It is a very dry zone vulnerable to wind and water erosion even with little or no pressure on the vegetation from grazing. The vegetation consists of deciduous shrubs, dominated by Acacia sp. interspersed with less frequent evergreen shrubs and succulents. It has very variable grass vegetation. The people of the area are pastoral and agro-pastoral. Large scale irrigated agriculture is gaining importance in some areas of the ecosystem. This ecosystem is the extreme lowland region of the country. The flora has developed an advanced xeromorphic adaptation. Shrubs and trees have developed dwarf growth and have small, sclerenchymatic or pubescent leaves. They have an adapted resistance to browsing through thorns and development of alkaloid content. The vegetation is very scattered above the ground with extremely well developed deep root systems. The topsoil is very often highly salty, thus the development of salt tolerance. The region is very low with erratic rainfall. The south and southeastern part of the ecosystem have two rainy seasons while the north and the Northeast have one (Zerihun Woldu, 1999; Demel Teketay, 1999). Harsh and hot temperature is the characteristic of the ecosystem with low and uneven distribution of rainfall. High evapotranspiration is another phenomena in the ecosystem, which causes excess monthly PET than rainfall.
Concerning the moisture zone, the ecosystem extends from semi-arid to hyper-arid. Accordingly, a great variation is observed in different climatic aspects of the ecosystem. Based on the temperature and moisture regime, the desert and semi-desert ecosystem can be explained by the context of arid (pastoral), semi-arid (pastoral/cultivation) and hyper-arid (EPA, 1998).
This ecosystem has mean annual rainfall of between 300-800 mm, a mean annual evapotranspiration of 1600-2100 mm and mean annual temperature of 16-27o C. The parts of the ecosystem that fall under the arid zone are characterized by mean annual rainfall of between 100-800 mm, mean annual temperature of 21-27.5o C and mean annual precipitation between 1700-2600 mm (Engeda Mersha, 2000).
Tertiary and quaternary volcanic are included in the parent materials. Colluviums from highland plateau, Aeolian deposits as well as marine deposit are also good characterstics. In general, sol types in the ecosystem vary depending on location and altitude. Vertisoles, Cambisoles, Calcisols, Gypsysols, Lithosoles, Regosoles, Solonchks and fluvistols can all be found in the ecosystem.
Varied water resource regimes are found in the ecosystem. The areas covering most of the drier lowland parts of the ecosystem situated in the southern parts of Oromia (Moyale area), the eastern most parts of Afar and some parts in the north-eastern Tigray are characterised by localized and moderately large quantities of ground water, especially along valleys. They have mostly intermittent streams with some perennials. The quality is fair-to-poor. The depth of the ground water is 0-270 m. Those areas covering the southeastern parts (the Ogaden) are characterized by localized and limited quantity of ground water with a depth of 0-300 m. The quality is fair-to-poor chemically.
True desert, where there is no vegetation, occurs only in the Northeast, the Danakil Depression (Zerihun Woldu, 1999). At low altitudes, semi-desert and bush land prevails. The desert ecosystem is found along borders of eastern and southern Ethiopia, below 900 meters above sea level. The semi-desert parts are found in the northern western and Northeastern parts of the country (Amhara, Tigray and Afar), Southern (Oromia and Southern Nations and Nationalities) and the South-eastern and eastern (Somali) parts (Zerihun Woldu, 1999). The northern parts of Afar and north-eastern Tigray are predominantly desert.
Highly drought tolerant shrubs, some succulents and few grasses characterize the ecosystem. The Charactersitic species are: Acacia spp., Boscia sp., Cadaba sp., Commiphora sp., Maerua sp., Ziziphus sp., Aloe sp., Commelina sp., Dactyloctenium sp., Euphorbia spp., etc. (Zerihun Woldu ,1999; Demel Teketay, 1999).
On rocky outcrops where succulents predominate may be found Euphorbia spp., Aloe spp., Caralluma spp., Sansevieria spp., Cissus spp., Commiphora spp., Dracaena ombet, Withania somnifera, Aenium spp.,etc. In areas where the run-off from the surrounding higher areas makes the condition suitable, a semi-desert thicket develops with Acacia spp., Commiphora spp., Ziziphus spp., Cadaba spp., Malruna spp., Jatropha spp., Sansevieria spp. (Zerihun Woldu, 1999; Demel Teketay, 1999; EPA, 1998).
Scattered shrubs and grasslands cover large part of the semi-desert areas. The shrubs are Grewia spp., Commiphora spp., Acacia spp. and Balanites spp. The most widely distributed grasses are Chrysopogon aucheri and Aristida spp. Blepharis spp. and Abutilon spp. are some of the herbs occurring in this ecosystem. In grasslands with saline soils, the occurrence of shrub species such as Atriplex spp., Suaeda spp., Limoniuin spp., clumbs of Tamarix spp. indicate the presence of ground water (EPA, 1998; Zerihun Woldu, 1999; Demel Teketay, 1999).
