Socotra Island xeric shrublands

The Socotra Island xeric shrublands is a terrestrial ecoregion that covers the large island of Socotra and several smaller islands that constitute the Socotra Archipelago. The archipelago is the western Indian Ocean, east of the Horn of Africa and south of the Arabian Peninsula. Politically the archipelago is part of Yemen, and lies south of the Yemeni mainland.

Socotra Island xeric shrublands
Socotra dragon tree (Dracaena cinnabari)
Topographic map of the Socotra Archipelago
Ecology
RealmAfrotropical
Biomedeserts and xeric shrublands
Geography
Area3,616 km2 (1,396 sq mi)
CountryYemen
Conservation
Conservation statusCritical/endangered
Protected0 km² (0%)[1]

Socotra has a uniquely diverse ecosystem. The islands are home to a high number of endemic species; up to a third of its plant life is endemic. It has been described as "the most alien-looking place on Earth."[2]

Socotra is considered the jewel of biodiversity in the Arabian Sea.[3] In the 1990s, a team of United Nations biologists conducted a survey of the archipelago's flora and fauna. They counted nearly 700 endemic species, found nowhere else on earth; only New Zealand,[4] Hawaii, New Caledonia, and the Galápagos Islands have more impressive numbers.[5] In 2008 Socotra was recognised as a UNESCO World Heritage Site.[6]

The ecoregion is at increasing risk from human activities. The WWF describes the Socotra Archipelago's conservation status as "Critical/Endangered".[7][8][9]

Geography

Socotra is the easternmost island in the Socotra Archipelago, and constitutes around 95% of the archipelago's landmass. The island measures 132 kilometres (82 mi) in length and 49.7 kilometres (30.9 mi) in width.[10] Socotra lies some 240 kilometres (150 mi) east of the coast of Somalia and 380 kilometres (240 mi) south of the Arabian Peninsula.[11]

A coastal plain, up to 5 km wide, extends around most of Socotra. The coastal plain's soils are mostly alluvial deposits of stone and coarse sand. The Noged Plain along the island's south shore has areas of sand dunes. The center of the island is a plateau, 300 to 700 meters in elevation, composed mostly of Cretaceous limestone. The Hajhir Mountains in northwestern Socotra are composed of precambrian granites and metamorphic rock, and include Mashanig, the island's highest point (1,519 m).[7]

The other islands in the archipelago are Samhah and Darsah, known as The Brothers, and Abd al Kuri. Abd al Kuri is closest to the African mainland, only 90 km away.

Politically, Socotra and most of the other islands are part of Yemen, and constitute Socotra Governorate. Abd al Kuri is part of Somalia's Puntland region. Geographically, Socotra and the rest of its archipelago are part of Africa – a geologic and biogeographic extension of the Horn of Africa.[7]

Climate

The climate of the ecoregion is tropical and mostly arid. It is influenced by the Indian Ocean's monsoon winds. The April-to-October southwest monsoon brings hot dry winds from the Horn of Africa. The Northeast monsoon arrives during the winter months, from November to March, and brings lower temperatures and more moisture.[7]

Average annual rainfall ranges from 150 mm on the coastal plain to more than 1,000 mm in the mountains. Rainfall is sporadic and unpredictable. Tropical cyclones are rare.[7]

Nighttime dew is an important water source for the islands' plants. The mountains intercept moisture-bearing winds, which creates more clouds, fog, and orographic precipitation. The mountains are also cooler than the lowlands. Small mountain streams sustain aquatic habitats, and can flow down through the lowlands during the cooler and wetter winter months and during high rainfall events.[7]

Flora

The long geological isolation of the Socotra archipelago and its fierce heat and drought have combined to create a unique and spectacular endemic flora. Botanical field surveys led by the Centre for Middle Eastern Plants, part of the Royal Botanic Garden Edinburgh, indicate that 307 out of the 825 (37%) plant species on Socotra are endemic, i.e., they are found nowhere else on Earth.[12] The entire flora of the Socotra Archipelago has been assessed for the IUCN Red List, with 3 Critically Endangered and 27 Endangered plant species recognised in 2004.[12]

