Astove Island
Encyclopedia
Aldabra Group
The Aldabra Group are part of the Outer Islands of the Seychelles, lying in the southwest of the island nation, 1000 kilometres from the capital, Victoria, on Mahé Island.The total land area of the Aldabra Group is 175.91 km²....
of the Seychelles
Seychelles
Seychelles , officially the Republic of Seychelles , is an island country spanning an archipelago of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar....
. It is 38 km SSE
SSE
-Computing:*Server-sent events, a technology to push content to web clients*Simple Sharing Extensions, a specification that extends RSS from unidirectional to bidirectional information flows*SPARQL Syntax Expressions*Microsoft SQL Server Express Edition...
of Cosmoledo Atoll
Cosmoledo
Cosmoledo is an atoll of the Aldabra Group and belongs to the Outer Islands of the Seychelles. The atoll is 14.5 km long east-west, and 11.5 km north-south. The total land area is about 5.2 km², while the lagoon measures 145 km² in area . It is located at . The closest island is Astove Island,...
, located at (10°06′S 47°45′E). It is a raised coral island
Raised coral atoll
A raised coral atoll is a typical atoll which has been lifted high enough above sea level by tectonic forces to protect it from scouring by storms and enable soils and diverse – often endemic – species of flora and fauna to develop...
of most peculiar form: a single stretch of land, more than 1 km (nearly one mile) at the widest, almost entirely encloses a shallow lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
. This has a maximum depth of 3 metres (10 ft), and the only exit is a winding passage in the southwest, called Gueule Bras Channel.
Astove Island measures nearly 6 km (3.7 mi) north to south and about 4 km (2.5 mi) at most east to west. The land area is 4.96 km², and the total area including the lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
9.5 km². The only settlement, on the western coast, has been abandoned since the 1980s. There is a grass airstrip on the north east point of the island, and remnants of a former coconut
Coconut
The coconut palm, Cocos nucifera, is a member of the family Arecaceae . It is the only accepted species in the genus Cocos. The term coconut can refer to the entire coconut palm, the seed, or the fruit, which is not a botanical nut. The spelling cocoanut is an old-fashioned form of the word...
and sisal
Sisal
Sisal is an agave that yields a stiff fibre traditionally used in making twine, rope and also dartboards. The term may refer either to the plant or the fibre, depending on context...
plantation
Plantation
A plantation is a long artificially established forest, farm or estate, where crops are grown for sale, often in distant markets rather than for local on-site consumption...
. Today, the island is rarely visited - usually by scientists researching the lagoon's ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
- but the near-vertical drop-off from its outer reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
edge is a popular location for diving
Diving
Diving is the sport of jumping or falling into water from a platform or springboard, sometimes while performing acrobatics. Diving is an internationally-recognized sport that is part of the Olympic Games. In addition, unstructured and non-competitive diving is a recreational pastime.Diving is one...
cruises.
Geology and geography
Astove's unusual structure has raised questions about its formation. It was suggested that Astove is not a true raised atollAtoll
An atoll is a coral island that encircles a lagoon partially or completely.- Usage :The word atoll comes from the Dhivehi word atholhu OED...
, but rather a reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
flat, with the lagoon being washed out later. In any case, the lagoon indeed grows slowly, owing to rainwater, acidic from the soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
's humic acid
Humic acid
Humic acid is a principal component of humic substances, which are the major organic constituents of soil , peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter...
s, dissolving the lagoon's carbonate rock
Carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...
bottom. This causes the unusual milky-white color of the lagoon's waters.
On the large land mass towards the western tip, exposed reef rock raises to some 5 m (15 ft) ASL
Above mean sea level
The term above mean sea level refers to the elevation or altitude of any object, relative to the average sea level datum. AMSL is used extensively in radio by engineers to determine the coverage area a station will be able to reach...
. Elsewhere it is largely covered by gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
ly debris. Dunes of up to 18 m (60 ft) line the windswept eastern rim of the island, and the eastern part of the lagoon is especially shallow due to the inblown dune sand. Astove Island's fringing reef
Fringing reef
A fringing reef is one of the three main types of coral reefs recognized by most coral reef scientists. It is distinguished from the other two main types in that it has either an entirely shallow backreef zone or none at all...
is just about 180 m (300 ft) wide. As noted above, beyond the reef's outer edge the seafloor drops down steeply, with a depth of 550 m (300 fathom
Fathom
A fathom is a unit of length in the imperial and the U.S. customary systems, used especially for measuring the depth of water.There are 2 yards in an imperial or U.S. fathom...
s) a mere 300 metres (900 ft) away from the shoreline.
The climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
is dominated by the southeast trade wind
Trade wind
The trade winds are the prevailing pattern of easterly surface winds found in the tropics, within the lower portion of the Earth's atmosphere, in the lower section of the troposphere near the Earth's equator...
s which are most pronounced between April and November. Temperatures are around 28 °C (83°F) in the shade during that time, and slightly higher during the northwest monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
season. At that time, tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
s with torrential rain sometimes hit the island, but overall it is very arid
Arid
A region is said to be arid when it is characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant and animal life...
.
