Location and General Description
This is the largest ecoregion of the Arabian Peninsula, stretching from the Yemeni border to the Arabian Gulf and from Oman to Jordan and Iraq. Within this area lies a vast wilderness of sand, ‘a desert within a desert so enormous and so desolate that even the Arabs call it the Rub’al-Khali or the Empty Quarter’ (Thesiger 1959). It is probably the biggest continuous body of sand anywhere in the world, with an area of over 500,000 km2, or about the size of France (Mandaville 1986). Egypt’s Sinai desert falls within this area, as well as much of southern and eastern Jordan, western Iraq, and northern Saudi Arabia. Bordering the Arabian Gulf, there is an extension into Qatar and, further east, the region covers almost all of Abu Dhabi in the United Arab Emirates (UAE). The Rub’al-Khali crosses over from Saudi Arabia into western Oman and eastern Yemen.
Three distinctive physical features are evident in this vast desert territory. A corridor of sandy terrain known as the ad-Dahna desert connects the large an-Nafud desert (c. 65,000 km2) in the north of Saudi Arabia to the Rub’al-Khali in the south. Cutting through this corridor is the Tuwayq escarpment, an 800 km arc of spectacular limestone cliffs, plateaus and canyons with fantastic shapes etched by the wind and sand on the high cliffs. The Wahiba sands of Oman form an isolated sand sea bordering the east coast and appear as a remarkable feature on any satellite image of the area.
The Rub’ al-Khali deserves further attention due to its dominant position in this ecoregion. It is a sedimentary basin elongated on a southwest to northeast axis across the Arabian shelf. Its surface elevation in the far southwest is 800m, declining evenly over 100km to near sea level in the northeast. The sand overlies gravel or gypsum plains and can vary in depth from zero to 250m, whereas in the eastern margins the dunes reach maximum heights of up to 250m. Dune types range from solitary barchan dunes to extensive longitudinal dunes (300 km long) in the southwest and colossal dune mountains in the northeast. The sands are predominantly silicates, composed of 80 - 90% quartz and the remainder feldspar, whose iron oxide-coated grains give the sands a strong orange-reddish colour (Mandaville 1986). The natural beauty of the sand desert is striking with its reddish dunes, sculptured by the wind and stretching as far as the eye can see to the horizon.
Brackish salt flats exist in some areas, the most famous being the quick sands of Umm al Samim. A travelling companion of Wilfred Thesiger told him of a raiding party perishing in the quick sands and how a whole flock of goats disappeared beneath the surface (Thesiger 1959).
Much of the Rub’al-Khali is classified as ‘hyper-arid’, where more than 12 consecutive rain-free months have been recorded in an area that lacks seasonal precipitation. Rainfall is generally less than 35mm per annum (Mandaville 1986), and daily mean relative humidity is about 52% in January and 15% in June-July. Daily maximum temperatures average 470C in July and August, reaching peaks of 510C. The daily minimum average is 120C in January and February, although frosts have been recorded. Daily extremes of temperature are considerable.
Saudi Arabia’s northern Harrat al Harrah Nature Reserve is subject to hot summer temperatures, averaging 27.80C, and very cold winters averaging 6.80C, with frost common in mid-winter and an average rainfall of less than 80mm (Nader 1995).
The Rub’al-Kali is notable for its very limited floristic diversity. There are only 37 species, of which about 20 have been recorded from the main body of the sands and 17 mostly around the outer margins; of these, one or two are endemic species (Mandaville 1986). Vegetation may be described as very diffuse but fairly evenly distributed sand shrubland, interrupted in some parts by near sterile inter-dune floors. Typical plants are Calligonum crinitum on dune slopes, saltbush (Cornulaca arabica) and tussocks of sedge (Cyperus conglomeratus). Other widespread associates are Dipterygium glaucum, Limeum arabicum and Zygophyllum mandavillei (Mandaville 1986). Trees are absent, except around the outer margins, and are typically Acacia ehrenbergiana and Prosopis cineraria in drainage lines and pans between dunes. Along the northern edge of the Rub' al-Khali sands in the UAE, Calligonum comosum is the characteristic woody perennial, growing in wind blown sand, with a succession of annual herbs that grow after rain, of which Danthonia forskallii is the most common (Satchell 1978).
