Aldabra

Last updated • 11 min readFrom Wikipedia, The Free Encyclopedia

Geology

The atoll reflects both fossil and geomorphological features, the former is the source of the biodiversity seen today. [25] The atoll consists of reef limestone of Pleistocene age (with irregular coral formations called "champignon", [26] made up of two layers of varying stages of crystallization [27] ) and this extends over an average width of 2 kilometres (1.2 mi) rising to a height of 8 metres (26 ft) above sea level, and forming the rim line (low cliffs with "deep notches, preceded by jagged pinnacles") of the shallow central lagoon. [17] [11] [25] [21] Geologically the limestone beds have been subjected to striation, sink holes and pits with prominent and continuous limestone bed on the eastern side above the sediment deposits.

The coastline has undercut limestone cliffs above a perched beach; it is in two clear terraces of 8 metres (26 ft) and 6 metres (20 ft) height above sea level. [7] [27] While the terrestrial topography (spread over an elevation range of 0–8 metres (0–26 ft) [26] ) is rugged and dictated by the geomorphic conditions, the land surface comprises limestone of about 125,000 years age, which has uplifted many times above the sea level. The surface conditions are criss-crossed and riddled with pot holes and pits. In the eastern zone of the lagoon, though the surface is continuous, sediment beds are also seen. The windward southern coast is made up of sand dunes. [7]

Climate and tides

Aldabra is situated in the dry zone of the south-west Indian Ocean. The northwest monsoon season is from November to March and brings the heaviest rainfall. In the remaining months, the south-easterly trade winds are dominant. Aldabra receives an annual average rainfall of 960 millimetres (38 in). [21] Cyclones are rare in the Seychelles due to its nearness to the Equator. [28] The reported monthly mean maximum air temperature recorded in December is 31 °C (88 °F). The mean minimum temperature recorded in August is 22 °C (72 °F).

The hydrodynamics of Aldabra's lagoon are dominated by the tides. The maximum tidal range at Aldabra is around 2.7m, which is exceptionally large for an atoll. Flow between the lagoon and open ocean is only possible through a small number of narrow channels, generating tidal currents of over 3 m/s at Passe Gionnet, and over 1 m/s at most other channels. Due to the restriction of water exchange through these channels, tides within the lagoon are lagged compared to the open ocean, and the tidal range varies across the lagoon. [29]

Aldabra sits in the path of the Indian Ocean's westward-flowing South Equatorial Current. Eddies are generated in the wake of Aldabra as the South Equatorial Current passes it, driving upwelling of deep waters that bring nutrients to the surface, and drive phytoplankton growth. [30]

Wildlife

EuropaIsland CoralMushrooms.JPG
Great Frigatebird at Genovesa.JPG
Left: coral "mushrooms". Right: nesting male great frigatebird.

The earliest study of the flora and fauna, and also the geomorphological structure was in 1910. There are 307 species of animals and plants on Aldabra. [31] Reptiles are the prominent terrestrial fauna. [7] Sir David Attenborough called Aldabra "One of the wonders of the world", and it is also known as one of "crown jewels" of the Indian Ocean. [32] [33]

Flora

A mangrove in Aldabra Atoll Aldabra Atoll-108994.jpg
A mangrove in Aldabra Atoll

The higher areas of Aldabra are covered in pemphis , a thick coastal shrub, while the lower areas, home to the giant tortoises, are a mixture of trees, shrubs, herbs and grasses. There have been recorded 273 species of flowering plants, shrubs, and ferns on the atoll. There are dense thickets of Pemphis acidula , and a mixture of grasses and herbs called "tortoise turf" in many areas. [7] [26] This flora includes 19 endemic species and 22 species that are only common to neighboring islands, and several of these species are on the IUCN Red List. The tropicbird orchid ( Angraecum seychellarum ) is the national flower of Seychelles and is found in the dry craggy limestone champignon of Aldabra. [34] Other endemic plants includes Pandanus aldabrensis , the Aldabra lily ( Aloe aldabrensis ) and a sub-species of tropicbird orchid, Angraecum eburneum. [35]

The lagoon is bordered by mangrove forests, and has large inland seagrass meadows as well as areas of coral reef and sand flats. [7] The mangroves, which thrive in tidal mudflat areas and saline conditions, are seen on the shores of the lagoon and are integral to the coastal ecosystem. There are seven species of mangrove on Aldabra, three of which are rarely occurring species. These include 'Mangliye blan' or white mangrove ( Avicennia marina ) which grows to 12 metres (39 ft), 'Mangliye lat' or black mangrove (Bruguiera gymnorhiza) which grows to 18 metres (59 ft) in a conical shape, 'Mangliye zonn' (Ceriops tagal) which grows to 7 metres (23 ft) with a buttressed trunk, and 'Mangliye rouz' or red mangrove ( Rhizophora mucronata ) which is the tallest species up to 20 metres (66 ft) in height. [36]