Open deciduous shrublands are the primary vegetation in the coastal plain and low limestone foothills. Many plants lose their leaves during the dry summer months, and succulent plants that store water in leaves and stems are common. The predominant shrubs are the endemics Croton socotranus and Jatropha unicostata. The succulent trees Euphorbia arbuscula, Dendrosicyos socotranus, and Adenium obesum spp. sokotranum rise above the shrub layer. Woody trees include Sterculia africana var. socotrana, Ziziphus spina-christi, and species of Boswellia and Commiphora. Grasses and herbs grow during periods of sufficient rainfall.[13]

Thickets of semi-deciduous shrubs and low trees occur on the limestone plateau and lower slopes of Hajhir Mountains. The predominant shrubs Rhus thyrsiflora, Buxus hildebrandtii, Carphalea obovata, and Croton spp.[13]

Dense shrub thickets of Rhus thyrsiflora, Cephalocroton socotranus, and Allophylus rhoidiphyllus grow on the high mountain slopes. Dragon's blood tree (Dracaena cinnabari), a strange-looking, umbrella-shaped tree, rises above the shrub layer. Other high-mountain plant communities include low shrublands dominated by Hypericum, anthropogenic pastures, and rock outcrops with lichens and low cushion plants, including the endemics Nirarathamnos asarifolius and species of Helichrysum.[13]

Dragon's blood tree is one of the most striking of Socotra's plants. Its red sap was thought to be the dragon's blood of the ancients, sought after as a dye, and today used as paint and varnish.[12] Socotra's various endemic aloes were also important historically for medicine and cosmetics. Other endemic plants include the giant succulent tree Dorstenia gigas, the cucumber tree Dendrosicyos socotranus, the rare Socotran pomegranate (Punica protopunica), Aloe perryi, and Boswellia socotrana.[14]

Fauna

The islands' fauna includes several endemic species.

The islands have 178 known bird species,[7] including six endemic species – the Socotra starling (Onychognathus frater), Socotra sunbird (Nectarinia balfouri), Socotra bunting (Emberiza socotrana), Socotra cisticola (Cisticola haesitatus), Socotra sparrow (Passer insularis), Socotra golden-winged grosbeak (Rhynchostruthus socotranus), and Socotra warbler (Incana incana).[14] Many of the bird species are endangered by predation by non-native feral cats.[5]

About 15 species of mammals live on Socotra. There are four species of bats, the Egyptian mouse-tailed bat (Rhinopoma cystops), Geoffroy's horseshoe bat (Rhinolophus clivosus), Somalian trident bat (Asellia italosomalica), and the endemic Hypsugo lanzai. The Egyptian mouse-tailed bat has been recorded on all the islands, and the other three species only on Socotra.[15] The other mammals were introduced by humans. The Socotran wild ass is a feral population of African wild ass (Equus africanus) that lives in the lowlands of Socotra. Introduced livestock include dromedary camels, cattle, and goats, which include both domestic and feral populations. Other introduced mammals include the domestic cat (Felis catus), lesser Indian civet (Viverricula indica), black rat (Rattus rattus), house mouse (Mus musculus), Etruscan shrew (Suncus etruscus), and Madagascan pygmy shrew (Suncus madagascariensis).[16]

Reptiles constitute a diverse Socotran vertebrate fauna with 31 species. If one excludes the two recently introduced species, Hemidactylus robustus and Hemidactylus flaviviridis, all native species are endemic. There is a very high level of endemism at both species (29 of 31, 94%) and genus levels (5 of 12, 42%). At the species level, endemicity may be even higher, as phylogenetic studies have uncovered substantial hidden diversity.[17] The reptile species include skinks, legless lizards, and one species of chameleon, Chamaeleo monachus.

There are many endemic invertebrates, including several spiders (such as the tarantula Monocentropus balfouri) and three species of freshwater crabs (one Socotra pseudocardisoma and two Socotrapotamon).[18] Socotra is also one of the homes of the butterfly Bicyclus anynana.[19]

The coral reefs of Socotra are diverse, with many endemic species.[14]

See also: List of spiders of Socotra

Human impacts

Over the two thousand years of human settlement on the islands the environment has slowly but continuously changed, and, according to Jonathan Kingdon, "the animals and plants that remain represent a degraded fraction of what once existed."[14] The Periplus of the Erythraean Sea says the island had crocodiles and large lizards, and the present reptilian fauna appears to be greatly reduced. Until a few centuries ago, there were rivers and wetlands on the island, greater stocks of the endemic trees, and abundant pasture. The Portuguese recorded the presence of water buffaloes in the early 17th century. Now there are only sand gullies, and many native plants only survive where there is greater moisture or protection from livestock.[14] The remaining Socotra fauna is greatly threatened by goats and other introduced species.

