Location and General Description
Socotra is the largest and most easterly island of this Indian Ocean archipelago, lying approximately 240 km east of the Horn of Africa and 480 km south of the Arabian Coast. The other main islands in the group are the Brothers, Abd al Kuri, Semhah, and Darsa. Abd al Kuri is the closest to the African mainland, only 90 km away. All except Darsa are permanently inhabited. They have a combined area of about 400 km2. Socotra and two of the Brothers are administered by Yemen, while Puntland (former Somalia) administers Abd al Kuri and some of the smaller islands. Socotra is often considered to be part of the Middle East and not Africa because it is administered by Yemen, but geographically and biogeographically it is a continuation of the Horn of Africa. The dominant landscape feature of Socotra Island is an extensive plateau of Cretaceous limestone averaging 300 to 700 m in elevation. The plateau rises near the Hagghier Mountains in the northwest (maximum elevation 1,519 m), which are composed of Precambrian granites and metamorphic rocks. The plateau then declines abruptly at the extreme western portion of the island, falling in steep escarpments to the coastal plains or directly into the ocean.
The coastal plains can reach up to 5 km in width and are found around most of the island. They consist mainly of alluvial soils of stone and coarse sand. Sand dunes can occur in some areas, particularly in the Noged Plain, a 60 km stretch of unbroken plain in the south. The climate of the ecoregion is strongly influenced by both the southwest (April-October) and northeast (November-March) monsoons. The southwest monsoons bring extremely strong, hot and dry winds from Africa. There is little precipitation and extreme desiccation during these months. The winter monsoon begins in November and lasts until March (Davis et al. 1994). Mean annual rainfall varies from about 150 mm on the coastal plains to more than 1,000 mm in the mountains. Rainfall is sporadic, and there are some years when no rain falls. Nocturnal dew is far more important to the water supply than monsoonal rain, especially in the high altitude mountain belt. The mountainous cloud zone provides ground water and running water for the entire island. The dew and mist is channeled into small brooks and rivulets that support aquatic fauna and may flow the entire way from the mountains to the sea in winter (Mies and Beyhl 1998). Mean average temperatures range from 27º to 37ºC maximum and 17º to 26ºC minimum along the coastal plain. It is substantially cooler in the Hagghier Mountains. The human population of Socotra is estimated to be between 20,000 and 80,000 (Davis et al. 1994) with lowest densities on the dry, rocky central plateau areas.
Socotra was not considered in the phytogeographical classification of White (1983), although he did note that the area ranked as a local center of endemism within the Somali-Masai regional center of endemism. Pronounced local variations in climate have resulted in a broad mosaic of plant communities on the island. The coastal plains and low inland hills consist of open deciduous shrubland dominated by the endemic Croton socotranus and scattered trees of Euphorbia arbuscula, Dendrosicyos socotranus, and Ziziphus spina-christi. Grasses and herbs develop after sufficient rainfall. The most widespread vegetation type is a distinctive species-rich open shrubland found on the coastal foothills and the limestone escarpments. Two endemics, Croton socotranus and Jatropha unicostata, are the main shrubs present and are the most abundant plants on Socotra. Succulent trees, such as Euphorbia arbuscula, Dracaena socotranus, and Adenium obesum spp. sokotranum and emergent trees, such as Boswellia spp., Sterculia africana var. socotrana, and Commiphora spp. are also present (Davis et al. 1994).
On the limestone plateau and upward to the middle slopes of the Hagghier Mountains there are areas of semi-deciduous thicket dominated by Rhus thyrsiflora, Buxus hildebrandtii, Carphalea obovata, and Croton spp. The higher montane slopes support a mosaic of dense thickets, dominated by Rhus thyrsiflora, Cephalocroton socotranus, and Allophylus rhoidiphyllus with the emergent dragon's blood tree (Dracaena cinnabari), low Hypericum shrubland, and in many areas anthropogenic pastures. Open rocks are covered by lichens and low cushion plants, including an endemic monotypic genus of Umbelliferae (Nirarathamnos asarifolius) and several endemic species of Helichrysum.
