Why savannahs are replaced by deserts. What is savannah and where is it located

The article defines what a savannah is. The climatic features of the natural zone are described, the characteristics of soils, flora and fauna are given.

This information will be useful for schoolchildren and students when preparing for a lesson, report or exam.

What are savannas

Savannahs are vast territories that occupy a significant part of the subequatorial belt, covered with high grassy vegetation and rare trees.

From the description of the natural zone of savannahs and light forests, the main points should be noted:

1. The grass cover is higher than in the steppes, and it is based on hard-leaved grasses.
2. Canopy density is either high or low so that the soil is visible.
3. There may be no trees at all, but there are areas that are almost sparse forest.

Geographical position

Location - subequatorial belt in both the northern and southern hemispheres. The map of natural areas shows that grassy spaces cover almost 40% of Africa's area, and separate territories are also located in Australia, Northeast Asia, and America.

In South America, the natural zone captures the Brazilian Highlands and the plains of the Orinoco River. In Brazil, the areas are occupied mainly by light forest, in the Orinoco basin there is almost no woody vegetation. South American savannahs have different names: Brazilian - campos, Venezuelan - llanos.

In Asia, the natural zone occupies separate parts of India, Burma, Ceylon, and Indochina.

In Australia, grassy areas are located in the northeast, characterized by a pronounced dry period.

savanna plants

The flora is represented by a high grass cover with separate trees and shrubs, small groups of trees.

elephant grass

Most of the plants are hydrophytes; there are also xerophytes adapted to the dry season. In the dry months, grasses burn out, many trees lose their leaves. Grasses stretch up to 3 m, and in the lowlands up to 5 m.

Typical plant species:

• elephant grass;
• oil palm;
• doom palm;
• pandanus;
• baobab is a thick tree with an unusually shaped trunk.

In more humid places, the grass cover becomes lower (up to 1.5 m), supplemented by acacias - trees with a dense spreading crown resembling an umbrella.

For more arid places, thorny semi-savannahs are characteristic. Trees almost the whole year without leaves, the grass carpet is rare, low (up to 1 m).

The flora is represented by low thorny tree species, succulents, cushion shrubs. Some scientists call these areas the African steppe.

Soils

The main ones are red-brown and lateritic soils, characterized by a sufficient content of humus due to the abundant decomposition of grass.

Due to periodic moistening in the soil layers, saturation with metal oxides proceeds actively, therefore, crusts often appear on the surface of the earth.

The seasonality of moisture affects the processes of soil formation. In the wet season, the soil layers are intensively leached, in the dry season, soil solutions rise due to the heating of the earth's surface. Therefore, the accumulation of humus, the blackening of soils, and the formation of chernozems are characteristic of dry savannahs, where the period without precipitation is long.

Relief

On the African continent, the zone of savannahs and light forests occupies the plateau of East Africa, the watershed plateaus of the Zambezi, Congo, Limpopo rivers, and individual sections of the high Kalahari plains.

Savannah in Tanzania

In South America, savannas are found on the Brazilian and Guiana highlands, the Gran Chaco plain, in the Orinoco basin.

In Australia, in the northeast plains.

Climate and climatic zones

Savannahs are located in the subequatorial climate zone. Two seasons are clearly identified: winter dry and summer wet. The annual temperature ranges from 18 to 32°C. Temperature fluctuations are slow, unexpressed.

The dry cool period lasts from November to April. The average temperature is 21°C. The weather is sunny, fires are frequent. No more than 4 inches (100 mm) of precipitation falls.

The dry season is the time for migrations. Huge herds of ungulates go in search of food and water, followed by predators. Woody species survive dry times thanks to deep root systems and dense refractory bark.

The hot humid period begins in May and lasts until October. Rainfall for the period reaches 10 - 30 inches (250 - 750 mm). Heavy rain falls in the afternoon.

During the rainy season, the life of the savanna is in full swing, the land is reborn after a drought, covered with a lush green carpet.

savanna dwellers

Savannah fauna is unique. Nowhere else on the planet is there such a variety of large ungulates and predatory animals.

