Biological diversity in the Horn of Africa
Last Updated: October 26, 2007
Table of Contents
The Horn of Africa has been a renowned source of biological resources for thousands of years. The ancient Egyptians, Greeks and Romans sent expeditions and caravans to the region for frankincense, myrrh and other natural commodities to be taken back North along the incense route through the Arabian deserts.
Centered on the arid Horn, east of the Ethiopian Highlands, this hotspot also covers the Rift Valley, which divides the Ethiopian Highlands into two major blocks, the xeric bushlands of northeastern Kenya and the southern coastal parts of the Arabian Peninsula. Politically, this includes most of Somalia, all of Djibouti, parts of Ethiopia, Eritrea, Kenya, Yemen and Oman, and a small piece of far eastern Sudan. Also included in this hotspot are the Socotra Archipelago off the coast of northeastern Somalia, and a few hundred tiny islands in the Red Sea. Although the entire hotspot covers more than 1.5 million km2, a relatively large portion of the land area has very limited flora (for example, the Danakil Depression), and most of the plants known from the region actually occupy only a small percentage of the area. The dominant vegetation type is Acacia-Commiphora bushland, although evergreen bushland, succulent shrubland, dry evergreen forest and woodland, semi-desert grassland and low-growing dune and rock vegetation also occupy portions of the region. Small areas of mangrove are found on both the African and Arabian sides of the hotspot, as well as riverine vegetation along major rivers such as the Wabe Shabelle and Awash.
The Horn of Africa is one of only two hotspots that is entirely arid; the other is the Succulent Karoo in southwestern Africa. It is believed that these two arid regions were united by an arid corridor during drier and colder periods in the Pleistocene, and possibly also in the earlier Tertiary. Several genera of flowering plants are entirely restricted to just these two 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.
Unique and Threatened Biodiversity
Although research into the flora of the Horn of Africa is still ongoing, the best possible estimates are that there are about 5,000 species of vascular plants in the region, just over half of which Ė about 2,750 species Ė are endemic. There are strong concentrations of endemic species in northern Somalia and in the Socotra Archipelago.
Socotra also has a relatively high level of generic ndemism, with 13 of the hotspotís nearly 60 endemic genera confined to the archipelago. Furthermore, the Horn of Africa is home to two endemic plant families: Barbeyaceae, which is represented by a single, relatively widespread evergreen species, Barbeya oleoides, and Dirachmaceae, represented by two threatened species, Dirachma socotrana (VU), on Socotra, and D. somalensis (EN) in central Somalia.
For thousands of years, several native tree species have provided the raw materials for some of the Horn of Africaís most important commodities, including frankincense (from Boswellia sacra in Somalia, Yemen and Oman, and B. frereana in Somalia), myrrh (from the widespread Commiphor myrrha and C. guidottii in Somalia and eastern Ethiopia) and dragonís blood or cinnabar (from Dracaena cinnabari, EN found on Socotra). All three are gum-resins obtained from these trees. Dragonís blood, is used as a medicine and dye. The production of frankincense and myrrh is still a major economic activity in Somalia and, to some extent, in Ethiopia and northern Kenya.
Among the hotspotís other notable plant species is the spectacular cucumber tree ( Dendrosicyos socotrana, VU), found only on Socotra, which has a massive water-storing trunk and tendrils on its branches. The daban or Bankoualť palm ( Livistona carinensis, VU) is interesting in that the other 30 or so species of Livistona occur in Southeast Asia and Australia. The daban, which is harvested for use in the construction of homes and drainage pipes, is now found only in a few isolated localities in northeastern Somalia, Djibouti and southern Yemen. The Yeheb nut ( Cordeauxia edulus, VU), an evergreen shrub or small tree with yellow flowers and edible, highly nourishing seeds is found in the dry bushlands of eastern Ethiopia and central Somalia, usually in areas of deep sand. It has been touted as a potential food crop for arid areas, but has proven difficult to cultivate.
Hundreds of new species have been discovered in Somalia alone in the last 20 years, most notable among them the Somali cyclamen (Cyclamen somalense). Known only from a small area in northern Somalia, the plant was a surprising discovery in tropical Africa, as the genus Cyclamen is otherwise found only in the Mediterranean region.
Of the 697 bird species regularly recorded in the hotspot, 24 are endemic. Seven of these species are found only in Somalia, including a bushshrike, the Bulo Burti boubou ( Laniarius liberatus, CR), which was described (and is still known only) from a single individual that was released (hence the specific name liberatus) after comprehensive study. Another six species are confined entirely to Socotra, including the golden-winged grosbeak ( Rhynchostruthus socotranus), the only representative of its genus. Four Endemic Bird Areas, as defined by BirdLife International, fall entirely within the hotspot.
One of the most notable endemic bird species in the hotspot is the Warsangli linnet ( Carduelis johannis, EN), locally common in high, steep escarpments along the Gulf of Aden in northern Somalia. Another important flagship species is the Djibouti francolin ( Francolinus ochropectus, CR), which is found only in two sites in Djibouti, ForÍt de Day, which is thought to be the only viable site for this imperiled species, and the nearby Mabla Mountains.
Nearly 220 mammal species are found in the Horn of Africa, although only about 20 are endemic to the hotspot. The most notable endemics are several antelope species, including the beira ( Dorcatragus megalotis, VU), dibatag ( Ammodorcas clarkei), Spekeís gazelle ( Gazella spekei) and silver dikdik ( Madoqua piacentinii, VU). The beira is confined to dry and inhospitable hills and mountains of northern Somalia, eastern Ethiopia and Djibouti, where it can survive without water. The slender dibatag, with its characteristic erect tail and long neck, is found in the bushlands of eastern Ethiopia and adjoining lowlands of northern and central Somalia. Both species have suffered from uncontrolled hunting and habitat degradation. The hotspot also has an endemic species of wild ass, the Somali wild ass ( Equus africanus somaliensis, CR), while the desert warthog ( Phacochoerus aethiopicus), a distinct species from the common warthog ( P. africanus), is found mainly in eastern Ethiopia, Somalia and northern Kenya.