Unique to Oman’s Wahiba sands are the long stretches of single species ghaf (Prosopis cineraria) woodlands, which can be up to 85 km in length and 20km wide. These woodlands provide vital shade and nesting habitat for birds (Brown 1988). The sands here are also thought to act as a major ecological barrier dividing the faunal species of the northern mountains from those of central and southern Oman. The best example is the division between Arabian tahr (Hemitragus jayakari), which occurs in the northern mountains but not south of the Wahiba, and the Nubian ibex (Capra ibex nubiana), which does occur south of the Wahiba but is in turn absent from the northern mountains (Munton 1988).
In Saudi Arabia, gazelle and oryx have been successfully re-introduced after motorized hunting parties had virtually exterminated them by the early 1970’s. At the Uruq Bani Ma’arid protected area, white (or Arabian) oryx (Oryx leucoryx) again roam the sands, as do sand gazelle (Gazelle subgutturosa) and mountain gazelle (G. gazella). Capra ibex nubiana survived the exterminations that befell the oryx and gazelle and are officially protected in 3 sites. Both Oryx leucoryx and Capra ibex nubiana are included on the IUCN Red List as endangered (IUCN 2001). Other characteristic mammals include Arabian wolf (Canis lupus arabs), Cape hare (Lepus capensis), striped hyaena (Hyaena hyaena), sand cat (Felis margarita), red fox (Vulpes vulpes) and caracal (Caracal caracal).
Greater flamingo (Phoenicopterus ruber) caused a stir when they bred in Abu Dhabi in 1993, representing the first documented occasion in Arabia since 1922 (Aspinall 1996). The rare houbara bustard (Chlamydotis undulata) also inhabits this area, as does the rare spiny tailed lizard (Uromastyx thomasi).
An amazing sight following heavy rain is the new vegetation that appears, as the seeds of annuals that have lain dormant explode into a carpet of flowers. Other life forms triggered by rainfall include the fascinating ‘instant crustacean’ (Triops); this creature lies dormant for years until the arrival of a rainstorm and then hatches, moults from its cyst and grows to its full size within the space of several days.
Over the last few decades the desert of the UAE has, unfortunately, witnessed local extinctions of Canis lupus arabs, Oryx leucoryx, Hyaena hyaena, jackal (Canis aureus) and honey badger (Mellivora capensis). Gazelle subgutturosa and G. gazella still survive, though with very small populations and restricted ranges. The sand cat (Felis margarita), Ruppell's fox (Vulpes rueppellii) and Lepus capensis are thought to be far less numerous than they were.
In Saudi Arabia, a comprehensive network of protected areas covers many key sites, based on a system plan (Child and Grainger 1990). These areas are managed by the National Commission for Wildlife Conservation and Development (NCWCD), assisted by its two prominent research centres, the King Khalid Wildlife Research Centre (KKWRC) and the National Wildlife Research Centre (NWRC) in Taif. The stony basaltic desert of Harrat al Harrah, whose northern boundary borders Jordan and Iraq, was established in 1987 as Saudi Arabia’s first national reserve (12,150 km2). The landscape is dominated by numerous uplifted extinct volcanic cones and black basaltic boulders of the middle Miocene, making vehicle access mostly impossible. The reserve provides habitat to over 250 species of plants, 50 species of birds and 22 species of mammals (Nader 1995; Seddon et al. 1997).
The Uruq Bani Ma’arid is a 12,000km2 reserve on the western edge of the Rub’ al-Khali. Projects to re-introduce Oryx leucoryx and Gazelle subgutturosa began here in 1995. The NCWCD established the Ibex Reserve (200 km2) south of Riyadh to protect Capra ibex nubiana which, in 1994, numbered c. 259 (Habibi 1994). This reserve also serves as a re-introduction site for Gazelle gazella which, by 1994, numbered c. 160 (Dunham 1997). The At-Tabayq Special Nature Reserve in northern Saudi Arabia is also a protected area for Capra ibex nubiana.
