What is a desert ecosystem

desert


The most famous desert on earth is the Sahara Desert, in which Markus Mauthe photographed these sand dunes. Despite their beauty, deserts are regarded by many as worthless and in the past they were often misused for nuclear weapons tests (and now for waste disposal). Other countries are trying to make the desert "usable" by populating it with "high-tech cities" - but these are dependent on fossil groundwater and are therefore not sustainable. © Photo: Markus Mauthe / Greenpeace.

The following text is a slightly revised and expanded version of the text from the book "Naturwunder Erde" (Markus Mauthe / Jürgen Paeger, Knesebeck-Verlag 2013).

Where it becomes too dry even for grasslands, the realm of the desert begins. These seemingly limitless landscapes combine almost unearthly beauty with the toughest living conditions and cover about a third of the earth's land areas - making them the largest ecosystem on earth. Experts differentiate between semi-deserts and full deserts (in the semi-deserts there is regular precipitation, the full deserts, on the other hand, can be without rain for many years) and the subtropical "hot deserts" from the "winter-cold deserts", in which it regularly freezes. The hot deserts that lie along the tropics include the largest desert area on earth, which extends from the Sahara to the Arabian Peninsula to the Pakistani-Indian deserts. When the word "desert" comes to mind, most people think of dunes first - but only a fifth of the desert area is sandy (and only half of it consists of dunes), rock and stone deserts, which take up more than two thirds of the desert area, are much more common.

The ecosystem

The decisive factor for the formation of deserts is drought: most deserts are where less than 200 to 250 millimeters of precipitation fall per year. In addition to the amount of precipitation, however, the rate of evaporation is also decisive: Deserts always arise where this is higher than the amount of precipitation, i.e. the water balance is negative on balance. Therefore, there are areas with annual rainfall of less than 200 to 250 millimeters that are not deserts because the rate of evaporation is even lower; conversely, there are regions with higher precipitation, which are still a desert due to the high rate of evaporation. Exceptions confirm the rule mentioned above.

A drought sufficient for the development of deserts can come about in different ways. The great desert belt from the Sahara to the Pakistani-Indian deserts, which consists of hot deserts and semi-deserts, lies along the tropics (hence these deserts are also called "tropic deserts"). Here the air sinks, which rises as moist air at the equator due to the strong solar radiation, rains down over the tropical rainforests and then moves towards the poles at high altitudes. In this way, the air cools further, becomes denser and finally sinks to the level of the tropics. When it sinks, it becomes warmer again, and since warm air can absorb more water vapor than cold air, all clouds are dissolved, rain cannot fall here. A closed desert belt does not arise in the northern hemisphere, however, because the monsoon winds in Southeast Asia bring enough rainfall to prevent the formation of deserts there. In the southern hemisphere, only Australia is big enough for a comparable event.

In South America and South Africa, on the other hand, the subtropical deserts arise under the influence of cold ocean currents (“coastal desert”): The water cools the air above, but warm winds from the interior prevent it from rising and raining down. In winter, however, fog regularly forms, which does not bring any measurable precipitation, but reduces evaporation and provides many plants and animals with important moisture. These areas, also known as “foggy deserts”, like the Atacama in South America and the Namib in South Africa, come closer to the equator than the other subtropical deserts. The winter-cold deserts beyond the subtropics, on the other hand, arise in the rain shadow behind mountains ("rain shadow desert" or "relief desert") or simply because of the distance from the oceans ("continental desert" or "inland desert"). These include the Central Asian deserts from the Takla Makan to the Gobi Desert and the highlands of Tibet and the Pamir; in North America the Mojave and the Great Basin Desert and in South America the Patagonian Semi-desert. There are also the “cold deserts”, which arise where the summers are too short and too cool to allow significant plant growth and the rainfall is insufficient to form an ice cover. Cold deserts occur in the north of Greenland and Svalbard as well as on the Russian archipelago Franz Josef Land in the Arctic Ocean. (Some geographers also count the polar ice as a "polar ice desert" as a desert, but since the amount of precipitation there is greater than the rate of evaporation, the polar ice is treated here as a separate ecosystem.)

