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Palearctic Deserts

By far the largest set of deserts in the world is found in the Palearctic realm. The region includes 21 lowland desert ecoregions that cover an immense corridor of deserts stretching from the Atlantic coasts of Morocco, to the Mediterranean coasts of the Sahara, to the Gobi in Central Asia, including the deserts of northern Africa, Arabia, Azerbaijan, Taklimakan, Central Persia and the Caspian lowlands (the full list of ecoregions includes Alashan Plateau semi-desert, Arabian desert and East Sahero-Arabian xeric shrublands, Azerbaijan shrub desert and steppe, Badghyz and Karabil semi-desert, Caspian lowland desert, Central Asian northern desert, Central Asian southern desert, Central Persian desert basins, Eastern Gobi desert steppe, Gobi Lakes Valley desert steppe, Great Lakes Basin desert steppe, Junggar Basin semi-desert, Mesopotamian shrub desert, North Saharan steppe and woodlands, Qaidam Basin semi-desert, Red Sea Nubo-Sindian tropical desert and semi-desert, Registan-North Pakistan sandy desert, Sahara desert, South Iran Nubo-Sindian desert and semi-desert, South Saharan steppe and woodlands, and Taklimakan desert).

The Palearctic realm also harbours three coastal deserts (Atlantic coastal desert, Gulf desert, and Red Sea coastal desert), as well as five montane sky-island relict ecosystems (Tarim Basin deciduous forests and steppe, Kuh Rud and Eastern Iran montane woodlands, Afghan Mountains semi-desert, Tibesti-Jebel Uweinat montane xeric woodlands, and West Saharan montane xeric woodlands). The Palearctic deserts cover a remarkable 16 million square kilometres, totalling 63 per cent of all the deserts on the planet. Of this area, 9 per cent is legally protected. Their population density is 16 persons per square kilometre, and their mean human footprint (15) is the second lowest on the planet, possibly because of the sheer inaccessibility and the extreme aridity of many of its large ecoregions.

The great shield desert of the Sahara has immense dune fields (Figure 4.6) covering 33 per cent of the total area, plains of metasediments, and sky-islands, mostly on volcanic rocks in the Tibesti, Ahaggar, and Ar ranges. The Red Sea rift separates the Sahara from the Arabian Peninsula. The deserts of West Asia and Central Asia (Figure 4.7) are all rain-shadow deserts, mostly encircled by young fold mountains. The Sahara alone occupies 4.6 million square kilometres, or 10 per cent of the African continent. It includes an undisturbed hyper-arid central area of sand and rock, but with small areas of permanent vegetation. Vegetation tends to be much more diversified in the Western Sahara, with xerophytes and ephemeral plants in the open desert plains, and halophytes in the moister areas. Currently the Sahara Desert is not well protected legally, but its inaccessibility plays a major role in conserving its ecosystems. The nomadic population depends on pastoral activities, hunting, and trade. Most settled people in the desert fringes do not venture into the interior. The high mountains shelter wild ancestors of many trees that have been domesticated for their fruits and nuts, such as pistachio and wild olive. Considering the hyper-arid conditions, the fauna of the Sahara is relatively rich; there are 70 mammalian species, 20 of which are large mammals; 90 species of resident birds, and around 100 species of reptiles.

In contrast with the Sahara and Arabian deserts, the deserts of Central Asia present fold mountains slopes cut by steep valleys with mostly ephemeral streams, fringing alluvial fans, and enclosed basins, some of which contain lakes (Caspian and Aral Seas, Lop Nor) or playas. The Turpan Depression in western China reaches 154 m below sea level. The predominant vegetation types are all formed by classic desert species of the Old World: grasses such as Panicum, Poa, and Stipa; sedges (Carex); desert bulbs such as wild tulips (Tulipa spp.); halophytes such as Salicornia and Atriplex; and shrubs such as Artemisia, Euphorbia, and Caragana. The most common vegetation found in the ecoregion is the desert sagebrush and other Artemisia species. The range of the flora goes from sagebrush to psammophytic (dune-adapted) plants, and includes salt-tolerant chenopod communities (Chenopodiaceae) in Afghanistan. The salt pans have almost no vegetation. In some parts, the deserts of Central Asia still support small populations of rare animals like wild Bactrian camels (Camelus ferus) and Asian wild asses (Equus hemionus) that have been largely extirpated in the wild.

The Mesopotamian shrub desert is transitional between the deserts to the south and the steppes to the north. The flora includes umbrella-thorn acacia (Acacia tortillis), shrubby rock-rose species (Cistus spp.), and many dwarf shrubs. Reeds and rushes grow in the wetland areas, while poplar (Populus euphratica) and tamarisk (Tamarix) grow along river channels.

The deserts of this biogeographic realm have evolved under continual grazing pressure and most plants are adapted to grazing pressure. However, overgrazing is a persistent threat; illegal hunting is a serious threat in the South Iran Nubo-Sindian desert; egg collection and nest disturbance affect nesting migratory waterfowl. Significant portions of the Azerbaijan shrub desert and the Central Asian northern desert are farmed under irrigation, causing water and soil pollution by the use of fertilizers and pesticides. The region as a whole contains most of the world's known oil reserves and large gas reserves, increasingly exploited and with associated urban areas and infrastructure development. Groundwater resources - vital but largely non-renewable - are also under heavy exploitation pressure.

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