SAVANNAS

Beneath an azure, cloudless sky, tall golden grass waves in a breeze which cools the hard sun-baked red soil (called laterite). As far as you can see the grassy vista disappears over the blue horizon, its flatness broken by flat-topped acacia trees and island-like anthills or the occasional water-hole.

Imagine that and you have in your mind's eye a typical savanna landscape. Savannas are a broad ecological zone which deserve special attention because they are the habitat we typically associate with the wildlife and people of Kenya.

Savanna is the name given to areas with distinct dry and rainy seasons and high temperatures year-round, usually above 70°F. Most rain falls in the hottest months, while the dry season occurs during cooler times of year.

Think of savanna as the habitat which fills the spaces between tropical forest and desert. In regions close to tropical forest, open woodland savanna prevails. In areas where rainfall is less regular, grasslands give way to thorn-scrub and widely scattered patches of grass. Most savanna plants are adapted to periods of drought.

Savannas are found on all the continents in regions which lie within the tropics (23°N to 23°S). The world's largest savanna region is in Africa; north, east and south of the West African rain forest. The northern border merges into the Sahara desert while the Kalahari desert claims the southern boundary. Worldwide, there are different types of savannah. All types are found in Kenya because it's altitude varies so much, from sea-level to almost 20,000 feet.

Savanna habitats range from thickly wooded grasslands to treeless plains. Much of this variety is natural but some treeless areas are man-created, initially deforested and afterward maintained by frequent fires, natural and man-made. Savanna grasses are taller than species of temperate plains species and grow in tufts rather than as uniform ground cover. Trees and bushes are widely spaced apart so they compete less for precious water and nutrients.

Dry season length determines vegetation types: areas with three to five dry months a year are moist humid wooded savannas. Those with six to seven dry months are shrub or grassland savanna. Areas that get between eight and ten dry months per year are thorn-scrub savannas. Annual rainfall varies from 8 inches (20 cm) in the most arid savanna regions up to 47 inches (120 cm) in the humid areas.

Tropical rain forest usually gives way abruptly to humid woodland savanna. The tall, closed evergreen canopy of the rainforest is replaced by scattered deciduous trees, many of which do not grow tall. Acacias rarely grow over 10 feet (3m). Some of them are so short they are obscured by grass that rapidly grows during the summer rains. Upon the return of the dry season, fierce fires rage through the tall dry grass and destroy all in their wake except for thick-barked trees.

Most savanna plants are adapted to fire. Numerous species even need fire to germinate their seeds. Even grasses, which bush-fires seem to vanquish utterly, quickly regenerate with the next rains. In Africa this type of savanna is known as 'miombo', south of the rain forest and as 'Guinea savanna' north of the rain forest.

In the miombo belt red laterite soils are common. Laterite is caused when iron oxidizes (rusts) in the soil. For the soil to laterize, there needs to be enough moisture to oxidize the iron. This limits laterite formations to hot, high rainfall areas, which is why they are so typical of savanna such as miombo.

Beyond the miombo savannah lies a belt called the 'Sudan savanna'. This is the classic dry savanna which covers much of Kenya. Grasses up to five feet tall cover the ground and acacias dot the landscape, together with thorny trees, huge water-bloated baobab trees and feathery palms. Fires maintain much of the landscape and a delicate balance exists between fire, climate, soil, vegetation and animals.

Toward desert, dry savanna merges into thorn-scrub and thorn-forest. Long, hot dry seasons, erratic rainfall and rapid evaporation limit plant growth. Grasses grow in short tufts, sparsely scattered among large expanses of bare earth.

Fire has little effect on vegetation of these types of savanna because plants are too far apart for fire to spread. Aridity and heat modify plant structure and grazing by animals has dramatic effects. In the natural situation animal activity enhances grazing conditions for other animals. Gazelles feed tender shoots that emerge only after the grass is cropped and trampled by wildebeest.

On the other hand, when humans use the land for their livestock, the fragile ecosystem is readily disrupted by too many grazing animals such as domestic cattle and sheep.

ECOLOGY OF SAVANNAS

If someone says, "No person is an island," they mean that someone cannot survive completely alone. We need other people.

In a similar way, an animal cannot be an island either. Each individual relies on others of its own species and on the roles of different species. Food is one obvious connection between species but the bonds of inter-dependence between species are often surprisingly tenuous. Interactions are grouped as to whether the two species affect each other negatively, positively or somewhere in between.

Competition is a common interaction where species negatively affect one another. Species compete for food, water, safe nesting sites or territory for courtship. Individuals within species also compete for these resources; for example, adult lions deprive cubs of a share of the kill when game is scarce.

Competition between species is most intense when resources are scarce, such as in a drought. For this reason, most species have varied diets which spread the possible risk of shortages and the consequences of severe competition. Use of different food types within the same habitat allows species to live together and avoid constant competition.

A second type of interaction is when one species affects the other negatively but itself gains. Predation or herbivory are textbook examples of this interaction, but parasitism (another form of predation) affects virtually every species. Herbivory is a win-lose interaction where the plant loses (leaves, fruit, etc.) and the animal gains.