Biologically, the Socotran Archipelago is well-known for its assemblage of endemic and unusual species. There are 850 recorded plant species, of which approximately 230 to 260 (about 30 percent) are endemic. There are also ten endemic genera: Ankalanthus, Ballochia, Trichocalyx, Duvaliandra, Socotranthus, Haya, Lachnocapsa, Dendrosicyos, Placoda, and Nirarathamnos; and one near-endemic family (Dirachmaceae). Some of the plants on Socotra represent the last surviving members of their genus. The limestone plateau and the Hagghier Mountains are the richest areas for endemic plant species, but endemics are found throughout the island in every type of vegetation. Due to habitat fragmentation and degradation, several endemic plant species are endangered. The endemic and monotypic Dirachma socotrana is considered Vulnerable by IUCN while Croton pachyclados survives only in one location (Mies and Beyhl 1998). Dendrosicyos is the only representative of the cucumber family to grow in tree form. Euphorbia abdelkuriensis grows only on Abd al Kuri. This endangered plant is an unusual Euphorbia, known for its spineless columnar stems, all linked by a single rootstock. In total, IUCN names 52 endemic Socotran plants in the Red List of Threatened Species (Hilton-Taylor 2000).
Plant species found here have evolved morphological and physiological adaptations to cope with the dry climate and fierce monsoonal winds. Adenium socotranum has a special cell sap cycling within the caudex which prevents overheating. The succulents display several morphological adaptations. Plant bodies are globular or columnar, with reduced surface areas that decrease transpiration. Glaceous wax surfaces and microanatomical epidermal emergences reflect radiation. Umbrella-shaped shrubs form dense thickets, with all plants reaching the same height, a structure that protects them from strong winds (Mies and Beyhl 1998).
The native plant species of Socotra reflect the island’s geological history and demonstrate links to the Horn of Africa, as well as to the more distant Macaronesian islands. Socotra separated from the Arabian mainland in the Tertiary by the same series of dislocations that produced the Gulf of Aden (Wranik 1998). Before that, its position on the supercontinent Gondwana meant that Socotra was nearest to Madagascar, India and the east coast of the African mainland. Many of the endemic plants found on Socotra were previously widespread. For example, Dendrosicyos socotrana was recorded in Djibouti and is now regarded as extinct on the African mainland. Similarly, the genus Dracaena contains xeromorphic species distributed in the Macaronesian islands, Madagascar, and along the African coast from southern Africa into Arabia. More research is needed to reveal the biogeographical circumstances that gave rise to these systematically unique and locally isolated species (Mies and Beyhl 1998).
There are only seven terrestrial mammals, most of which are introduced, although a bat Rhinopoma sp. and a shrew Suncus sp. are possibly endemic. Of the 178 known bird species, 6 are endemic: island cisticola (Cisticola haesitatus), Socotra warbler (Cisticola incanus), Socotra bunting (Emberiza socotrana), Socotra sunbird (Nectarinia balfouri), Socotra starling (Onychognathus frater), and Socotra sparrow (Passer insularis) (Stattersfield et al. 1998). At least 30 species are known to breed on Socotra, including a significant population of Egyptian vultures (Neophron percnopterus) (Wranik 1998). Two of the six endemic bird species, island cisticola (Cisticola haesitatus) and Socotra bunting (Emberiza socotrana) are classed as Vulnerable by IUCN (2000). An estimated 10-11 endemic subspecies have been identified, some of which may warrant full species status (Sagheir and Porter 1998). While the Socotra cormorant (Phalacrocorax nigrogularis) is distributed throughout the Middle East, it is considered Vulnerable by the IUCN. Among the terrestrial reptiles, 21 of the 24 currently known species are endemic and the three remaining species were most likely introduced (Joger 1999). Among the reptiles, the burrowing legless Pachycalamus brevis is considered to be a relic of an ancient and once widely distributed Afro-Arabian herpetofauna. There are no amphibians, despite adequate water and the arid-adapted species present on the nearby African and Arabian mainlands. It is possible that severe drought in the past eradicated any amphibians that may have colonized the island (Wranik 1998).
The original climax vegetation on Socotra has been altered through a combination of grazing and the cutting of wood for fuel. However, recent reviews (Davis et al. 1994) indicate that the vegetation is still in quite good condition. Some subsistence cultivation is practiced, but this is minimal given the harsh climate. The main land use is pastoralism. The habitat is fragmented and only small areas of the natural vegetation remain.
There are no legally protected areas, although there are many areas of the highlands that are privately owned where grazing is controlled by traditional means. These provide habitat refuges. The island’s isolation has also helped to curb degradation and preserve the natural habitats. From April to October, the violent monsoon winds prevent the approach of any airplanes or ships. However, all natural habitats and many naturally occurring species on Socotra have been greatly fragmented over thousands of years by anthropogenic activities. Some populations of endemic animal and plant species are now extremely small and occupy only scattered parts of the island. The more densely populated coastal plains are more degraded than the mountains and interior plains. The mangrove tree Avicennia marina has been all but eradicated by wood collection and browsing so that the last stands are found only on the remote western shores of Socotra (Mies and Beyhl 1998). Additionally, monsoonal rains may cause severe erosion damage.