Fauna

Aldabra giant tortoise Aldabra Atoll-108993.jpg
Aldabra giant tortoise
Souimanga sunbird (Cinnyris souimanga) Souimanga Sunbird (Cinnyris souimanga).jpg
Souimanga sunbird (Cinnyris souimanga)

The atoll has distinctive fauna including the largest population of giant tortoises (Aldabrachelys gigantea) in the world (100,000 animals). [37] [38] Tortoise size varies substantially across the atoll, but adult tortoises typically have a carapace length of 105 centimetres (41 in) and can weigh up to 350 kilograms (770 lb). They are herbivores and feed on plants, trees and algae that grows in the freshwater pools. The tortoises mate between February and May, the females then lay their eggs from June to September in areas with suitable soil layers. They lay eggs the size of golf balls [12] in a clutch of three to five eggs every few years in high-density areas and 14-16 eggs in low-density areas. The females can lay several clutches in a year and [39] the incubation period is 73–160 days. The small vulnerable juveniles are prey to coconut crabs, land crabs, rats and birds. [12] In the past giant tortoises have been relocated to other islands in Seychelles and also to Victoria Botanical Gardens in Mahé. [12] One of the longest-lived Aldabra giant tortoises was Adwaita, a male who died at the age of about 250 years at Kolkata's Alipore Zoological Gardens on 24 March 2006.

Blacktip reef sharks (Carcharhinus melanopterus) Aldabra Atoll-108988.jpg
Blacktip reef sharks (Carcharhinus melanopterus)

Aldabra is a breeding ground for the hawksbill sea turtle (Eretmochelys imbricata) and green sea turtle (Chelonia mydas). [33] Aldabra has one of the largest populations of nesting green turtles in the Western Indian Ocean. Aldabra has a large population of the world's largest terrestrial arthropod, the coconut crab (Birgus latro); and hosts the white-throated rail, the only surviving flightless rail species in the Indian Ocean. [2] Sharks, manta rays, and barracuda populate the seas surrounding the island. During the Pleistocene the dominant land predator was the crocodilian Aldabrachampsus which is now extinct. Three extant species of lizards occur, the skink Cryptoblepharus boutonii and the geckos Phelsuma abbotti and Hemidactylus mercatorius. Pleistocene fossils also indicate the former presence of an Oplurus iguana and other skink and gecko species. [40] There are three endemic species of bat from Aldabra: Paratriaenops pauliani , [41] Chaerephon pusilla and the Aldabra flying fox (Pteropus aldabrensis), as well as the more widely distributed Mauritian tomb bat (Taphozous mauritianus). [42] There are 1,000 species of insects, many of them endemic. [7] Many species of butterflies also flutter around Aldabra. [43]

Endemic birds include the Aldabra drongo (Dicrurus aldabranus), the Aldabran subspecies of the white-throated rail (Dryolimnas cuvieri aldabranus), the last surviving flightless bird of the Indian Ocean region, and the endemic Aldabra fody (Foudia aldabrana), [26] The islands are important breeding grounds for thousands of seabirds, including several species of tern, red-tailed tropicbirds, white-tailed tropicbirds, red-footed boobies, and the world's second largest breeding populations of great and lesser frigate birds. [7] [33] [44] The bird fauna is most similar to Madagascar or Comoros and other birds found here include greater flamingos, the Malagasy pond heron, Comoros blue pigeon, Malagasy kestrel, Malagasy coucal, Madagascar nightjar, Malagasy bulbul and souimanga sunbird. [45] [46] [47] [48]

At least 13 species of cetaceans, including dolphins, orcas, and especially humpback whales, [49] [50] have been identified in the waters. [51] [52] Dugongs, thought to be regionally extinct in the 18th century, [53] have been confirmed multiple times in recent years. [54] [55] [56]

Conservation

Conservationists feared a major threat to the atoll's biodiversity in the 1960s when, as part of the British Indian Ocean Territory, the British made plans to set up a military establishment on the atoll. Due to national and international opposition this plan was cancelled in 1967. This incident became known as the 'Aldabra Affair' in England.

Invasive alien species such as rats, cats and goats that were introduced in the past threaten the native biodiversity of the atoll. Goats were eradicated from the atoll in 2012 after a long-term eradication program. Cats have been removed from all of the islands except Grande Terre Island, which allowed for the reintroduction of the Aldabra rail to Picard Island. Research into a feasibility study to eradicate rats from the atoll has been undertaken.

