Location and General Description
This ecoregion occupies the majority of the Horn of Africa to the east of the Ethiopian highlands, including the Ogaden Desert and northeast Kenyan semi-deserts. A narrow corridor of the ecoregion penetrates the floor of the Ethiopian section of the Rift Valley, separating the northern and southern Ethiopian highlands, and a finger extends north to the Eritrean/Sudanese border. The ecoregion is mainly flat and low-lying (over half lies below 500 m) rising towards the west and north. However, it is defined more by rainfall and vegetation type than by altitude, and thus extends from sea level on the coast of Somalia to over 1,500 m in the Rift Valley and Sidamo region of southern Ethiopia.
The mean maximum temperatures are around 30°C, and the mean minimum temperatures are 15° to 18°C. Annual rainfall varies from below 100 mm in the Ogaden Desert to around 600 mm in areas bordering the Ethiopian highlands. There are only three permanent rivers of any significance: the Awash, Wabi Shebele and Jubba, all of which originate in the Ethiopian highlands. The area is underlain by post-Cretaceous rocks which are mainly marine in origin, over which soils indicative of high aridity - xerosols and yermosols - have developed. The Somali hinterland, or Haud, is characterized by deep, infertile sands. Pre-Cambrian granites form inselbergs, or ‘burs’, in southern Somalia.
The ecoregion is sparsely populated, with typical densities of less than 20 persons per km2. In the heart of the Ogaden Desert and some of the other significantly dry regions of the former Somalia and eastern Ethiopia there are no permanent inhabitants.
Phytogeographically, the ecoregion lies within the bushland and thicket of the Somali-Masai regional center of endemism, and Somali-Masai semi-desert grassland and shrubland (White 1983). The most common tree species belong to the deciduous genera Acacia and Commiphora. The understory consists of shrubby herbs less than one meter high, such as Acalypha, Barleria and Aerva. At lower elevations where rainfall is less consistent, vegetation becomes semi-desert scrubland. Acacia and Commiphora are joined by Euphorbia and Aloe, as well as grass species such as Dactyloctenium aegyptium and Panicum turgidum (Tilahun et al. 1996). Important evergreens include Boscia, Dobera, Salvadora, Grewia and Cadaba. Crotalaria and Indigofera are also found. The uniform appearance of this vegetation hides considerable variation in floristic composition. Forest vegetation once surrounded the bases of the inselbergs and lined permanent watercourses, but has largely been destroyed by human activity (Friis 1992).
Because this ecoregion is such an ancient and stable habitat, a high number of arid-adapted species are found here, many of them endemic. There are 1,250 plants recorded from the Somalia-Masai phytochorion (White 1983), but it is not known how many of them are endemic. There are also a large number of endemic reptiles, with 33 species believed to be confined to this ecoregion, and a further c. 20 near-endemics also found in the surrounding lowland ecoregions of the Horn of Africa.
The Horn of Africa is a notable center of endemism for mammals, particularly for antelopes such as the dibatag (Ammodorcas clarkei, VU), beira (Dorcatragus megalotis, VU), hirola (Damaliscus hunteri, CR) and Speke's gazelle (Gazella spekei, VU). There are also a number of smaller mammals including four Gerbillus species, one Microdillus species, one white-toothed shrew (Crocidura greenwoodi, VU), and the walo (Ammodillus imbellis, VU), a gerbil known only from Somalia. The Somali warthog (Phacochoerus aethiopicus delamerei, VU) is also near endemic to this ecoregion.
Between 400 and 1,000 hirola (Damaliscus hunteri, CR) live in the vicinity of Bura east of the Tana River in Kenya. The dibitag (Ammodorcas clarkei, VU) is found only in Somalia and eastern Ethiopia. Widely distributed but threatened ungulate species include dorcas gazelle (Gazella dorcas, VU) and Soemmering's gazelle (Gazella soemmeringi, VU). In the north of Ethiopia a small population of African wild ass (Equus africanus somaliensis, CR) inhabits the Yangudi Rassa National Park and a herd of Grevy's zebra (Equus grevyi, EN) occurs in the Alledeghi Game Reserve. Small numbers of the formerly widespread but now endangered subspecies Swayne's hartebeest (Alcelaphus buselaphus swaynei, EN) inhabit the Senkelle Wildlife Sanctuary and Nechisar National Park in Ethiopia's Rift Valley. The beisa oryx (Oryx gazella beisa) was formerly widespread throughout Somalia. Excessive hunting had exterminated this antelope over much of its Somalian range by the 1980s but it is still distributed over a wide range in Ethiopia, the largest population found in the Awash Valley north of the Awash N.P. The gerenuk (Litocranius walleri) also occurs and has a wider distribution, extending further south into Kenya (Stuart and Stuart 1996). The greater and lesser kudu (Tragelaphus strepsiceros, Timberbis) can be found in areas of Acacia-Commiphora woodland in the Ethiopian section of this ecoregion. Sizable populations of the lesser kudu are found in Ethiopian protected areas such as Awash, Omo and Mago National Parks. The greater kudu’s distribution within Ethiopia is patchier, but significant populations can be found in hilly areas in Nechisar, Omo and Mago N.P.s (East 1999).
Elephants (Loxodonta africana, EN) and buffalo (Syncerus caffer) were previously widespread in the wetter portions of this ecoregion. Elephant populations are decreasing, with limited numbers found in protected areas. The Babile Elephant Reserve in Ethiopia was established to protect the only known population of the isolated, ecologically distinct subspecies Loxodonta africana orleansi (Barnes et al. 1999). This subspecies was once also found in the Somali Alifuuto (Arbowerow) Nature Reserve but no recent information is available (World Bank 1993). Lion (Panthera leo, VU), leopard (Pathera pardus, EN), cheetah (Acinonyx jubatus, VU), and striped and spotted hyaenas (Hyaena hyaena and Crocuta crocuta) are the main large carnivores in this ecoregion. The endangered wild dog (Lycaon pictus) is also found in the Ethiopian section of this ecoregion, most commonly in Mago and Omo N.P.s (Woodroffe et al. 1997).
