In parts of the world where it is too dry for trees to grow, yet moist enough to prevent the land from becoming desert, grasses are the dominant plants. Grasses are unusual in that their stems grow from a point near the ground.
This means that unlike most other plants, which grow from their tips, grasses are unharmed by grazing. In fact, grazing animals help grasses to maintain their dominance by stunting the growth of competing plants.
This creates a vast, open habitat in which there is plenty of plant food — for those that can digest it — but little shelter from the elements.
Before the advent of farming, grassland covered large parts of the temperate world, notably in the northern hemisphere. These vast grasslands — which include the prairies of North America, and the steppes of Europe and Central Asia — are nearly all in the center of large landmasses, far away from coasts and their moisture—laden winds. Summers are often warm, but winters can belong and cold, with biting winds. An unusual feature of this kind of habitat is that the majority of the plant matter is hidden away below ground — the exact opposite of the situation elsewhere on land.
This is because grass plants direct much more energy into growing roots than into producing leaves, and their roots form a continuous mat that protects the surface of the ground by holding the soil in place. If grassland is burned, or hit by drought, it can soon recover because the grass can draw on its buried reserves in order to start growing again.
The root mat makes a useful source of food for insects and other small animals, it is also a perfect medium for burrowers, because it is easy to dig through and, unlike loose soil, rarely caves in. Above ground, the food supply is closely tied to the seasons. In temperate grassland, most of the year’s water usually comes in the form of spring rain or melting snow. This creates a flush of growth during spring and early summer, which is the time that most grazing animals breed. By late summer, the grass is brown and dry, although for a while grass seeds make a valuable autumn harvest. Winter is a difficult time for all grassland animals, but particularly for grazers because they often have to survive on low-grade food that is hidden under snow.
Savanna is tropical or subtropical grassland that contains scattered shrubs and trees. The grasslands of East Africa are a familiar example, with their diverse wildlife and distinctive vegetation (particularly the flat-topped acacia trees). Compared with temperate grassland, savanna is very variable: in some savanna habitats, trees are few and far between: in others, they form scattered thickets, merging into open woodland.
Trees have a major impact on the savannas animal life. This is because they produce a wide variety of food. including wood, leaves, flowers. and seeds: and that also create shelter and breeding sites for animals that live off the ground. The balance between trees and grass is a delicate one that is sometimes changed by the animals themselves.
For example, elephants destroy trees by pushing them over so that they can reach their leaves. However, elephants also help trees to reproduce because they ingest the trees’ seeds, which are then passed in their dung — an ideal medium for promoting seed growth. Browsing mammals often keep trees in check by nibbling saplings before they have had a chance to become established. Fire also helps to hold back trees, and its effect is most apparent in places where trees grow close together.
Unlike temperate grassland habitats, savanna is usually warm all year round. There is often a long dry season, when most trees lose their leaves. followed by a wet, or “rainy” season. Which produces a rapid burst of growth that turns the landscape green. During this wet season, plant-eating animals rarely have to contend with a shortage of food: in the dry season, the threat of starvation is never far away, and many animals travel long distances to find water and food.
LIFE IN GRASSLAND
Despite centuries of human disturbance, grassland supports some of the largest concentrations of animal life on earth.
Survival in grassland habitats is far from easy, however: aside from the lack of shelter and plant diversity, there are hazards such as drought and fire to contend with. Added to this is the ever-present risk of attack by some of the world’s fastest and most powerful predators.
Life in open grassland is often dangerous because there are few places to hide. To increase their chances of survival, many large plant-eaters live in herds. This makes it more difficult for predators to attack, because while most members of the herd are eating, some are always on the lookout for danger.
Today’s largest herds are found on Africa’s plains. Here, migrating wildebeest can form herds over a quarter of a million strong. and 40km (25 miles) long, although even these herds are small compared with some that existed in the past. During the 19th century, springbok herds in southern Africa sometimes contained more than 10 million animals.
