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Savannas

Savanna landscape

*Savannas - or 'savannahs' in the pre-1956 spelling - are the most important vegetation type in the lowlands of the seasonal tropics and sub-tropics. The name is derived from a 16th Century Spanish word zavanna (modern Spanish: sabana), and it was first recorded in 1535 by Oviedo as coming originally from Carib, a language found in the southern West Indies (see: 'A Brief History of Savanna Studies'). Savanna vegetation comprises a mixture of grasses and herbs with woody elements, ranging from dwarf shrubs to large umbrella-shaped trees. The ratio between the grass and the trees can vary enormously, from only scattered Acacia trees in the Serengeti Plains of Tanzania to the forested savannas of India, South East Asia, and Northern Australia. Savannas are distinguished from both true forests and deserts by their near-continuous cover of grasses, which also ensure that running fire is a particularly important feature of savanna ecology. The grasses belong mainly to the C4 group of photosynthetic plants, which are especially adapted to living in hot dry environments.

*Savannas occupy a very wide area of the tropics and the sub-tropics where there is a combination of a low total annual rainfall of between 400 mm and 1500 mm (extremes at 200 mm and 2000 mm) and a marked seasonality in the rainfall distribution, with a normal dry season of between 4 to 8 months (extremes at 2 to 10 months) of the year. This very long dry season is a second reason why fire is an annual or biennial event in the savannas, and both natural fires, from lightning strikes, and human-induced fires are common throughout all savanna regions. The careful management of these fires is important for maintaining the balance between the grass and the trees, and some wrong-headed attempts to remove fire completely from savanna environments may lead to the excessive build-up of ground fuels and much more damaging fires. Biologically, savanna plants possess many adaptations to both fire and drought, including thick rough bark, thorns, reduced and leathery leaves, underground stems, complex root systems, and specialized patterns of living. Savannas are therefore well-adapted to moderate levels of fire and they have always burnt, even before hominids and humans first used fire in the African plains.

*The open grassy character of some savannas is also associated with a distinctive and often very visible mammal fauna of noteworthy predators, including the African lion (Panthera leo) and the cheetah (Acinonyx jubatus), the world's fastest mammal over a short distance. These hunters prey on large herds of grazers and browsers, such as wild cattle, antelope and deer, as well as a range of savanna specialists, like the giraffe (Giraffa camelopardalis) and the zebras(Equus grevyi and Equus burchelli). The most important savanna animals of all, however, are probably the insects, especially the ants and the termites, which play such a vital role in both herbivory and seed dispersal. Many savanna landscapes are characterized by tall termite mounds and towers. This is yet another example of the ecological maxim that it is really 'the little things that run the world'.

*The very wide climatic tolerance of the savannas is the prime reason why they dominate the tropical world. Savannas occupy no less than 45 per cent of South America, 65 per cent of Africa, and 60 per cent of Australia. They range from the 'Big Thicket' in Louisiana and Texas, north of the Tropic of Cancer, to well south of the Tropic of Capricorn at Port Elizabeth in South Africa. Savannas are also widespread in the Caribbean, the Indian Sub-continent, and in both mainland (Burma, Thailand, Laos, Cambodia and Vietnam) and maritime (Indonesia) South East Asia. Moreover, the majority of people living in the tropics, comprising no less than one-fifth of the world's population, inhabit the savannas, which form the core of the world's monsoonal lands that overall support some 50 per cent of the global population. The savannas in consequence are the single most important terrestrial environment, and are both older than and as diverse as the tropical rain forests. They have also become the classic example of a non-equilibrium environment (see: 'A Brief History of Savanna Studies'; 'Savanna-Forest Non-Equilibrium'; and 'Towards Understanding a Non-Equilibrium World').


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