Between the fierce aridity of the desert belt and the constant humidity of Tropical Forests (After Man)|the tropical forest regions lies an area where the rainfall is intermittent and erratic. The dominant plants are grasses and the habitat is one of open plains with scattered scraps of brush and woodland. As the region lies wholly within the tropics, the sun is therefore directly overhead at any one place on two occasions each year. Most of the rainfall comes at these times because the tropical convergence of global winds and the wet conditions associated with them move north and south with the sun twice every year. The intervening dry seasons are due to the dry high-pressure belt associated with the deserts moving over the grassland area.
The dominance of grasses over trees has more to do with the level of soil moisture than it has to do with the total amount of rainfall. Typically only the upper soil layers contain water, whereas the lower strata, where tree roots would be found, remain dry all year. Some areas of grassland receive substantial amounts of rain, but because it occurs only at certain times of the year trees are unable to establish themselves.
Because of the general dryness of the region it is highly susceptible to fire. Indeed it is repeated destruction by frequent grassland fires that has produced the plants that are characteristic of the habitat. The trees are particularly hard and fire-resistant and the grasses grow from their bases rather than from the tips of their leaves and stalks. They also spread by means of underground runners, which allows their instant recovery after fire has swept the area and destroyed the exposed parts.
The rapid recovery of trees and grasses from damage permits the grasslands to support large numbers of grass-eating animals despite the frequent fires. Only the upper parts of the leaves and stems are eaten, leaving the growing bases and the underground runners intact. Another feature of the grasslands that has an important influence on the fauna is the sparseness of cover. A grazing animal can therefore be seen by a predator from a great distance, and conversely the grazing animal can see danger coming. Hence both grazers and predators in these regions are highly adapted for speed and pursuit, and have long legs and quick reactions. Some birds, too, have found that they can survive on the grasslands using only their legs to take them out of trouble without recourse to flight.
Another feature of tropical grassland life is migration. Because of the seasonality of rainfall, different areas of the grasslands provide food at different times of the year and hence great migrations of grazing herds occur throughout the year. Migration also takes place to the grasslands from other parts of the world. Many birds summer in the temperate woodlands, far to the north, and fly south to the grasslands to escape the winters.
As with the desert belts, the total land area of the globe lying within the tropical grassland climatic belt has diminished since the Quaternary due to the constant northward movement of the Australian continent. Although the largest extent of tropical grassland is on the African subcontinent, considerable expanses also exist on the South American island continent south of the equatorial rainforests.
The grasslands, both tropical and temperate, are the home of the running animals. The long vistas and the general lack of cover make concealment difficult and speed is the most practical means of defense.
The grasslands first appeared on a large scale about 80 million years ago, when a general reduction of global temperatures, causing a drop in average rainfall, produced a reduction in the area of forest found on Earth. At this time the mammals that had been in existence for about 20 million years developed running forms in large numbers for the first time. The grasses, representing a vast untapped food source, had however a high silica content, which made them much tougher than the leaves of trees to which the browsing mammals were accustomed. To deal with this more fibrous material new tooth structures appeared that had hard enamel ridges and cusps to grind down the grass before it was swallowed. New, elaborate digestive systems were also evolved to deal effectively with it,
By the Quaternary, the long-legged grazers, the ungulates such as the zebra, Equus, and the gazelle, Gazella, were the most successful animals of the tropical grasslands. However the rabbucks, which originated in the temperate woodlands, after man's extinction, spread southwards, round the mountain barriers, into the African and Indian subcontinents, where they flourished and competed so effectively with the ungulates that in time they largely replaced them.
Although many forms of rabbuck inhabit the same region, because of their different feeding habits they do not compete directly with one another.
Although the two principal predators of the tropical grasslands of the African subcontinent are both primates, they have evolved along very different lines and hunt different prey.