Five monotypic mammal genera are endemic to the hotspot, including the aforementioned beira and dibatag, as well as three small mammals: the Somali pygmy gerbil ( Microdillus peeli), the ammodile ( Ammodillus imbellis, VU) and Spekeís pectinator ( Pectinator spekei).
The hamadryas or sacred baboon ( Papio hamadryas), which was held sacred in ancient Egypt and often mummified, is today endemic to the arid Horn, living on hillsides and escarpments bordering the southern Red Sea and the Gulf of Aden.
The Horn of Africaís highest levels of endemism occur among reptiles, with more than 90 of around 285 species found nowhere else. The hotspotís six endemic reptile genera include Haackgreerius, a monotypic genus of skink found in Somalia, and Aeluroglena, which is represented by a single species of snake, A. cucullata. Half of the endemic genera are restricted to Socotra, including the two Haemodracon gecko species and two snake genera, Ditypophis and Pachycalamus, represented by single species.
Unlike the reptiles, amphibians are relatively poorly represented in the arid Horn, with nearly 30 species recorded, of which at least six are endemic. There is only a single endemic genus, Lanzarana, which is represented by one species, Lanzaís frog ( L. largeni) of Somalia. Despite suitable habitats, no amphibians are known to exist on Socotra.
There are an estimated 100 species of freshwater fish in the Horn of Africa, about 10 of which are endemic. These endemics include three cave-dwelling species (each the only representative of an endemic genus) found only in Somalia, two of which Ė the Somalian blind barb ( Barbopsis devecchii, VU) and Somalian cavefish ( Phreatichthys andruzzii, VU) Ė are blind. 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.
The Horn of Africa is under heavy pressure from human activity, and is one of the most degraded hotspots in the world, with only about 5 percent of original habitat in relatively pristine condition. Nearly all of the land area is used for grazing, mainly by camels, goats and sheep. Overgrazing and subsequent land degradation is a problem in large areas of the hotspot, particularly near watering points. Shifting cultivation is particularly destructive in parts of central and southern Somalia, where bushland and woodland are cut and burned for the cultivation of cassava. Stands of many unique tree species, including the dragon tree on Socotra and the daban palm in Somalia, are increasingly becoming overmature with little regeneration.
The greatest threat to vegetation and biodiversity in Somalia is the uncontrolled production of charcoal, to cover both domestic needs and for export to countries in the Arabian Gulf region. The most sought after tree for charcoal production is Acacia bussei. Although the tree itself is not a threatened species, woodlands formerly dominated by Acacia bussei are rapidly dwindling as the destruction of big trees changes the composition and structure of the ecosystems. Agricultural schemes in the Rift Valley and along rivers in Somalia and Ethiopia also threaten the biodiversity of riparian habitats.
However, perhaps the most serious barrier to conservation activities in this hotspot is the lack of governance and political instability. Overseas investment in, and aid to, the region has been particularly scarce, following US military involvement in Somalia and the notorious Black Hawk Down fiasco.
On Socotra, the major threats are infrastructure development, including the building of a new port, an airport and new roads. Although there is great potential for sensitively managed ecotourism on the island, the influence of this development on biodiversity must be carefully monitored.
Finally, uncontrolled hunting, particularly of ungulates, is a serious threat in many parts of the hotspot.
Conservation Action and Protected Areas
The Horn of Africa's 41 protected areas cover nearly nine percent of the land area, although only about a third of this is in protected areas in IUCN categories I to IV. As in other areas of the African Continent, protected areas vary in their degree of protection and effectiveness across the region. In Oman, the massive 24,785 km2 Arabian Oryx Sanctuary is a Natural World Heritage Site famous for the successful reintroduction of the Endangered Arabian oryx ( Oryx leucoryx), and Jebel Samhan National Nature Reserve is the home of a population of Arabian leopard (Panthera pardus minor, CR), a Critically Endangered subspecies. Ethiopia has several national parks and sanctuaries, including Awash National Park and Chew Bahir Wildlife Reserve, mainly designed to protect remaining populations of desert-dwelling ungulates. Kenya has the remote Malka Mari National Park in the northeastern corner of the country. Although three protected areas are officially recognized in Somalia, there has been no formal protection of these areas since the breakdown of the federal government in 1991. In Djibouti, the small (100 km2) For√™t de Day National Park is the country's only reserve.
With such an incomplete protected areas system, current conservation activities in the Horn of Africa are completely inadequate for long-term preservation of its biodiversity. A massive increase in conservation action and targeted funding throughout the region is essential not only in its own right but also to provide the environmental and social sustainability necessary to restore governance through its nations. The only part of the hotspot that is receiving any major international attention is the Socotra Archipelago, which has recently been added to UNESCO's World Network of Biosphere Reserves. In addition, the United Nations Development Program (UNDP) has begun a major program for "Sustainable Development and Biodiversity Conservation for the People of the Socotra Islands," a five-year program financed with U.S. $5 million from UNDP and the governments of Italy and Yemen specifically to support the people of Socotra through conservation and sustainable use of the islands' unique biodiversity and natural resources.
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Conservation International (Content Partner); Mark McGinley (Topic Editor). 2007. "Biological diversity in the Horn of Africa." In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [Published in the Encyclopedia of Earth October 26, 2007; Retrieved January 14, 2008].