Wild animals inhabiting the ecosystem are the Wild Ass, Sommering’s Gazelle, Beisa Oryx, Grevy’s Zebra, Ostrich, Bustards, Secretary Bird, Carmine bee-eater, Abyssinian roller and Pygmy falcon.
8.4 Uses and values
The whole of the eastern lowland areas consisting of the Afar Depression and the Ogaden is under nomadic pastoral use, exception being a very small area in Awasa and Kelafo. In these small areas, traditional irrigated agriculture still exists and in the higher upstream modern mechanized cotton farming has developed. From Lake Turkana area (Southern Omo Zone), extending east to southern Sidamo and Bale Zones is also under pastorals. Downstream along the Omo River a small amount of traditional flood agriculture occurs.
The eastern part of the ecosystem, the Danakil Depression is mainly grazing land. The same is true with the Ogaden and the Maji-Lower Omo plains, which is the wettest of the arid areas (Zerihun Woldu, 1999).
Rangelands are providing forage for livestock and wildlife. Pastorals depend for their life on livestock products and by-products. In addition, rangelands supply minerals, soil, plant, water, wildlife, wind, radiant energy, fish, gums, resins and aesthetics. Oil and gas are also found in the ecosystem. The geothermal and fossil fuels are characteristic wealth in the ecosystem. Mineral deposits such as limestone, marble stone, salt, potash, sulphur, gold, etc. are available in the ecosystem (EPA, 1998).
The desert and semi-desert ecosystem is important for its wealth of endemic plant species. A good example is the flora of the Ogaden region, which is one of the richest in the world. The ecosystem is also considered the basis of an expanded eco-tourism. Big game and bird fauna are abundant in the Rift Valley and the lowland. A number of national parks including Awash and Omo National Parks are found in the ecosystem. This protection of wild animals in the ecosystem might be because of the cultural attribute, which the community assigns to hunters as low class people-despise (Barkhadle, 1999). In addition, the ecosystem is the center of endemism, source of eco-tourism and area of a number of archaeological discoveries.
8.5 Threats and rates of change
The ecosystem is located in arid zone, thus vulnerable to wind and water erosion. In some places, like areas around water point, overgrazing is likely to occur. Vulnerability is further aggravated by the alluvial nature of the soil. Still more, the salinity of the soil in some places being an observed environmental problem, will be a serious constraint under the increasing irrigated agriculture. In some areas of the ecosystem, desertification is advancing. If either human intervention is not checked and/or the region’s climate becomes marginally drier, there is a very high probability that desertification will occur.
The Ogaden and the Danakil Depression being subjected to heavy grazing are environmentally deteriorated. This environmental problem is further exacerbated by the highly competing irrigated crop cultivation. In central, western and some parts of Ogaden, the ground water is charged with salts and water in some shallow-dug wells has high salt content due to saline deposits.
In regions such as Afar, the high number of domestic animal and the insufficient availability of grass during the dry season exacerbated the situation. Other than the destruction of the vegetation, overgrazing has lowered the water table, increased run-off and caused soil erosion. The degradation of the vegetation by overgrazing has also resulted to the invasion the ecosystem by Parthenium hysterophorus and Prosopis juliflora, which threatened the rangeland plant species. When bush invasion reaches high proportion, it is mainly used for browsing. This has a negative impact on the livestock population, especially, on the Borena Breed that is being pushed out by browsers such as camels and goats.
Fragmentation and overgrazing of the rangeland has also affected wild animals. In this ecosystem, Wild Ass is critically endangered and it appeared in the 1996 IUN list of threatednd animals.
8.6 Conservation status
The semi-desert and scrub ecosystem is being subjected to increased grazing, threatened by bush encroachment and invasive exotic species. In the Northeast depression, Guma Valley, a preserve of about 102 Km. was set aside to protect some fine Acacia ehrenbergiana and A. mellifera. To the west is Dobbi Valley with Tamarix–Saueda vegetation needing protection. Part of this ecosystem may be protected in conjunction with a game preserve for the rare Wild Ass, in the area of Mt. Curub and the Sardo village.
In the Ogaden region of the ecosystem only, which is floristically the most rich in endemism in Ethiopia, are found six of the seven endangered endemic plant species of the ecosystem (Ensermu Kelbessa et al., 1992). In areas inhabited by humans and wildlife, where the ecosystem is deteriorating, there is high possibility of reversing the situation if adequate research and implementation of the results could be conducted.