Unfortunately, since the beginning of the 20th century, wildlife has been seriously suffering due to the activities of poachers and indefatigable hunters, laying roads, and allocating large areas for cattle breeding and agriculture.

horse antelope

The list of animals that disappeared due to hunting activities includes:

• white-tailed wildebeest;
• horse antelope;
• zebra quagga.

Ungulates

The largest group of savannah ungulates lives in Africa.

The most common:

• blue wildebeest;
• zebras;
• Thompson's gazelles;
• Grant's gazelles;
• impalas;
• Cannes;
• cow antelope;
• swamps;
• giraffes;
• buffaloes;
• warthogs;
• African elephants.

Antelope Kudu

Rare ungulates, found only in reserves, are kudu, oryx.

Black and white rhinos are on the verge of extinction. Their luxurious, as you can see in the photo, horn is a valuable prey for poachers.

In the reserves, great efforts are being made to preserve these animals.

Predators

Predatory animals are as diverse as herbivores.

African leopards

On the African plains are common:

• lions;
• spotted hyenas;
• hyena dogs;
• leopards;
• cheetahs;
• caracals;
• Nile crocodiles.

In the American steppes live:

• jaguars;
• ocelots;
• maned wolves;
• cougars.

Dingo dog

In Australia:

• monitor lizards;
• dingo dogs.

Birds

The variety of African birds is amazing, attracting tourists from all over the world.

African ostrich

On the trees, baboons and numerous species of monkeys coexist with birds. Flamingos are the decoration of reservoirs.

Nandu ostriches are inhabitants of the Brazilian steppes, emu ostriches are Australian.

Insects

Among insects that eat the green parts of plants, one can note:

• locusts (the most common family);
• bronzovok;
• cicadas;
• Khrushchev;
• caterpillars;
• leaf beetles;
• goldfish;
• stick insects.

Of the processors of dead organic matter, the following are common:

• termites (in savannas, the largest number of termite mounds, often of enormous size);
• crickets;
• worms;
• cockroaches;
• centipedes;
• darklings;
• land molluscs.

Termites are the main food source for Australian and South American anteaters.

Every year more and more deserts come to the savannas. This is especially noticeable in Africa. The main reason why savannahs are replaced by deserts is human activity. A person takes too much water from reservoirs for his needs, because of which the vegetation experiences a severe moisture deficit.

Other causes of desertification are global warming and intensive animal husbandry. Grazing cattle eat grass so actively that the grass cover does not have time to recover.

Savannahs and deserts on the continents where they exist are almost always adjacent and replace each other. This is due to the fact that each of them is located in the same climatic zone. Under the influence of the precipitation system of these zones, they become savanna or desert.

Formation of savannas and deserts

Savannahs form in areas where there are two distinct seasons of the year: rainy and dry. Also in this zone, a high temperature of about 30 ° C almost always remains. Almost always, such conditions are observed in the subequatorial or tropical zones.

There are savannahs on four continents:

• Africa.
• Asia (India, Pakistan).
• Australia.
• South America.

Deserts, in turn, are most often formed in the tropical zone, since here the temperature is almost the same or even higher than in the subequatorial zone. However, the winds that bring precipitation bypass the tropical zone, so there is very little precipitation here.

This explains why these two natural zones replace each other on the continents.

Change from savannah to desert

In some parts of the Earth, especially in northern Africa, there is a tendency for the desert to expand at the expense of the savannah. This happens for several reasons.

First, as a result of global warming, the drought season has been dragging on lately. Therefore, in the transition zone between savannahs and deserts, the sands of the Sahara cover the fertile soil, making it difficult for plants to sprout after the rainy seasons.

Secondly, the human factor plays a big role here, since people in the savannah zone graze too many livestock that eat all the grass entirely. Including, their food is cereals with seeds. And in the savanna, the ecosystem is designed so that local herbivores (zebra, wildebeest, etc.) eat different parts of plants, which gives the latter time to spread.