Until recently, Aldabra had remained free of invasive birds. However, the Madagascar fody ( Foudia madagascariensis ) has spread to Aldabra after being introduced to Assumption Island. An eradication program for this bird on both Assumption and Aldabra is almost complete. Due to the limited space of its habitat, extreme weather conditions, epidemic and limited range could also pose serious threats to the entire ecology of the atoll. [39]

Aldabra also accumulates large amounts of plastic pollution on its coastline and, despite a major clean-up effort in 2019, over 500 tonnes is thought to remain on the atoll, with significant impacts on wildlife. [57] Most of this pollution originates from the regional fishing industry, although a significant proportion is also transported from southeast Asia through ocean currents. [58]

The coral reefs at Aldabra were severely affected by the 2016 El Niño event. Although thermal stress was not exceptionally high (less than 4 Degree Heating Weeks), hard coral cover on Aldabra's seaward-facing reefs was reduced by over 50% (35% in the lagoon), and soft coral cover fell by over 90%. [59] Fortunately, there are some signs of recovery, as the abundance of juvenile corals has since increased significantly, particularly within the lagoon, [60] although Aldabra's reefs of course remain critically threatened by climate change.

Protection

Aldabra atoll was designated a UNESCO World Heritage Site on 19 November 1982. It is one of the two UNESCO World Heritage Sites in the Seychelles;, [61] and is managed by the Seychelles Islands Foundation (SIF). [62] The marine protected area extends 1 kilometre (0.62 mi) into the sea to ensure preservation of its marine fauna. [7] [26] Eco tourism is controlled and introduction of invasive species is restricted. [7]

Based on the evaluation process, UNESCO inscribed the site, a legally protected special reserve of 35,000 hectares (86,000 acres), on the list of World Heritage Sites under three criteria: Criterion (vii): Aldabra Atoll encompasses a large expanse of relatively untouched natural beauty where a number of important animal species and some plant species thrive, along with remarkable land formations, and its process provides a unique spectacle of natural phenomena; [7] Criterion (ix): The atoll is a superlative example of an oceanic island ecosystem in which evolutionary processes are active within a rich biota. The size and morphological diversity of the atoll has permitted the development of a variety of discrete insular communities with a high incidence of endemicity among the constituent species that are typical of island ecosystems. The natural processes take place with minimal human interference and can be clearly demonstrated in their full complexity; [7] and Criterion (x): Aldabra provides a natural laboratory for the study of the process of evolutionary ecology and is a platform for key scientific discovery. The atoll constitutes a refuge harboring viable populations of a range of rare and endangered species of plants and animals, including the last giant tortoise and flightless bird populations of the Western Indian Ocean, a substantial marine turtle breeding population, and large seabird colonies which number in the tens of thousands. The substantial tortoise population is self-sustaining and all the elements of its inter-relationship with the terrestrial environment are evident. [7] [63]

BirdLife International declared Aldabra as an Important Endemic Bird Area (IBA) in 2001 due to its large seabird colonies under categories A1, A2, A4i, A4ii and A4iii, covering an area of 33,180 hectares (82,000 acres) overlapping with the special reserve area of 35,000 hectares (86,000 acres) of Aldabra Atoll. [26] [64]

Aldabra became a Ramsar Convention Wetland Site of International Importance in 2010. [17] Covering 25,100 ha (over half the area of the whole atoll) the wetland ecosystem of Aldabra includes the extensive shallow lagoon inside the atoll, which is carpeted with lush seagrass beds and patchy coral reefs, the intertidal mud flats, the coral reefs outside the lagoon, freshwater pools, beaches, and 2000 ha of mangrove stands. These wetlands support several endangered species including the increasing number of turtles at the atoll, dugongs and many other bird, fish and invertebrate species.

Aldabra was designated as a site under the Indian Ocean South East Asia (IOSEA) turtle network, in their 2014 convention.

Demography

A small scientific research station of the SIF is based in La Gigi village on Picard Island. The permanent staff (currently 12 [65] ) conduct research to study Aldabra's biodiversity.

