HORN OF AFRICA
The Horn of Africa was already a renowned biological hotspot 5,000 years ago, when the ancient Egyptians sent expeditions to the “Land of Punt” to bring back unique natural commodities, such as frankincense and myrrh. During the times of the ancient Greek and Romans these products were brought to Europe by caravans along the incense route through the Arabian deserts. Even the isolated island of Socotra, with its famous cinnabar (dragon's blood) and Aloe, was part of this trading system more than 2,000 years ago.
The Horn of Africa is here defined as the arid Horn and basically covers the area east of the Ethiopian highlands (though it includes the Rift Valley, which divides the Ethiopian highlands into two major blocks), including also the xeric bushlands of northeastern Kenya, and the southern coastal parts of the Arabian Peninsula. The arid Horn covers most of Somalia (including Somaliland and Puntland), Djibouti, and parts of Ethiopia, Eritrea, Kenya, Yemen, Oman, and, very marginally, Sudan. The area covers around 1.65 million km2, but a relatively large proportion of the land, such as the Danakil depression in Ethiopia, has a very depauperate flora, and the approximately 5,000 species of vascular plants known from the region actually occupy only a small percentage of the area. Included in this Hotspot is the Socotra archipelago, with the main Socotra Island and three smaller islands off the coast of northeastern Somalia covering about 3,636 km2, and a few hundred small islands in the Red Sea, of which Dahlak Island is the largest (643 km2).
Phytogeographically, all of the arid Horn as defined here belongs to the Somalia-Masai region of endemism (White 1983; White and Léonard 1991), a region that also extends further south through the Kenyan lowlands into northern Tanzania. The Horn of Africa in the present sense can be regarded as the core part of the Somalia-Masai region.
The most widespread vegetation type of the arid Horn is Acacia-Commiphora bushland (about 30 species of Acacia and 50 species of Commiphora are endemic to the area), but also evergreen bushland, succulent shrubland, dry evergreen forest and woodland, semidesert grassland, and low-growing dune and rock vegetation occupy considerable areas. Succulents are common, including numerous endemic species of, for example, Euphorbia and Aloe. Small areas of mangrove are also found, both on the African and the Arabian side, as well as riverine vegetation along the major rivers: Wabi Shabeelle, Jubba and Awash.
The altitude within the arid Horn ranges from 155 m below sea level at Lac Assal in Djibouti to about 2,400 m above sea level in the mountains of northern Somalia, but most of the area is below 500 m. The Haghier Mts on Socotra reach just above 1,500 m altitude and the highest escarpments in Hadramaut on the Arabian side reach about 2,000 m. An unusual feature of the region is that land plants, such as the Doum palm (Hyphaene thebaica, Arecaceae), can have an altitudinal range from about 100 m below sea level to about 1,000 m above sea level. The altitudinal delimitation between the arid Horn flora and the highland floras in Ethiopia, Eritrea and Yemen is not always clearly distinguishable in the field and also varies between about 1,500 to 2,000 m according to local climate, geology and topography. Still, there is a clear-cut separation between highland (Afromontane) flora and lowland (Somalia-Masai) flora (Friis et al., in press).
The arid Horn, including the southern Arabian Peninsula and Socotra, appears to have been elevated by about the upper Eocene some 40 million years ago. Geological evidence indicates that the Arabian Peninsula and Africa separated by about 10 million years ago, forming the incipient Red Sea and Gulf of Aden. When the continental fragment Socotra was last connected to Africa is less certain, but may have been considerably earlier. The massive limestone series covering most of the island (Beydoun and Bichan, 1970) are connected with corresponding series in the escarpments of northern Somalia and the southern Arabian Peninsula.
Today, the major part of the arid Horn is covered by limestone, sandstone or gypsum, whereas pre-Cambrian rocks form, for example, the prominent inselbergs in southern Somalia, as well as the rugged pinnacles of the mountains of Socotra. Large areas are covered by deep sand, partly derived from Quaternary coralline rocks, or alluvial soils, and the fossil dune formations running along the coast of central and southern Somalia are prominent features of the landscape. Lava of more or less recent origin is found in the Rift Valley and the Afar Depression, and in parts of the southern Arabian Peninsula.