Thirdly, man uses groundwater for various purposes. Because these dry season waters help the root systems of perennials survive until the rainy season, when man takes them, the grasses die and the desert expands.

1. Name the natural areas of Africa. What are the features of their placement on the mainland?
2. What is the relationship between climate zones and natural areas?
3. What are the essential features of the zones of equatorial forests, savannahs, tropical deserts.

Equatorial forests are located on both sides of the equator in the Congo (Zaire) basin and along the Gulf of Guinea north of the equator. The formation of the zone is due to the large amount of heat and moisture throughout the year.

The equatorial forests of Africa are diverse in composition. There are about 1000 species of trees alone. The upper tier is formed by ficuses, palm trees, etc. In the lower tiers grow bananas, tree ferns, lianas, which, hanging in garlands from trees, make the forest thicket impassable in places.

The equatorial forest is the birthplace of many valuable plants, for example, the most common of all palms - oilseed, from the fruits of which palm oil is obtained. The wood of many trees is used to make expensive furniture and is exported in large quantities outside the mainland, for example, ebony, which has black or dark green wood.

Many animals of the equatorial forests live in trees. In addition to birds, rodents and insects, numerous monkeys live on the trees - monkeys, chimpanzees, etc.

Terrestrial inhabitants include bush-eared pigs, small ungulates (African deer, etc.). On the forest edges and near the shores of water bodies, there are the rarest animals on earth - pygmy hippos (up to 80 cm tall) and giraffe relatives - okapi, living only in Africa. A large predator of the equatorial forests is the leopard. In remote, inaccessible places, the largest great apes, gorillas, have been preserved, which are not found anywhere else. Snakes and lizards are found in loose soil and forest litter.

Rice. 56. Rhinoceros in one of the national parks of the African savannas

Ants are common in all tiers of the forest. Some types of ants (the so-called wandering ants) move in long columns, destroying all living things in their path. Numerous termite insects feed on plant debris.

The tsetse fly brings great harm to the population. It is a carrier of a pathogen that causes disease and death in cattle and horses, and in humans, life-threatening sleeping sickness.

Unusual for us is not only the vegetation and fauna of the equatorial forests, but also other natural phenomena. There reigns eternal summer, the eternal equinox. And we have only two days a year when day equals night. In low latitudes it gets dark quickly, just as quickly in the morning the sun awakens all nature. No less unusual is the view of the starry sky. The Southern Cross is striking, and the North Star is close to the horizon.

Savannahs in Africa occupy vast expanses - about 40% of the mainland. No other continent has such a large percentage of the savannah area. In appearance, the savanna differs sharply from the equatorial forests. A person who is in the forest is cut off from light and sun, he is surrounded by huge trees and shrubbery, he is, as it were, at the bottom of a green sea, in eternal twilight. People who find themselves in the savannah after a dark, gloomy forest are amazed at the abundance of light and open cheerful spaces. Forest and savanna are two different worlds.

The soils and vegetation of the savannas depend on the length of the rainy season. Closer to the equatorial forests, where the rainy season lasts 7-9 months, red ferrallitic soils form. Herbs reach 3 m in height. Among the continuous sea of ​​grasses, groves with sparsely growing trees, huge baobabs with spreading branches, oil palms, and a doom palm are scattered in places.

Where the duration of the rainy season is less than 6 months, typical savannahs are common with reddish-brown soils, with not very tall grasses. On the boundless grassy space, various acacias with a flat, umbrella-shaped crown stand out.

Rice. 57. The fauna of Africa is unique

On the border with semi-deserts, where scarce rains fall for only 2-3 months, deserted savannahs are formed with dry thorny bushes and sparse hard grasses. There are also spurges - tree-like plants with fleshy stems and branches, devoid of leaves and covered with thorns, which, adapting to a dry climate, accumulate moisture in them.

The rainy time, which comes after the zenithal position of the Sun, comes suddenly. As if by magic, the savannah is covered with luxurious grass, and the trees awaken to life after sleep. Herds of antelopes, rhinos, elephants, zebras, etc. appear.