In North America, bison herds probably reached similar sizes before hunting brought the species to the edge of extinction. Life in herds does have its problems, one of which is the risk that an animal might wander off and become lost.
Most herding species have scent glands on their hooves so that if an animal becomes isolated it can follow the scent tracks to rejoin the herd. Another problem is giving birth. To prevent their young from being trampled or attacked, many grazers give birth in cover, and rejoin the herd a few weeks later. Some however, are born in the open and have to be able to keep up with the herd when they are just a few hours old.
In grassland and savanna, there is a premium on speed. It is no accident that the worlds fastest land animals, such as the cheetah and the pronghorn, are found in this habitat. Natural selection favors predators that are fast enough to catch food, and prey animals that are fast enough to escape. Most of the fast runners are mammals; however, grassland also has nature’s fastest- running birds, including ostriches, rheas, and emus – giant species that have lost the ability to fly.
These birds can reach speeds of up to 70kph (44mph). More importantly, they are able to maintain such speeds for up to 30 minutes — long enough to outrun most of their enemies unless a predator launches an attack from a very close range.
Despite the many fast runners, grassland life often appears tranquil. This is because running is extremely energy-intensive, and animals run only when they absolutely have to. Prey animals have invisible “security thresholds” that vary according to the threat they face.
For example. gazelles often let lions approach to within about 200m (650 ft) because they are instinctively aware that lions that are visible at this distance are unlikely to be stalking prey. A solitary cheetah, on the other hand will send a gazelle herd sprinting, even if it is seen to be 4 times further away.
Some grassland animals find safety not by running away but by retreating into burrows below ground. There, they can stay out of reach of most predators and find some protection from the worst of the elements. Subterranean animals include a wide variety of species, ranging from mammals to insects. Some animals, particularly snakes, do not excavate their own burrows; instead, they adopt existing ones.
The largest burrows, made by African aardvarks, are big enough to accommodate a person, and are a serious hazard to vehicles: the most extensive are made by prairie dogs and other rodents. Before farming became widespread in North America’s prairies, some prairie-dog burrow systems covered several thousand square kilometers and housed millions of animals.
Termites are also accomplished builders, constructing giant, elaborate, subterranean nests that extend high above ground level.These nests house large, cooperative communities that can contain over 30 million inhabitants. Along with ants, they make up a very large part of the habitats animal life, and provide food for the large insect—eaters.
Although grass is rich in nutrients and easy to find, it is difficult to digest. Many mammals, including humans, cannot break it down at all because it contains large amounts of cellulose — a carbohydrate that most animals cannot digest. Grazing mammals, however, have special micro-organisms in the gut that break down cellulose so that the body is able to use it.
Some non-mammal species also use microbes to digest plant material. In tropical savanna for example, termites rely on them to break down dead leaves and wood. The animals that are most efficient at using cellulose are ruminant mammals — antelopes, buffaloes, and giraffes, for example — which helps to explain why these animals dominate grasslands.
The ruminants complex stomach acts like a fermentation tank, working to extract the maximum amount of nutrients from food. The animal assists in the process by regurgitating its food and chewing it a second time, making it even easier to break down. Non-ruminant plant-eaters, such as zebras, have less efficient digestive systems and must therefore eat more to survive. In pure grassland, plant-eaters compete for the same food, although each may have a preference for a different type of grass.
In savanna, the presence of trees and shrubs makes for a wider range of food, and browsing mammals have minimized competition further by evolving specific ways of feeding. This means that a remarkable number of species can live side by side. For example, the small Kirk’s dik dik antelope feeds on shoots and fruit, and rarely touches grass. While the much larger eland will eat almost anything from fruit and seeds to roots scraped up from the ground.
Scavengers which also play an important part in the habitat’s ecology, include birds as coyotes, jackals,and hyenas. Most airborne scavengers are vultures, but there are several species of scavenging storks one of which is the marabou fork.