The climate of the arid Horn can generally, and not surprisingly, be described as hot and dry. It is not uncommon for temperatures to reach above 40 degrees during several months of the year. Characteristically, there are two rainy seasons, one in April–May and one in September–November, but there are many deviations from this. The fog-oasis of eastern Yemen and Oman has summer rain during the southwest monsoon, whereas winter rain occurs along the escarpments of Eritrea and northern Somalia. Precipitation from mist plays an important role along the Arabian coast, as well as in the higher parts of Socotra and Somalia. The rocky outcrops in southern Somalia are more humid than can be expected from rainfall data alone.
The arid Horn is a kind of northeastern antipode to the other arid African hotspot, the Succulent Karoo in southwestern Africa. These two arid regions are believed to have been united by an “arid corridor” during repeatedly drier and colder periods during the Pleistocene, but most probably also earlier during the Tertiary. Some genera of flowering plants are entirely restricted to these regions, such as Kissenia with one species in the arid Horn and one in the Succulent Karoo, and Wellstedia with six species in the arid Horn and one in the Succulent Karoo. A few species of plants and animals in the Horn of Africa may also have their closest relatives in the southern United States and Central America. For example, Chapmannia, a genus of mostly woody legumes, has five species in the arid Horn, one in Mexico, Guatemala and Venezuela, and one in Florida (Thulin, 1999), and the scorpion genus Heteronebo has two species in the Socotra archipelago and about a dozen species in the Caribbean. This pattern, which may be explained by the Tertiary “North Atlantic land bridge”, is thought to be common among organisms that diversified during the Tertiary in xeric and seasonally dry vegetation (Lavin et al., 2000).
The Horn region is sparsely populated with generally less than 20 inhabitants per km2, and nomadic pastoralism is commonly practised. Well over half the area (Somalia, eastern Ethiopia and north-eastern Kenya) is mainly inhabited by Somalis, a culturally, linguistically and religiously homogeneous group. In southern and northeastern Ethiopia, northern Kenya, as well as in Djibouti and the lowlands of Eritrea, the situation is more complex, with many other East Cushitic groups, such as Borana and Afar, living together. In southwestern Ethiopia, there are also many groups that speak languages of the Omotic family. The native language on Socotra, Socotri, is a West Semitic language with archaic features. It is most closely related to Mahri and some other languages spoken in southeastern Yemen and Oman, along with the ubiquitous Arabic.
Estimating the number of species of vascular plants in the arid Horn region is not easy. One reason is that the area falls under five different Flora projects: Flora of Tropical East Africa (1952-), Flora of Ethiopia and Eritrea, Flora of Somalia, Les plantes vasculaires de la République de Djibouti, and Flora of the Arabian Peninsula and Socotra. Of these, only the Djibouti flora, with a relatively low number of species, has been completed (Audru et al., 1994), whilst the other projects are ongoing. According to the admittedly rough, but best possible, estimate available there are about 5,000 species of vascular plants in the region, and of these about 2,750 are endemic. In terms of plant endemism, this figure is higher than that for the Succulent Karoo (2,539 species), but then the area covered by the arid Horn is more than 10 times as large. Many of the species in the arid Horn have very restricted areas of distribution, however, with strong concentrations of endemics in northern Somalia (Thulin, 1994; Friis et al., in press) and in the Socotra archipelago (Miller and Bazara'a, 1998).
There are nearly 60 endemic genera of vascular plants in the arid Horn (out of a total of about 970 genera), 13 of which are endemic to the Socotra archipelago alone. It is also striking that the native flora of the Socotra archipelago comprises so few armed plants. For example, the single endemic Acacia on Socotra, A. pennivenia (VU), is unarmed, whereas the thorny bushlands of the continental Horn abound in armed endemic species of Acacia.
Of the 170 families in the region, two are endemic. These are Barbeyaceae and Dirachmaceae, both woody, Barbeyaceae with a single species, Barbeya oleoides, which is relatively widespread in evergreen bushland and dry evergreen forest, and Dirachmaceae with two species, Dirachma socotrana (VU) on Socotra and D. somalensis (EN) in central Somalia. Barbeyaceae, with small unisexual flowers without petals, have usually been associated with Urticales. Dirachmaceae, with relatively large bisexual flowers with prominent petals, have been associated with Geraniales or Malvales. However, molecular evidence clearly indicates that Barbeyaceae and Dirachmaceae, despite their completely different morphology, are closely related (Thulin et al., 1998).