Nowhere in the world is there such an accumulation of large animals as in the African savannah: various antelopes, striped zebras, giraffes, which, stretching their long necks, eat leaves from tall trees. There are also other large herbivores in the savannah - elephants (weight up to 4.5 tons), buffaloes, rhinos, which are almost exterminated by man. Along the banks of rivers and lakes there are hippos (weight up to 3 tons). Such an accumulation of large animals is possible due to the abundance of a variety of food. Herbivores are accompanied by numerous predators - cheetahs, leopards, jackals, hyenas. Among them, the most powerful and formidable is the lion. Crocodiles live in the rivers, the largest of them - the Nile ones - reach a length of 5-6 m.

African savannahs are unusually rich in birds. Here is the smallest - a beautiful sunbird, and the largest bird on Earth - an African ostrich, a marabou bird, which is found only in Africa. Of the predatory ones, the secretary bird with long legs, like those of a crane, stands out for its appearance and habits. She hunts for small rodents, reptiles, especially snakes. The bird catches up with the snake and tramples it with its feet.

There are extremely many termites in the savannah, their strong high buildings of various shapes - termite mounds - are a characteristic detail of the zone.

In the dry season, large animals, birds migrate to humid places, invertebrates, amphibians hibernate or take refuge in shelters.

The natural conditions of the savannahs are favorable for growing cultivated plants of hot countries: cassava (an evergreen shrub whose roots are rich in starch), sweet potatoes (sweet potatoes), corn, peanuts, in the eastern part of the savannas - cotton, and in more humid places - rice.

Tropical deserts in Africa cover a huge area. Deserts and savannas account for a large area of ​​the mainland. Therefore, Africa is called the mainland of the classical development of savannahs and deserts.

The largest area of ​​the desert is in North Africa. The annual amount of precipitation in the Sahara is almost everywhere less than 100 mm. In the inner parts of the rainfall sometimes does not happen for several years. Clouds are rare, so the sun's rays heat the earth's surface especially strongly. In summer, the heat reaches 40-50°C in the shade. Escaping from the scorching sun, the local population puts on loose long clothes. It is especially difficult to bear the heat because of the hot and dry winds, the heat dries out the skin. A person feels very tired, thirsty, loses his appetite.

Rice. 58. Sandy desert in the Sahara

Large daily and significant annual temperature amplitudes cause strong physical weathering. Often in the Sahara you can hear explosions reminiscent of the rumble of a distant cannonade. These rocks crack and collapse, turning into a pile of stones, rubble and sand. In the Sahara, huge areas are occupied by rocky deserts. Clay and sandy deserts alternate with them, where dunes and dunes pile up in places.

The vegetation of the Sahara is extremely sparse, and in some places, especially in the central part, it is completely absent. In some places separate bunches of herbs and thorny bushes grow. Rich vegetation develops only in oases. The animals of the Sahara, like other deserts, are adapted to the conditions of the desert climate. So, antelopes are able to run long distances in search of water and food. Lizards, turtles and snakes can go without water for a long time. Various beetles, locusts, scorpions are numerous. From predators there are hyenas, jackals, foxes.

In South Africa, the desert zone occupies the coast of the Atlantic Ocean (Namib Desert). It is characterized by a unique and amazing velvichia plant. Its short trunk rises only 50 cm above the ground. Two dense leathery leaves extend from its top, reaching a length of up to 3 m. The leaves grow continuously, dying off at the ends. The age of velvichia can reach 150 years. To the east and north, the deserts of South Africa turn into semi-deserts, dominated by thorny cushion plants, as well as euphorbia and aloes. Wild watermelons with juicy fruits are also characteristic, often replacing water for the local population and animals.

1. On the map, determine in which climatic zones there are tropical deserts and semi-deserts.
2. Using maps, determine the climatic conditions of the desert zone.
3. Why are savannahs replaced by deserts?
4. What are the characteristics of rivers in the tropical desert zone?