezing at
night. Desert animals have adapted to these extremes by living in burrows that insulate them from the heat and cold.