Dave Taylor's African Safari: Abiotic Factors of the Serengeti-Mara Ecosystem - Water

Abiotic Factors of the Serengeti-Mara Ecosystem
Water
Page 11

Summary

Water is one of the most important abiotic factors in any ecosystem. Life simply cannot exist without it. Not all animals drink water, but those that never or only rarely drink water, such as the tiny dikdik, get all they need from the plants they eat. Others get their required moisture from the blood of their prey.

Most animals do, however, need to drink water. In the Serengeti-Mara, water can be found in several forms. There are a few permanent rivers and lakes. There are also seasonal streams, marshes and pools. There are underground streams. There are freshwater and "soda" lakes. Moisture is stored in trees, leaves, fruits and gourds as well.

Water Brings Both Life and Death

Water brings life but it also brings death. Thousands of wildebeest drown each year on their migration as they cross swollen rivers. Mud holes trap even powerful elephants. Predators, especially in the dry season, lie in ambush waiting for the thirsty plant eaters to approach.

When water sources dry up, the animals and plants have to find ways of coping. If an organism cannot survive the droughts, it cannot exist here. Some, like the wildebeest, migrate and go in search of water. Others aestivate. Freshwater clams, bull frogs and catfish all burrow deep into the mud where they sleep away the dry season. Some simply die, leaving their fertile seeds or eggs behind to develop when the rains return.

Rainfall Patterns

The source of most of the region’s water is the seasonal rains. There are two rainy seasons in the Serengeti-Mara ecosystem. The long rains occur from March through May. The short rains fall between November and December and in some years may not occur at all. During these times, the dry, southwestern, short grass plains receive most of their 500 mm (19 inches) of annual rainfall. The northern plains of the Mara average 1 200 mm (47 inches) of rain a year.

The long rains are the most reliable and it is during this season that the wildebeest herds calve on the short grass plains of the Serengeti. There is just enough rainfall here to encourage the grass to grow. The grass is the type the herd most prefers and it is only when the rains fail and the grass no longer replenishes itself, that the herds move on. The dry season has arrived (see also The Producers (Plants): Grass).

Rains on the Plains

During the long rains, the short grass plains provide a newborn wildebeest with just about everything it needs. The grass nourishes its mother and the rain provides her with enough moisture to keep up her milk production. There is little cover for predators to hide and lots of open space to flee, should danger appear.

Further north in the Mara, there is too much rain. The land is soggy and foot rot sets in if the herds stay too long. The grass is long, giving predators the advantage. Even the much smaller, resident, Mara herd of wildebeest moves north into the Maasai ranchlands where it is drier.

The Mara and the Northern Serengeti beckon the huge herds north when the long rains end and the southern plains dry up. The migration follows the rain as it moves north with the sun’s heat. The amount of rain that the north receives also falls off and so the plains are not as wet and the grass not as high as during the long rains.

Some years the short rains fail, but most years they fall in November and December. It is these rains that push the herds to the south again. If they fail, the herd lingers longer in the Mara region and even there it can get dry. It is during times of severe drought that wildebeest travel the most, seeking water. It is then that they are most likely to seek out the year round rivers as their main source of drinking water.

And it is then that they are most vulnerable to attacks from crocodiles.

Rivers

The map of the Serengeti-Mara region shows several rivers crossing the plain. All drain into Lake Victoria to the east.

The most reliable of these rivers, as a source of year round water, is the Mara which flows through the eastern portion of the Mara National Reserve and then across the top of the Serengeti. The Mara receives the most consistent rainfall of any of the area’s rivers and seldom dries up.

The Grumeti River crosses the northern third of the Serengeti and is an important source of water when the migration begins. Portions of it do dwindle and dry up as the season progresses and in some years it can dry up completely. It has several tributaries such as the Orangi, Nyabogati and Ngare Nanyuki Rivers, which have their sources in the Ngorongoro Conservation Area. During the dry season they, too, become mere trickles and may dry up.

Rivers to the south, such as the Mbalageti River, are even more likely to dry up completely during the dry season.

Even when apparently dry, these rivers still can be a source of water. Underground reservoirs and streams still hold moisture that trees growing along the banks can tap. The trees' leaves, fruits and trunks then become a source of water for browsing animals, insects and elephants.

Lakes

There are four lakes in the Serengeti-Mara Ecosystem. Within Serengeti National Park are Lake Magadi and Lake Ndutu. Just outside the park, in the Ngorongoro Conservation Area, lies Lake Masek. Lake Natron is located in the Loliondo Controlled Area, just south of the Kenya border. Lake Natron is seldom visited by the migrating herds because of the high hills surrounding it.

Magadi, Ndutu and Masek are all soda lakes with a high content of alkaline salts in them. This high salt content has its origin either in volcanic ash or mineral springs or both. The alkalinity becomes higher as the water evaporates and the salts make up a higher percentage by volume of the remaining water. Even so, these lakes provide a source of water, well into the driest parts of the year. Around Ndutu, for example, are several springs which feed the swamps, providing water for those animals that do not migrate north. During the worst of the dry season Ndutu itself may be reduced to a dry white pan. Nearby Masek fairs a little better. It is deeper and larger and holds more water so it lasts longer in the drought.

Lake Magadi is a very small lake to begin with, but because it is isolated from the others and more central in the ecosystem, it is important as a waterhole along the migration. Magadi is a Maasai word for salt and it too becomes very salty as it dries up.

There is another Lake Magadi in Ngorongoro Crater (see also Ngorongoro, A Day In the Life of an African Eden). It, too, lives up to its Maasai name. It is larger than its namesake in the Serengeti.

The most alkaline lake of them all is Lake Natron. It has the highest content of dissolved sodium carbonate of any of the lakes in the region. It is so concentrated that it irritates human skin should you plunge your hand into it. (I can attest to the fact that the dust blowing off these lakes is salty enough to irritate eyes, nostrils and lungs, but I have yet to stick my hand in the water!)

Lake Natron, like Ngorongoro’s Lake Magadi and other soda lakes in the Great Rift Valley, is an important feeding ground for lesser flamingos. Some three to four million of these birds rely on the rich algae growth in the soda lakes. A million flamingos consume over 66 000 tonnes (72,000 tons) of algae and diatomite a year!

It should be remembered that, despite their rich chemical content, these lakes are undrinkable by wildebeest and other mammals (and most birds). When these animals are seen by these bodies of water they have come to drink at the freshwater streams and springs that feed the lakes.

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