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The savanna ecosystem is one of the largest terrestrial biomes in the world with a remarkable expanse throughout tropical environments of South America, Africa, Asia, and Australia. It is estimated that throughout the world savannas cover a total land area of about 12.7 million square miles (33 million square kilometers). The term savanna is of Amerindian origin used in Haiti and Cuba to denote a treeless plain. This term was later incorporated into Spanish. Although there is no agreement on a single definition for this biome, the term is broadly used to describe all tropical and subtropical ecosystems characterized by continuous herbaceous cover of heliophilous grasses that show seasonality related to water and temperature and in which woody species are significant but do not form a closed canopy or continuous cover. As such, savannas contain a continuous cover of perennial grasses of three to six feet in height interspersed with shrubs or open canopy trees that are drought, fire, and browse resistant.
Several classifications have been identified dependent upon the composition of the flora, for example the savanna woodland, savanna parkland, savanna grassland, low tree, and shrub savannas. The classification may also involve the use of the dominant tree layer, for example, palm savannas, pine savannas, acacia savannas, brachystegia-julbernadia savannas, and so on.
The savanna biome is generally located between the tropical rain forest belt and the desert biome to the north or south of the equator. Not enough rain falls in this biome to support tropical rain forests. Precipitation is estimated at 30 to 50 inches of rainfall per annum with mean monthly temperatures of about 65 degrees F. The soils are often lateritic oxisols resulting in their low fertility. 0ur ecological knowledge of this biome comes from research that has been conducted largely in Africa and Australia. In southern and eastern Africa, the vernacular term for savanna is miombo woodlands. This is the primary woodland ecosystem in this region and is dominated by the tree species of the genera Brachystegia, Julbernardia, and/or Isoberlina, all of the legume family Fabaceae, subfamily Caesalpinioideae.
The miombo woodland biome in eastern and southern Africa covers almost 1.5 million square miles (2.7 million square kilometers) containing over half of Africa’s tropical dry forests stretching over a number of countries such as Kenya, Tanzania, Zimbabwe, Botswana, South Africa, Malawi, Mozambique, Zambia, and Namibia. Elsewhere in the world, the llanos of the Orinoco basin of Venezuela and Colombia are grass savannas maintained by the annual flooding and long periods of standing water, which inhibit the growth of trees. In Brazil, the cerrado is open woodland that is extremely rich in species and second only to the tropical rain forest in fauna and flora diversity. The tropical savanna of northern Australia is one of the largest in the world, with dense grass inter-spaced with scattered eucalyptus trees providing a home to many animals including several species of kangaroos. Savanna woodlands are also found in large areas of India.
In spite of the fact that savannas are found throughout the world, the savannas many people are most familiar with are the east African savannas covered with acacia trees. This is probably because the largest chunk of savanna woodlands (about 40 percent) is found in Africa. The famous game parks of Kenya, Zimbabwe, Botswana, South Africa, and Tanzania such as the Serengeti Plains of Tanzania are located in this biome. Here, one finds animals such as lions, zebras, elephants, and giraffes, and many types of ungulates (animals with hooves) graze and hunt. The diversity of species is remarkable, with a large range of highly specialized plants and animals that depend on each other to keep the environment in balance. It is estimated that there are over 40 different species of herbivore mammals that live on the savannas of Africa. The variety of these herbivores provides a wide range of food for carnivores, like leopards, cheetahs, hyenas, lions, and jackals.
However, an ever-expanding human population is putting pressure on the savanna biome. Large areas of the savanna are under stress and disturbance from human activities such as grazing, fuel wood, and timber collection and land clearing for cultivation. Threats to African savanna from intense human activity have received little attention in comparison to those affecting tropical rain forests. Yet Africa’s savanna habitants are being degraded rapidly. Poaching for ivory to satisfy the international market has also resulted in the decimation of the elephant and rhinoceros to the point of near extinction. Deforestation is another major threat to the survival of the savanna environment in Africa. Research is currently shading important knowledge about the fragility of the savanna ecosystem. Once the vegetation is cleared either due to overgrazing or deforestation, the savanna quickly converts to semiarid or desert environment.
- Martin Adams, “Savanna Environments,” in William M. Adams, Andrew S. Goudie, and Antony R. Orme, eds., The Physical Geography of Africa (Oxford University Press, 1996);
- David Beerling and Colin P. Osborne, “Opinion: The Origin of the Savanna Biome,” Global Change Biology (v.12/11, 2006);
- James Fairhead and Melissa Leach, Misreading the African Landscape: Society and Ecology in a Forest-Savanna Mosaic (Cambridge University Press, 1996);
- David Harris, Human Ecology in Savanna Environments (Academic Press, 1980);
- Mark Owens and Delia Owens, Secrets of the Savanna: Twenty-Three Years in the African Wilderness Unraveling the Mysteries of Elephants and People (Houghton Mifflin, 2006).