A total of 219 mammals in 115 genera are known from the arid Horn and of these 20 are endemic, the most notable ones being a number of antelopes, such as Beira (Dorcatragus megalotis, VU), dibatag (Ammodorcas clarkei, VU), Speke's gazelle (Gazella spekei, VU), silver dikdik (Madoqua piacentinii, VU), and Salt's dikdik (Madoqua saltiana). In addition, there is an endemic subspecies of wild ass (the Somali wild ass, Equus africanus somaliensis, CR). There are five endemic mammal genera in the Horn, all of them monotypic, including the aforementioned Beira and dibatag, and three small mammal genera, represented by single species: the Somali pygmy gerbil (Microdillus peeli), Ammodile (Ammodillus imbellis, VU), and Speke's pectinator (Pectinator spekei). Indeed, the arid Horn has been identified as an important area for rodent conservation (Amori and Gippoliti 2001). No native mammals are known from the Socotra archipelago, except possibly some species of bats, and it has been assumed that this is because Socotra was isolated from the continental Horn too early for colonization by mammals.
There are 704 bird species regularly recorded from the arid Horn and 25 of these are endemic. Four Endemic Bird Areas (EBAs) fall entirely within the hotspot: Socotra(with six species confined entirely to this EBA); North Somali mountains (three species); Central Somali coast (two species); and Jubba and Shabeelle valleys (four species). There are seven species of birds found only in Somalia, including the Bulo Berti Bushshrike (Laniarius liberatus, CR) – known only from a single individual and described from molecular, photographic and vocalization data (Smith et al., 1991) – the Warsangeli linnet (Carduelis johannis, EN) and five species of larks. Among the six species of birds restricted to Socotra is the Soqotra sunbird (Nectarinia balfouri), a relatively abundant species regularly seen visiting the red flowers of species of Aloe and Ballochia. Another Socotra endemic, the golden-winged grosbeak (Rhynchostruthus socotranus), is the only representative of its genus.
It is estimated that there are around 100 species of freshwater fish in about 48 genera in the arid Horn, of which 10 species are endemic. The endemics include three cave-dwelling species restricted to Somalia, two of which, Somalian blind barb (Barbopsis devecchii, VU) and Somalian cavefish (Phreatichthys andruzzii, VU), are blind. The three cave-dwelling species are each placed in their own endemic genus. No native freshwater fishes are known with certainty from Socotra, but populations of Aphanius dispar have been introduced to some waters as part of an anti-malaria program. However, the endemic freshwater crab Potamon socotrensis is common in small mountain streams, and a second endemic species, Socotra pseudocardiosoma, placed in its own genus, has recently been described (Cumberlidge and Wranik 2002).
For thousands of years the Horn of Africa has been famous as the source of frankincense (mainly from Boswellia sacra in Somalia, Yemen and Oman, and B. frereana in Somalia), myrrh (mainly from Commiphora myrrha, widespread in the Horn, and C. guidottii in Somalia and eastern Ethiopia), and dragon's blood or cinnabar (from Dracaena cinnabari, EN, on Socotra), all of which are commodities of gum-resins obtained from these trees.
In the Egyptian temple at Deir al-Bahari, near Thebes, the walls are decorated with coloured relief frescoes commemorating an expedition sent out to the Land of Punt by the Queen of Egypt, Hatshepsut, about 1500 BC. Quantities of incense were brought back by this expedition, as were living trees in tubs that apparently were then planted in the temple courtyard. The exact position and extension of Punt is uncertain, but judging from the commodities obtained from there it has to have been within the Horn of Africa in the present sense. Later, frankincense and myrrh were transported by huge caravans along the highly organized incense route from southern Arabia to ancient Greece and Rome (Groom 1981).
The production of frankincense and myrrh is still of major economic importance in Somalia, and to some extent in Ethiopia and northern Kenya. The frankincense trees in northern Somalia are owned by extended families and clans. To tap the trees, incisions are made into the inner bark of trunks and branches. The resin is left to dry and is later collected. Both Boswellia frereana and B. sacra are evergreen trees that often grow on limestone rocks, but B. frereana has the ability to grow even on vertical cliff faces, and the collecting of the frankincense from this species, therefore, is a particularly hard and dangerous occupation.
Dragon's blood from Socotra (then called Dioscorida) was in high demand by the ancient Greek and Romans, for its medicinal properties and as a red dye. However, it lost its importance long ago and, during the last centuries, dragon's blood has mainly been obtained from the related and more easily accessible dragon tree on the Canary Islands. Dracaena cinnabari is prominent in the vegetation of Socotra, where the gum-resin is still used as medicine and dye. It is a spectacular tree with repeatedly ramifying branches forming a dense, umbrella-like canopy.