The hot deserts are among the hottest habitats on earth due to the lack of cloud cover. Temperatures in summer can exceed 50 degrees Celsius even in the winter-cold deserts. They cool down considerably at night, resulting in high daily temperature fluctuations. In winter, the temperature in cold deserts can drop to minus 40 degrees Celsius. In the desert, the flora hardly plays a role: the landscape is dominated by the rock. Even in the Sahara, sandy deserts only make up a fifth of the total area, the rest consists of rock, rubble and stone desert, known in the Sahara as "Hammada". The shapes of these landscapes are shaped by the wind, which spreads sand and dust and removes rocks like a sandblasting blower. Since sand is often blown close to the ground, this is how, for example, the "mushroom rocks" are created, the lower part of which has become thin as a stem due to the "sandblasting", while the "mushroom hat" remains unpolished above. The only desert on earth that is a pure sand desert is the Rub al-Khali (the "empty quarter") on the Arabian Peninsula, the greater part of which also consists of dunes. In the Sahara, the most famous sandy deserts (called "Erg" here) are the Erg Chebbi in Morocco and the Great Erg of Bilma in Niger. In addition to the sand, rock, scree and stone deserts, there are also gravel deserts ("Reg" in the Sahara), alluvial plains in which - if there is no runoff - salt lakes arise (it is the strong evaporation that causes the content of salts and minerals in the water is constantly increasing) and mountain deserts, such as in the Hoggar Mountains in the Algerian Sahara.

In full deserts, plants are most likely to grow in the mostly dry river beds (“wadis”) - and of course in the oases, habitats where spring water escapes. Reeds and various types of palm grow here. Elsewhere, larger plants, especially umbrella acacias and tamarisks, only occur where their roots can reach groundwater. The mountain deserts also offer opportunities for vegetation, because mountains are also rain catchers in the desert; In addition, the rainwater cannot "run" in the mountains, but sometimes even collects in natural cisterns ("Guelta" in the Sahara, the most famous of which is the Guelta d'Archei in the Ennedi Mountains in Chad, where there are even some once more widespread in the Sahara - crocodiles live). The salt lakes such as the Schotts in the Sahara or the Great Salt Lake in the USA are extreme habitats: only bacteria, cyanobacteria and green algae live in them. In the semi-deserts there is a richer vegetation. Small and medium-high bushes and cacti of all sizes cover at least part of the ground. The semi-desert of the Great Basin is roughly defined by the say brush, a type of wormwood, whose leaves wilt when dry. The cacti (and other very similar plants, such as the spurge family of the genus Euphorbia) defy drought by storing water - the saguaro cactus, which is so typical of the deserts of North America, can contain several hundred liters. Other plants fall into a "drought" or survive as seeds, bulbs or tubers and germinate en masse after rainfall. Above all, these short-lived plants ensure that flowering deserts are among the most colorful spectacles in nature.

The low plant productivity of the deserts also prevents abundant wildlife. To avoid the heat, many animals are nocturnal, during the day they crawl into the ground. In the semi-deserts there are also some larger herbivores such as saiga and oryx antelopes, gazelles or the Argali sheep in the high plateaus of Asia. Occasionally, they even attract large carnivores such as the snow leopard. The pill-turner, which lives on the excrement of herbivorous mammals, was revered by the ancient Egyptians as a symbol of the resurrection: such reverence for the recycling principle is unique to this day. The largest herbivores, the Arabian dromedary and the Bactrian camel ("trample") from the steppes and semi-deserts of Asia, were domesticated independently of each other in the third millennium BC and have been used as pack animals and farm animals ever since. The wild dromedary is extinct, the Bactrian camel is also threatened with extinction and is now only found in the Gobi.

Keyword: The material cycles of the earth

The fertility of soils is determined by their nutrient content. In addition to water and carbon dioxide, plants must primarily absorb nitrogen, phosphate and potassium as well as trace elements; these come from the mineral and organic components of the soil. When plants and parts of plants die, the nutrients are returned to the soil so that they are in a cycle. However, some of the nutrients are always removed from the system, for example when dissolved nutrients are washed out with rainwater and transported into the sea via rivers. This nutrient discharge must be balanced out if the ecosystem is not to lose fertility over the long term. Outputs and inputs form global nutrient cycles, such as nitrogen and phosphate cycles. In these, biological and geological cycles work together (the mineral components come from rocks). The Saharan dust, for example, is rich in iron and phosphorus (which can be traced back to an inland sea that once existed here, the paleo or mega-Chad). Iron is required by plants for the formation of chlorophyll, phosphorus is part of the DNA and the energy carrier ATP in living beings. When dust from the Sahara fertilizes the Amazon rainforests, it is part of the earth's iron and phosphate cycle.