Most of the endemic species of animal and plant are associated with dry habitats, but the riverine habitats along the Jubba and Wabi Shebele support two strictly endemic birds, the Degodi lark (Mirafra degodiensis, VU) and the Bulo Burti bush-shrike (Laniarius liberatus, CR), qualifying as an Endemic Bird Area (Stattersfield et al. 1998). The Abyssinian yellow-rumped seedeater (Serinus xanthopygius), the short-billed crombec (Sylvietta philippae, DD), and Sidamo bushlark (Heteromirafra sidamoensis, VU) are all restricted to this ecoregion as well, while the sombre chat (Cercomela dubia, DD), white-winged collared-dove (Streptopelia reichenowi), Salvadori’s weaver (Ploceus dicrocephalus), and the scaly babbler (Turdoides squamulatus) are considered near-endemic.
The habitats of the ecoregion are mainly dry woodlands and scrub, with a gradation to grasslands and deserts in the driest places. Most of these areas remain unfragmented and intact, as the human population is low and agriculture is concentrated along watercourses and close to the coastline. This ecoregion has been severely affected by political instability and war over the past few decades. Large mammal populations have been depleted throughout the ecoregion, especially in Somalia where there has been no federal government since 1991.
Stable government will need to return to Somalia before large-scale conservation work can occur in the Somalian portion of this ecoregion. The Ethiopian portion is currently stable but conflicts between protected areas and people continue. The endemic, desert-adapted large mammals and the restricted range birds are priority species for conservation efforts.
There are several protected areas in this ecoregion, many of which harbor the last remaining populations of desert-dwelling ungulates. However, these parks are generally not well protected, managed, or funded, making them "paper parks" rather than effective sites for conservation. Protected areas in Ethiopia include Yangudi Rassa, Nechisar, Awash, Omo, and Mago National Parks, as well as Chew Bahr Wildlife Reserve and Babile Elephant Sanctuary. In Kenya, the Malka Mari National Park falls within this ecoregion and in Somalia, the Alifuuto (Arbowerow) Nature Reserve is found here, although there is no recent information about the status of this site.
Types and Severity of Threats
Habitats have become degraded in many places through grazing by livestock and fuelwood collection, particularly close to villages and towns. This overuse may be accelerated in areas where irrigation schemes have been launched (e.g. around Lake Ziway in Ethiopia). In Ethiopia, recent agricultural schemes have denuded the landscape in the Gode plain along the Wabi Shebele River and in the Awash Valley in the Afar region. These large, mechanized farms use pesticides, herbicides and fertilizers, and have continued their activities largely unmonitored. Infrastructure for harvesting natural gas is being developed in the region of Ethiopia closest to the Somali border (Tilahun et al. 1996).
Protected areas are also degraded by human use, including national parks that are well-established. Recent reports indicate that more people are moving into Mago, Omo and Nechisar National Parks, with serious effects on the vegetation and wildlife. Negotiations are underway with stakeholders in an effort to resettle people living within park boundaries.
Throughout this ecoregion, riverine vegetation is often extremely degraded. Some economically important species, such as the yeheb nut (Cordeauxia edulis) may be declining due to overgrazing. There has been a long history of human habitation in the Horn of Africa. However, droughts dramatically decrease livestock numbers and allow native vegetation to recover. Native plants and animals are also adversely affected by drought, but not as severely as livestock (Kingdon 1989).
Populations of most endemic and near endemic large mammals have declined dramatically and are fragmented due to illegal hunting following decades of political instability and open warfare over large parts of the region. Conflict with farmers is a serious threat to the dwindling elephant population (Barnes et al. 1999). Better management of existing protected areas and improved law enforcement are the main conservation requirements. If stable government ever returns to parts of Somalia, it will be imperative to support the authorities to re-establish protected areas and to try and safeguard the remaining populations of large mammals. Regional conservation planning should be encouraged but will obviously be difficult to achieve. Relations between Ethiopia and the former Somalia have been strained since the two countries fought over the Ogaden Desert in the late 1970s; Ethiopia and Eritrea fought a border war as recently as the late 1990s; and there is politically-motivated instability in the border regions between Kenya and Ethiopia, Kenya and Somalia, and Eritrea and Sudan.
Justification of Ecoregion Delineation
This ecoregion follows the northeastern part of the ‘Somali-Masai Acacia-Commiphora bushland and thicket’ vegetation unit of White (1983). This was separated from similar communities to the south based on different bioclimatic patterns and floral and faunal compositions in the Horn of Africa. The northern portion of this ecoregion subsumes large portions of White’s ‘Somalia-Masai semi-desert grassland and shrubland.’ It is bound on the west by the Omo River, and to the south by the Guiba and Tana Rivers and inland areas of sandy and rocky plateau. These areas reflect the southern extent of some species like the dibitag (Ammordorcas clarckei), and the northern extent of others, such as the common eland (Taurotragus oryx), Bohor reedbuck (Redunca redunca) and Burchell’s zebra (Equus burchelli). The ecoregion is noted for high levels of endemism in mammals, reptiles, and birds. Kingdon (1989) identifies the Horn as a regional center of endemism.
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Prepared by: Chris Magin, Miranda Mockrin
Reviewed by: In progress