In Somalia, the Daban, as it is called there, is highly threatened and the total population is now probably less than 40 trees in the two known localities (the author counted 11 mature trees at one locality in 1995 and 28 mature trees at the other locality in 2000). The trees have been used for house building, drainage pipes, and so on, and regeneration is prevented as the leaves of young plants are grazed or used for the production of mats or baskets.
The Yeheb nut (Cordeauxia edulis, VU) is a wonderful evergreen shrub or small tree with yellow flowers found in the dry bushlands of eastern Ethiopia and central Somalia, usually in areas of deep sand. The seeds are edible and nourishing and highly appreciated as food, and the foliage provides a red dye. The plant has attracted considerable interest as a potential food crop for arid areas, but is not easy to cultivate.
During fieldwork conducted for the Flora of Somalia project over the last 20 years or so, hundreds of new species have been discovered, many of them apparently narrow endemics. Without comparison, the one species among these that has attracted most attention is the Somali cyclamen (Cyclamen somalense). This was first found in 1986 on one of the misty limestone escarpments in northern Somalia and is still known only from a very small area. The horticulturally important genus Cyclamen is otherwise distributed in the Mediterranean region, extending eastwards to Iran, and the occurrence of a species of this genus in tropical Africa was most unexpected, although there is a fairly strong Mediterranean element in the flora of this part of Somalia.
In addition to the plants and plant products that the ancient Egyptians brought back from the Land of Punt, they also brought back animals, and the most famous of these is the Hamadryas or sacred baboon (Papio hamadryas). This monkey is today endemic to the arid Horn, where it lives on hillsides and escarpments bordering the southern part of the Red Sea and the Gulf of Aden. The sacred baboon is an omnivore for which desert dates (Balanites and Ziziphus) are of great seasonal importance, but crops are raided in settled areas and often lead to conflict. The animals were held sacred in ancient Egypt and were also mummified. During the earliest Egyptian civilisation the distribution of the sacred baboon may have extended to the Egyptian border area, though they were later imported to the temples, and the relief frescoes at Deir al-Bahari show baboons swarming over the ships in Hatshepsut's expedition.
Besides the baboon, the Horn is an important region for a number of threatened antelope species, particularly the beira and dibatag antelopes (in both cases, the Somali name for the species has become the common name). The beira, with its prominent ears, is confined to dry and inhospitable hills and mountains of northern Somalia, eastern Ethiopia and Djibouti (where its presence was only confirmed in 1993), where it is able to survive without water. The larger, but more slender dibatag, with its characteristic erect tail and long neck, is found in the Acacia-Commiphora bushlands of eastern Ethiopia and adjoining lowlands of northern and central Somalia. Both the beira and the dibatag have suffered from uncontrolled hunting and habitat degradation.
Another important flagship is the desert warthog, Phacochoerus aethiopicus, recently confirmed to represent a distinct species from the common warthog, P. africanus, and distributed mainly in eastern Ethiopia, Somalia and northern Kenya (d'Huart and Grubb 2001). In addition, species such as Speke's gazelle, Salt's and Silver dik-diks, and Beisa oryx (Oryx beisa) have their ranges entirely or almost entirely within the Horn, while Swayne's hartebeest (Alcelaphus buselaphus swaynei, EN), a subspecies of the common hartebeest, has its range restricted entirely to the southern Rift Valley of Ethiopia, despite once occurring throughout the Rift Valley eastwards into northwestern Somalia (East 1999).
Among the 31 endemic birds in the region, the Warsangeli linnet is one of the most notable. This small bird, with colors in white, black and various shades of grey and reddish brown, is locally common in high steep escarpments along the Gulf of Aden in northern Somalia. Ash and Miskell (1998) reported it as resident in two sites (Daalo and Mash Caleed) in the 1980s, and the bird has been seen in both of these sites in recent years. The species is likely to occur along this whole stretch of escarpment, where access is nearly impossible. The Ethiopian bush-crow also deserves a special mention as a flagship species. This small starling-like crow is found in Acacia bushland in a very restricted area of southern Ethiopia. It is still locally common, but ongoing degradation of the habitat may become a threat in the near future. Finally, the Djibouti francolin (Francolinus ochropectus, CR) is known from only two sites in Djibouti: Forêt de Day, which is thought to be the only viable site for this imperilled species, and the nearby Mabla Mountains.