Transport

Aldabra is not easily accessed. No airstrips, helipads or landing jetties have been permitted on the atoll. The nearest airfield is on Assumption is 50 kilometres (31 mi) south-east of Picard Island. Bi-monthly supply ships operating from Mahé provide essentials to the research station. [11]

Tourism

Cruises are operated by several companies [66] [67] along with dive boats which may visit the atoll on expedition tours. Visits to the island by people other than the scientists and staff of the SIF are strictly controlled and only guided tours are provided with prior permission. [66] As of 2012, a yearly average of 900 tourists visit the atoll. Within the atoll, paved walking paths exist from the village of La Gigi, which leads to a promontory from where scenic views of the large lagoon (during low tides) and the mangrove species are seen. [66]

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References

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  56. ↑ Amla, Hajira (2 March 2015). "A creature of mystery – rare dugong is sighted in Seychelles at Aldabra". Seychelles News Agency . Retrieved 4 September 2017.
  57. ↑ Burt, April J.; Raguain, Jeremy; Sanchez, Cheryl; Brice, Jude; Fleischer-Dogley, Frauke; Goldberg, Rebecca; Talma, Sheena; Syposz, Martyna; Mahony, Josephine; Letori, Jake; Quanz, Christina; Ramkalawan, Sam; Francourt, Craig; Capricieuse, Ivan; Antao, Ash (2020-09-10). "The costs of removing the unsanctioned import of marine plastic litter to small island states". Scientific Reports. 10 (1): 14458. doi:10.1038/s41598-020-71444-6. ISSN   2045-2322. PMC   7483532 . PMID   32913284.
  58. ↑ Vogt-Vincent, Noam S.; Burt, April J.; Kaplan, David M.; Mitarai, Satoshi; Turnbull, Lindsay A.; Johnson, Helen L. (2023-02-01). "Sources of marine debris for Seychelles and other remote islands in the western Indian Ocean". Marine Pollution Bulletin. 187: 114497. Bibcode:2023MarPB.18714497V. doi: 10.1016/j.marpolbul.2022.114497 . ISSN   0025-326X. PMID   36646000.
  59. ↑ Cerutti, Julia M. B.; Burt, April J.; Haupt, Philip; Bunbury, Nancy; Mumby, Peter J.; Schaepman-Strub, Gabriela (February 2020). "Impacts of the 2014–2017 global bleaching event on a protected remote atoll in the Western Indian Ocean". Coral Reefs. 39 (1): 15–26. doi:10.1007/s00338-019-01853-1. ISSN   0722-4028. S2CID   253816988.
  60. ↑ Koester, Anna; Ford, Amanda K.; Ferse, Sebastian C. A.; Migani, Valentina; Bunbury, Nancy; Sanchez, Cheryl; Wild, Christian (2021-12-07). Keshavmurthy, Shashank (ed.). "First insights into coral recruit and juvenile abundances at remote Aldabra Atoll, Seychelles". PLOS ONE. 16 (12): e0260516. Bibcode:2021PLoSO..1660516K. doi: 10.1371/journal.pone.0260516 . ISSN   1932-6203. PMC   8651144 . PMID   34874982.
  61. ↑ MairBeckley 2012, pp. 27, 48.
  62. ↑ Seychelles Islands Foundation. Accessed 22 June 2008
  63. ↑ "List of World Heritage Sites". Unesco. Retrieved 23 March 2013.
  64. ↑ "Aldabra Island xeric scrub". World Wildlife.org. Retrieved 25 April 2013.
  65. ↑ Info on staff
  66. 1 2 3 MairBeckley 2012, p. 187.
  67. ↑ "Lonely Planet review for Aldabra Atoll". Lonelyplanet.com. Retrieved 25 March 2013.

Bibliography

Aldabra Atoll
An Outpost for Evolution at Aldabra Atoll.jpg
Satellite view of Aldabra
Seychelles location map.svg
Red pog.svg
Aldabra Atoll
Location of Aldabra Atoll in Seychelles
Geography
Location Indian Ocean
Coordinates 09°25′S46°21′E / 9.417°S 46.350°E / -9.417; 46.350
Archipelago Seychelles
Adjacent to Indian Ocean
Total islands46
Major islands
  • Picard
  • Malabar
  • Grande Terre
  • Polymnie
Area155.4 km2 (60.0 sq mi)
Length34 km (21.1 mi)
Width13 km (8.1 mi)
Coastline85 km (52.8 mi)
Highest elevation16 m (52 ft)
Highest pointunnamed dune
Administration
Group Outer Islands
Sub-Group Aldabra Group
Sub-Group Aldabra Atoll
Districts Outer Islands District
Largest settlementLa Gigi(pop. 12)
Demographics
Population12 (2016)
Pop. density0.08/km2 (0.21/sq mi)
Ethnic groups Creole, French, East Africans, Indians.
Additional information
Time zone
ISO code SC-26
Official website www.seychelles.travel/en/discover/the-islands/outer-islands
Criteria Natural: vii, ix, x
Reference 185
Inscription1982 (6th Session)
Outer islands of Seychelles / Coralline Seychelles (Zil Elwannyen Sesel)
Outer island Groups
Coat of arms of Seychelles.svg
Islands and Atolls
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