A big cat depends on other animals for prey. The giraffe depends on acacia trees for its browse. That is easy to see. We begin to appreciate more the balance of nature if we remember that the vulture, hyena and other scavengers depend on the kills made by the big predators.

Thus, a third type of common interaction is where one species gains and the other neither wins or loses. For example, the hyena and vulture benefit from a lion kill and interact with lions but probably have little effect on the lions' survival chances. In these 'commensal' interactions, one species benefits and the other is unaffected.

Many birds depend on insects which are disturbed from vegetation by movements of large grazers, particularly elephants. The insects are quickly caught by sharp-eyed oxpeckers and bee-eaters. Other birds follow at more sedately pace, to make meals of insects and larvae attracted to the herbivores dung. This is typical commensalism: birds benefit from the activities of elephants but do not affect the huge mammals in any way.

Another form of interaction involves mutual benefits to both species hence is known as 'mutualism'. Cattle egrets spend their lives perched on the backs of the large beasts, where they peck at bothersome parasites and flies. Both cattle egrets and their mobile dining tables benefit but they can survive perfectly well without each other.

This is not always so. Some mutualistic relationships are inter-dependent to the point where the two species completely rely on one another, a state known as 'symbiosis'. This arises from step-by-step evolution, when species change in tandem, so they become utterly dependent on one another to meet their needs.

For example, many acacia species have poisonous leaves and seeds in case their thorns fail to deter a hungry herbivore. However, some species are are non-poisonous even though they are subject to pressure from herbivores. How do they survive herbivory?

The non-toxic acacias have an ingenious defense: ants which inhabit their thorns and branches. Sugar-water is supplied from special pits by the plants—only to sustain their miniature armies. In return the ants fiercely attack anything that tries to feed on their host plant. So protective are the ants that they mobilize if someone just touches the tree or stands beneath it. Hordes of ants rush out to sting and bite the unfortunate animal until it runs away. Smaller animals may even be stung to death.

Other symbiotic relationships seem even more fantastic. Many species of ants and termite cultivate fungi for food and eat nothing else. In turn, the fungus is totally dependent on the ants. It grows only in the composted leaves and constant environment the insect farmers provide deep within underground chambers.

Instances of symbiosis are mostly between insects and plants or fungi, and tend to be rare among vertebrates. Plants and insects have been on earth longer than other groups. This and the rapid evolution of insects compared to larger animals has promoted coevolution and led to intricate inter-relationships.

In savannas multitudes of interactions have affect the lives of coexisting species. The position a species occupies in the savanna community can be thought of as a profession—the job a species does or how it fits into the scheme of things. Ecologists call this position a 'niche', which is the sum of all the different activities a species might carry out. The niche is a useful abstract concept to help us understand how so many species can live together yet not out-compete each other out of existence.

ECOLOGICAL NICHES OF THE SAVANNA

Diet is the best guide to a species' niche. There as many variations in diet as there are species. Different species' diets always vary in some respect to minimize competition. For many species with similar food preferences to inhabit the same area , there is must always some difference in the diet that allows them to coexist.

The herbivorous fauna of the savanna are an excellent case of this phenomena, known as 'niche displacement'. For example, small antelope feed on small tufts of grass and herbaceous plants whereas larger antelope graze grass tops and browse on shrubs and small trees. Still larger animals, such as giraffe, browse the highest crowns of trees and so avoid food shortages which might affect smaller herbivores.

A 'guild' is a set of species with similar types of niches in the savanna ecosystem. Among mammals there is the guild of tiny rodents that eat seeds and grain. Other small mammals, such as shrews, are members of the insectivore guild. Larger mammals, the bovids, antelope and elephant are herbivorous on grasses, herbs, shrubs and trees.

Yet other large mammals, lion, cheetah and leopard, are predators. To avoid competition they specialize on prey of different body sizes. Lion take bigger animals such as zebra or wildebeest and cheetah choose small fleet-footed gazelles. Leopards specialize on wild pigs and primates, especially baboons.

Many bats are in a unique guild—nocturnal aerial insectivore. Birds, on the other hand have diversified into many different ecological roles.

From the principle of 'niche displacement' it is clear different species of birds coexist because they each exploit well-separated habitat niches. Many birds are herbivorous. These include ground-dwelling savanna birds, such as ostriches, guinea fowl and aquatic birds like geese and flamingos. Other aquatic birds, such as cormorants, pelicans and fish eagles eat fish or other water animals.

Raptors, shrikes and specialists such as the ground hornbill are predators while forest birds like parrots and mousebirds usually eat fruits and seeds. Brilliant sunbirds feed on flower nectar in a niche analogous to hummingbirds in the New World. Other birds are important scavengers, especially vultures and the maribou stork.

Reptiles and amphibians tend to be scavengers, insectivorous or carnivorous. Insects and other invertebrates are their main food. Since their diets often overlap, reptiles and amphibians coexist by habitat separation. One group inhabits dry areas and the other is restricted to damp areas.

Insects and other arthropods are vital to the ecology of the savanna. Most are herbivores and highly specialized on one or two plant species. Many insects and worms are parasites or disease vectors, while others perform crucial services as flower pollinators, seed dispersers, scavengers or decomposers.