Recently, conservation initiatives have been undertaken to identity and eventually ensure the protection and proper management of representative areas of Socotran habitat. A list of Important Bird Areas exists (Evans 1994), and there is a proposal to designate the whole island as a biosphere reserve and UNESCO World Heritage Site. The United Nations Development Program and Global Environment Facility are currently running a biodiversity project in Socotra, along with the Environmental Protection Council of Yemen. This project aims to establish a zoning system with protected areas, foster sustainable development, train Socotrans in environmental survey techniques and set up a Socotra Conservation Fund. The conservation fund will be used by Socotrans to support the long-term implementation of the zoning plan and promote sustainable development. The zoning plan would set aside the entire 500 m coastal strip of the islands as a national park, with no development allowed except in the port and town of Socotra. The Yemeni government has officially designated these coastal protected areas.
Types and Severity of Threats
The main threats to biodiversity are excessive woodcutting for timber and fuel in the vicinity of the main settlements; unplanned infrastructural development on Socotra, especially the coastal zone; an expanding population of goats, and to a lesser extent cattle, which may increase to such an extent that the natural vegetation may be permanently damaged if the sinking of bore holes and provision of supplementary fodder become common activities. Meaningful conservation on Socotra must balance development of a formal system of protected areas while ensuring that traditional systems of management are still honored.
The small populations of some of the endemic resident bird species and endemic plants are indicative of the threats to their long-term survival on the island. This is particularly true for some of the breeding seabirds, whose numbers have been reduced by rats and other introduced predators.
Justification of Ecoregion Delineation
Biogeographically an extension of the Horn of Africa, the Socotra archipelago is undeniably a unique island ecosystem. It is characterized by interesting plant assemblages, high levels of plant endemism (28-32%) at both the species and generic levels, and is also an important area for endemic birds and reptiles.
Davis, S.D., V.H. Heywood, and A.C. Hamilton (eds.). 1994. Centres of plant diversity. Vol 1: Europe, Africa, Southwest Asia and the Middle East. World Wide Fund for Nature and IUCN, Oxford, UK. 354 pp.
Evans, M. (comp.) 1994. Important Bird Areas of the Middle East. BirdLife Conservation Series No. 2. BirdLife International, Cambridge, UK. 410 pp.
Hilton-Taylor, C. 2000. 2000 IUCN Red List of Threatened Species. IUCN, Gland, Switzerland and Cambridge, United Kingdom.
Joger, U. 1999. The reptile fauna of the Socotra archipelago. In: G. Rheinwald (ed.) Isolated vertebrate communities in the tropics. Proc. 4th Int. Symp., Bonn. Bonn Zool. Monogr. 45.
Mies, B.A., and F.E. Beyhl. 1998. Vegetation ecology of Soqotra. Pp. 35-82 in: H.J. Dumont (ed). Proceedings of the first international symposium on Soqotra Island: present and future, Aden 1996. Soqotra Technical Series, Volume 1. UNDP, NY, USA.
Sagheir, O.A., and Porter, R.F. 1998. The bird biodiversity of Soqotra. Pp. 199-212 in: H.J. Dumont (ed). Proceedings of the first international symposium on Soqotra Island: present and future, Aden 1996. Soqotra Technical Series, Volume 1. UNDP, NY, USA.
Stattersfield, A.J., M.J. Crosby, A.J. Long, and D.C. Wege, 1998. Endemic Bird Areas of the World. Priorities for Biodiversity Conservation. BirdLife Conservation Series No. 7. BirdLife International, Cambridge, UK. 846 pp.
White, F. 1983. The vegetation of Africa, a descriptive memoir to accompany the UNESCO/AETFAT/UNSO Vegetation Map of Africa (3 Plates, Northwestern Africa, Northeastern Africa, and Southern Africa, 1:5,000,000). UNESCO, Paris.
Wranik, W. 1998. Faunistic notes on Soqotra Island. Pp. 135-198 in: H.J. Dumont (ed). Proceedings of the first international symposium on Soqotra Island: present and future, Aden 1996. Soqotra Technical Series, Volume 1. UNDP, NY, USA.
Prepared by: Mike Evans
Reviewed by: In progress