The arid Horn is a hotspot under heavy pressure. Close to 100 percent of the land is used for grazing, mainly by camels, goats and sheep, and the area, therefore, is in a seminatural state and very little can be said to be pristine nature. Overgrazing, leading to a gradual degradation of the vegetation is a problem over large areas and is particularly severe near watering points. The stands of many unique species of trees, such as the Dragon tree on Socotra and the Daban palm in Somalia are increasingly becoming overmature with little regeneration. One study has estimated that the region is represented by around 24.2% undisturbed habitat, 70.3% partially disturbed and 5.5% human-dominated habitats (Hannah et al. 1995), but this is almost certainly an underestimate of the disturbance, and perhaps only 5% can be regarded as undisturbed. However, the category partially disturbed would include all degrees of degradation from almost completely destroyed to nearly pristine habitats.
Shifting cultivation, where areas of bushland and woodland are cut and burned for the cultivation of cassava over a period of a few years, is particularly destructive in parts of central and southern Somalia. However, in Somalia the worst threat to the vegetation and the biodiversity is the uncontrolled production of charcoal. Charcoal is now not only produced to cover the domestic needs within the country, but has become a major export item to the countries in the Arabian Gulf region. The tree most sought after for charcoal is Acacia bussei, and the woodlands formerly dominated by this tree are now rapidly dwindling. Acacia bussei itself is not threatened as a species, but the destruction of the big trees changes the environment completely and adversely affects numerous other species of plants and animals. Agricultural schemes in the Rift Valley and along the Wabi Shebelle and Awash Rivers in Ethiopia and along the Wabi Shabeelle and Jubba Rivers in Somalia also threaten the biodiversity, particularly of riparian habitats.
On Socotra, there has been considerable infrastructure development in recent years, including the building of a new port, an airport with tarmac runway, and new roads. The potential for sensitively managed ecotourism is great, but the ways in which these developments actually influence the biodiversity of the island require careful monitoring.
Despite these threats, at least among the plants we cannot point to a single species that is known to be extinct. Taverniera sericophylla, an endemic on Socotra earlier reported as extinct (Groombridge 1994), is still thriving in parts of the island (Thulin pers. obs.), and several other endemic species on Socotra reported as Critical (Lucas and Synge 1978), such as Dirachma socotrana and Punica protopunica (VU), also persist in viable populations. In Somalia, there are a few flowering plants that have not been seen for more than 100 years, such as the endemic Somali lupine (Lupinussomaliensis), but there is still hope that it survives in some inaccessible place. The situation for larger animals, in contrast, is much worse and uncontrolled hunting, particularly of the ungulates, is a real problem in many parts of the region with many species classed as threatened.
Despite the vast size of this hotspot, less than 9% has some form of legal protection in 41 protected areas classed in IUCN categories I to VI, a percentage that falls to just over 3% when including only those protected areas in categories I to IV. . There are several national parks and sanctuaries in the Ethiopian part of the region, such as Awash National Park (750 km2) and the Chew Bahir Wildlife Reserve (2,730km2), mainly aimed at protecting remaining populations of desert-dwelling ungulates; the problems and challenges faced are to a large degree similar to those described for the Ethiopian Highlands. In Kenya there is the remote Malka Mari National Park (870km2) in the northeastern corner of the country. In Somalia there are no areas with formal protection after the breakdown of the federal government in 1991, although three protected areas are officially recognized. In Djibouti, the Forêt de Day National Park (100km2), home to the aforementioned Djibouti francolin, is the country's only reserve. The governments of Somaliland and Puntland have ministries for the environment, but their resources are totally inadequate. The Socotra Archipelago has recently been added to UNESCO's World Network of Biosphere Reserves, which should help in the implementation of conservation management and in the development of ecotourism in the area. In Oman, the massive Arabian Oryx Sanctuary (24,785km2) is a Natural World Heritage Site famous for the successful reintroduction of Arabian oryx (Oryx leucoryx, EN), and Jebel Samhan is a National Nature Reserve mainly due to its population of Arabian leopard (Panthera pardusnimr, CR).
In general, conservation activities in the Horn of Africa hotspot are completely inadequate for the long-term preservation of its biodiversity. Today, only Socotra can be said to receive any serious international attention. Recently, for example, a major programme for Sustainable Development and Biodiversity Conservation for the People of the Socotra Islands was signed by the United Nations Development Programme (UNDP). The programme, financed by UNDP, the Government of Italy and the Yemen Government, will collectively contribute over US$5 million to continue to support the people of Socotra through conservation and sustainable use of the islands' unique biodiversity and natural resources for the next five years. Much needs to be done before the arid Horn can regain the kind of international prominence that it had 2,000 years ago.