Savannas are tropical or subtropical grasslands ranging from dry scrubland to wet, open woodland. Savannas occur in Asia, from India to Southeast Asia, and in Africa, from the Sahara and Kalahari deserts to the southern tip of Africa. The Ilanos of Venezuela and the campos of Brazil are regions of savanna in South America.
Rainy season and long periods of drought are typical of savannas. The amount of rain in a savanna can be as high as 150 cm a year, but most of it falls heavily during thunderstorms in the short rainy season. In Africa, the rainy season usually lasts from January to April. During the rest of the year, the savanna may be very dry. The extreme climate demands a wide range of adaptations in the organisms of the savanna.
In order to survive, plants must be resistant to drought, fires, and grazing animals. Many of the plants grow runners. Runners are long horizontal stems above or below the ground. Runners are used by some plants to reproduce; they spread quickly for several meters. When a fire occurs, the runners are protected.
Savanna grasses grow in tufts. Tufts are large clumps of tall, coarse grasses. The savanna trees and shrubs have thorns or sharp leaves that keep them from being eaten by grazing animals, such as gazelles. Another adaptation is the ability to grow rapidly. This adaptation enables the plants to recover quickly from the damage caused by fire and animals.
The concentration of animal populations in smaller areas around streams and water holes is also influenced by the rainy season of the savanna. Because of this, some animals are adapted to make use of the available food in what is called a vertical feeding pattern. In a vertical feeding pattern, animals eat vegetation at different heights. This decreases competition for food. The patterns allows more animals to live on limited resources because the animals can have smaller, more specific niches.
Many larger animals migrate to areas of the savanna where rain has fallen, often traveling great distances to find water. In Africa, lions, cheetahs, and other predators prey on migrating herds of wildebeest, zebras, and various species of antelope. Sometimes habitat loss due to human activities disrupts the migration patterns of these animals. Survival for many species depends on wildlife preserves and global conservation efforts.