In Jordan, the Royal Society for the Conservation of Nature (RSCN) is the body responsible for the creation and management of protected areas. Within this ecoregion, the Shaumari Wildlife Reserve protects vegetation, Gazelle spp., and the re-introduced onager (Equus hemionus). In 1978, this reserve was one of the first re-introduction sites in the Arabian Peninsula for Oryx leucoryx. In southern Jordan, the RSCN is working to establish a Wadi Rum Nature Reserve.
Types and Severity of Threats
Common threats to biodiversity in this ecoregion are wildlife poaching, overgrazing, and damage to vegetation caused by off-road driving.
A number of threats face UAE’s desert environment, including overgrazing by camels and goats, damage to vegetation through off-road driving, and habitat disturbance and fragmentation in the form of roads, agricultural projects and oil and gas surveys. No formal protected areas exist at the time of writing but a number of protected areas are in the planning for Abu Dhabi.
Hatough-Bouran & Disi (1991) describe how flora and fauna in the eastern deserts of Jordan are threatened by overgrazing. Socio-economic changes involving livestock subsidies and the introduction of water tankers have resulted in increased herd sizes and a more sedentary lifestyle amongst the Bedu. Similar overgrazing problems are reported for Saudi Arabia by Thouless et al. (1991), and such pressures are common elsewhere in this ecoregion.
Primary Sources and Justification for Ecoregion Delineation: This ecoregion boundary was formed using Zohary’s (1973) geobotanical map of the Middle East. It corresponds to Zohary’s classified regions of Saharo-Arabian desert vegetation of Anabasetea articulatae and Sahara-Arabian interior sand desert vegetation of Haloxylo-Retametalia raetami (including Haloxylion persici arabicum.
Justification of Ecoregion Delineation
This ecoregion boundary was formed using Zohary’s (1973) geobotanical map of the Middle East. It corresponds to Zohary’s classified regions of Saharo-Arabian desert vegetation of Anabasetea articulatae and Sahara-Arabian interior sand desert vegetation of Haloxylo-Retametalia raetami (including Haloxylion persici arabicum.
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Brown, K. 1988. Ecophysiology of Prosopis cineraria in the Wahiba Sands, with reference to its reafforestation potential in Oman. Journal of Oman Studies, Special Report 3:257-270.
Child, G., and J. Grainger. 1990. A system plan for protected areas for wildlife conservation and sustainable rural development in Saudi Arabia. National Commission for Wildlife Conservation and Development, Riyadh, Saudi Arabia.
Dunham, K. M. 1997. Population growth of mountain gazelles Gazella gazella reintroduced to central Arabia. Biological Conservation 81: 205-214.
Habibi, K. 1994. The desert ibex. Immel Publishing, London, England.
Hatough-Bouran, A., and A. M. Disi. 1991. History, distribution and conservation of large mammals and their habitats in Jordan. Environmental Conservation 18:19-32.
IUCN. 2001. The 2000 IUCN Red List of Threatened Species. IUCN, Gland, Switzerland and Cambridge, United Kingdom.
Mandaville, J.P. 1986. Plant life in the Rub’ al-Khali (the Empty Quarter), south-central Arabia. Proceedings of the Royal Society of Edinburgh 89B:147-157.
Munton, P. 1988. An overview of the ecology of the sands. Journal of Oman Studies Special Report 3:231-240.
Nader, I.A. 1995. Harrat al Harrah first national reserve in Saudi Arabia. Arabian Wildlife 2(2):14.
Satchell, J.E. 1978. Ecology and environment in the United Arab Emirates. Journal of Arid Environments 1:201-226.
Seddon, P. J., Y. Heezik, and I. A. Nader. 1997. Mammals of the Harrat al Harrah Protected Area, Saudi Arabia. Zoology in the Middle East 14:37-46.
Thesiger, W. 1959. Arabian sands. Penguin Books, London, England.
Thouless, C. R., J. G. Grainger, M. Shobrak, and K. Habibi. 1991. Conservation status of gazelles in Saudi Arabia. Biological Conservation 58: 85-98.
Zohary, M. 1973. Geobotanical foundations of the Middle East: Vol.1, Gustav Fischer Verlag, Stuttgart, Germany.
Prepared by: Robert Llewellyn-Smith
Reviewed by: In process