Conservation and threats

As a result of the 2015 Yemen civil war in mainland Yemen, Socotra became isolated and the gas prices became high, causing residents to turn to wood for heat. In December 2018, UAE sent cooking gas to Socotra residents to curb deforestation caused by the cutting down of trees for wood.[20]

World Heritage Site

The island was recognised by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a world natural heritage site in July 2008. The European Union has supported such a move, calling on both UNESCO and International Organisation of Protecting Environment to classify the island archipelago among the environmental heritages.[6]

References
  1. Eric Dinerstein, David Olson, et al. (2017). An Ecoregion-Based Approach to Protecting Half the Terrestrial Realm, BioScience, Volume 67, Issue 6, June 2017, Pages 534–545; Supplemental material 2 table S1b.
  2. Huntingford, George Wynn Brereton (1980). The Periplus of the Erythraean Sea. Hakluyt Society. p. 103. ISBN 978-0-904180-05-3.
  3. FACTBOX-Socotra, jewel of biodiversity in Arabian Sea. Reuters, 2008-04-23
  4. Taonga, New Zealand Ministry for Culture and Heritage Te Manatu. "1 – Native plants and animals – overview – Te Ara Encyclopedia of New Zealand". www.teara.govt.nz. Retrieved 2017-06-09.
  5. Burdick, Alan (25 March 2007). "The Wonder Land of Socotra". T Magazine. New York: New York Times. Retrieved 9 November 2009.
  6. "EU to protect Socotra archipelago environment". Saba Net. Yemen News Agency (SABA). 15 April 2008.
  7. "Islands east of the Horn of Africa and south of Yemen". WWF. Retrieved 4 February 2019.
  8. "Socotra Island xeric shrublands". Global Species. Retrieved 4 February 2019.
  9. "Socotra Island xeric shrublands". EOL. 6 May 2014. Retrieved 4 February 2019.
  10. Abrams, Avi (4 September 2008). "The Most Alien-Looking Place on Earth". Dark Roasted Blend.
  11. "Socotra islands scenery in Yemen". en.youth.cn. China Youth International. 25 April 2008.
  12. Miller, A.G.; Morris, M. (2004). Ethnoflora of the Socotra Archipelago. Royal Botanic Garden Edinburgh.
  13. Fund, W. (2014). Socotra Island xeric shrublands. Retrieved from http://editors.eol.org/eoearth/wiki/Socotra_Island_xeric_shrublands
  14. Kingdon, Jonathan (1989). Island Africa: The Evolution of Africa's Rare Plants and Animals. Princeton, New Jersey: Princeton University Press. pp. 38–42. ISBN 978-0-691-08560-9.
  15. Benda, Petr & Nasher, Abdul Karim & Van Damme, Kay & Vallo, Peter & Reiter, Antonín. (2017). Bats (Mammalia: Chiroptera) of the Eastern Mediterranean and Middle East. Part 14. Bat fauna of the Socotra Archipelago, Yemen. 10.13140/RG.2.2.14724.63360.
  16. Josef Suchomel, Josef (2012). "Mammals of Socotra". Friends of Soqotra. Accessed 15 August 2020.
  17. Vasconcelos R, Montero-Mendieta S, Simó-Riudalbas M, Sindaco R, Santos X, et al. (2016) Unexpectedly High Levels of Cryptic Diversity Uncovered by a Complete DNA Barcoding of Reptiles of the Socotra Archipelago. PLOS ONE 11(3): e0149985. https://doi.org/10.1371/journal.pone.0149985
  18. Apel, M. and Brandis, D. 2000. A new species of freshwater crab (Crustacea: Brachyura: Potamidae) from Socotra Island and description of Socotrapotamon n. gen. Fauna of Arabia 18: 133-144.
  19. Bicyclus, Site of Markku Savela
  20. "UAE organisation delivers cooking gas to Socotra residents to prevent deforestation". The National. 2 December 2018.
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