Significance for the earth's ecosystem

The extreme temperature fluctuations in the deserts accelerate the weathering of rocks, and the sandblasting fan of the wind, which is hardly slowed by vegetation, helps. Sand and dust stay in the air for different lengths of time: While sand is usually only transported over shorter distances and then deposited, for example in the form of dunes, the finer dust (geologists consider all rock particles smaller than 0.063 millimeters in diameter to be dust) remain in the air for a long time and are transported over long distances as suspended cargo. Every year one to two billion tons of dust are distributed over the earth (from time to time the “Saharan dust” also reaches Germany). In the desert environment it is deposited as a fertile layer of loess. In the Chinese loess plateau, formed from the dust of the inner-Asian deserts, they have reached a thickness of up to 400 meters and have created their own culture with dwellings dug in the loess. In West Africa, too, desert dust brings valuable nutrients. In the seas it brings nutrients for algae and cyanobacteria, which bind carbon dioxide and are the basis of the food chain in the ocean. But dust transport even works across the continents: every year, for example, up to 50 million tons of dust are transported from the Sahara to the Amazon rainforest (240 million tons to Central and South America in total), where it compensates for the nutrient losses and thus preserves it contributes to this ecosystem.

The desert and man

Hunters and gatherers conquered the semi-deserts early on, as shown by rock paintings and finds in the South African Karoo - they probably took advantage of the occasional abundance after rainfall and followed the antelopes and gazelles. Later, the hunters and gatherers of the ancient world were often driven away by nomadic pastoral tribes, so that today some Aboriginal groups in Australia are among the last remaining hunters and gatherers of the deserts. Camels, dromedaries, sheep and goats were and are the most important grazing animals of the nomads. However, the oases were much more pleasant: water - there is plenty of light and warmth - enabled at least two harvests a year. Under the palm trees - from Mauritania to the Middle East date palms, on the southern edge of the Sahara Doum palms - fruit and vegetables and grain were grown; Fruit, vegetables and grain were also exchanged with the nomads for camels, sheep and goats. Oases were also important junctions for trade routes through the deserts.

With the demarcation of borders as a result of European colonization, the nomadic way of life in Africa and Asia became more difficult, in the Soviet Union and in communist China it was largely destroyed by collectivization. The use of the same pasture areas, however, led to the fact that their vegetation was destroyed. Semi-desert became desert. In the course of the oil boom, too, many oases from the Sahara to Arabia to Central Asia were given up. Instead, new cities fed by deep fossil groundwater resources were built as “high-tech oases” and attempts were made to green the desert with the help of this water. The latter often led to salinisation of the soil; and where it was successful it depleted the groundwater. Aside from their mineral resources, deserts today are often viewed as worthless and considered ideal places to dispose of litter or establish military test sites. Many atom bomb tests have been carried out in deserts. In the semi-deserts, on the other hand, attempts are primarily made to prevent the spread of the deserts (“desertification”). The increasing sedentariness of former nomads does not help.

What we can do for the desert

Since no value is attached to the deserts, their pollution from waste, the development of oil and gas reserves or from atomic bomb tests hardly meets with criticism - what has no economic value has no right to exist for many (which is of course extremely poor thinking) . The fact that deserts are fascinating habitats hardly counts, even if they are slowly being recognized as a tourist attraction. A camel caravan into the desert is one of the few unadulterated nature experiences that are still possible today. In the Sahara, however, this has become difficult in recent years due to the increasing presence of Islamists - "every European they get hold of is like winning the lottery for them" (Michael Martin), because ransom money can be extorted with tourists as hostages .
The nomadic use of pastures in the semi-deserts is the most ecologically justifiable use of these areas; and helping those who continue to aspire to this way of life to improve their living conditions is key to maintaining it. The prerequisite for this is the recognition of the value of the traditional way of life: the cultural diversity of mankind, which is declining from adaptation to the most diverse habitats, is just as threatened today as biological diversity. The knowledge of traditional peoples could one day become very valuable again when we run out of fossil fuels. However, these efforts are endangered by climate change: If, as the forecasts say, extreme weather events such as droughts continue to increase, the semi-deserts will become increasingly inhospitable, even for nomads. Activities against climate change - using energy more efficiently, using renewable energy sources - therefore also help the inhabitants of the desert.

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Jürgen Paeger 2013 - 2020