Flora
Astove Island has a very thin soilSoil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
layer overlying its rocky core, which is pockmarked with caverns. Guano
Guano
Guano is the excrement of seabirds, cave dwelling bats, and seals. Guano manure is an effective fertilizer due to its high levels of phosphorus and nitrogen and also its lack of odor. It was an important source of nitrates for gunpowder...
of nesting seabird
Seabird
Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...
s has accumulated in these, and the western part has been worked over and some of the guano was mined in the past. Large stretches of the reef rock were stripped bare of vegetation, but some Pisonia grandis
Pisonia grandis
Pisonia grandis is a species of flowering tree in the Bougainvillea family, Nyctaginaceae.-Description:The tree has broad, thin leaves, smooth bark and bears clusters of green sweet-smelling flowers that mature into sticky barbed seeds....
and White Milkwood (Sideroxylon inerme
Sideroxylon inerme
Sideroxylon inerme is a Southern African coastal tree, with dense foliage, black berries and small, dainty white flowers. The tree's formal name means "Iron-wood" in Greek, referring to its very hard timber....
) persisted. The general vegetation on much of the island's western side is herbaceous plant
Herbaceous plant
A herbaceous plant is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground...
s however, mainly the leadwort
Plumbago
Plumbago is a genus of 10-20 species of flowering plants in the family Plumbaginaceae, native to warm temperate to tropical regions of the world. Common names include plumbago and leadwort...
Plumbago aphylla, as well as Stachytarpheta
Stachytarpheta
Stachytarpheta is a plant genus in the verbena family . The flowers are rich in nectar and popular with many butterflies, such as the South Asian Crimson Rose , Malabar Banded Swallowtail , and Grass Yellow .-Selected species:* Stachytarpheta cayennensis* Stachytarpheta indica*...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
and the crowfoot grass
Dactyloctenium
Dactyloctenium is a grass genus in the family Poaceae. The approximately ten species in this genus are collectively known as crowfoot grasses...
Dactyloctenium pilosum. Coconut Palms
Coconut
The coconut palm, Cocos nucifera, is a member of the family Arecaceae . It is the only accepted species in the genus Cocos. The term coconut can refer to the entire coconut palm, the seed, or the fruit, which is not a botanical nut. The spelling cocoanut is an old-fashioned form of the word...
(Cocos nucifera) remaining from the struggling plantation along the western shore, Sisal
Sisal
Sisal is an agave that yields a stiff fibre traditionally used in making twine, rope and also dartboards. The term may refer either to the plant or the fibre, depending on context...
(Agave sisalana), and Wild Cotton (Gossypium
Gossypium
Gossypium is the cotton genus. It belongs to the tribe Gossypieae, in the mallow family, Malvaceae, native to the tropical and subtropical regions from both the Old and New World. The genus Gossypium comprises around 50 species , making it the largest in species number in the tribe Gosssypioieae....
) are also found here and there. Maize
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
(Zea mays) was planted by the guano miners, but presumably this plant which depends on constant care has disappeared since.
The eastern dunes are overgrown with the dropseed
Sporobolus
Sporobolus is a genus of grasses in the family Poaceae. They are usually called dropseed grasses or sacaton grasses. They are typical prairie and savanna plants, and they occur in other types of open habitat in warmer climates....
grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...
Sporobolus virginicus
Sporobolus virginicus
Sporobolus virginicus, known by numerous common names including Marine Couch, Sand Couch, Coastal Rat-tail Grass, Salt Couch Grass, Saltwater Couch and Nioaka, is a coastal non-bunching tussock grass with a wide distribution.-Description:It is a perennial tussock grass from 10 to 50 centimetres in...
near the sea, and on the higher parts bwa matlo (Bay Cedar, Suriana maritima
Suriana
Suriana is a monotypic genus of flowering plants containing only Suriana maritima, which is commonly known as Bay Cedar. It has a pantropical distribution and can be found on coasts in the New and Old World tropics. Bay Cedar is an evergreen shrub or small tree, usually reaching a height of and...
) shrubs are found. In more sheltered places, a regular scrubland of vouloutye (Scaevola taccada
Scaevola taccada
Scaevola taccada, also known as Beach Cabbage, Sea Lettuce, Beach Naupaka, Naupaka kahakai , Merambong and Ngahu is a flowering plant in the family Goodeniaceae found in open coastal locations of the tropical areas of the Indo-Pacific...
) and Tree Heliotrope (Heliotropium foertherianum
Heliotropium foertherianum
Heliotropium foertherianum is a species of flowering plant in the borage family, Boraginaceae. It is native to tropical Asia, including southern China, Madagascar, northern Australia, and most of the atolls and high islands of Micronesia and Polynesia. Common names include Tree Heliotrope, Velvet...
), with some Pisonia, occurs. Here, the main herbs are fimbries (Fimbristylis
Fimbristylis
Fimbristylis is a genus of sedges. A plant in this genus may be known commonly as a fimbry, fimbristyle, or fringe-rush. There are 200 to 300 species distributed worldwide. Several continents have native species but many have been introduced to regions where they are not native. Many are...
) and the parasitic
Parasitic plant
A parasitic plant is one that derives some or all of its sustenance from another plant. About 4,100 species in approximately 19 families of flowering plants are known. Parasitic plants have a modified root, the haustorium, that penetrates the host plant and connects to the xylem, phloem, or...
vine
Vine
A vine in the narrowest sense is the grapevine , but more generally it can refer to any plant with a growth habit of trailing or scandent, that is to say climbing, stems or runners...
Cassytha filiformis.
The flats around the lagoon show a mixture of grassland (mainly Pembagrass, Stenotaphrum dimidiatum) and Pemphis acidula scrub; in places, it is difficult to penetrate to the lagoon through the mass of Pemphis and bwa matlo. Small Grey Mangroves (Avicennia marina
Avicennia marina
Avicennia marina, commonly known as grey mangrove or white mangrove, is a species of mangrove tree classified in the plant family Acanthaceae...
) line much of the southern half of the lagoon rim.
Fauna
Astove Island is home to four landbirds:- Cosmoledo Abbott's Sunbird (Cinnyris sovimanga buchenorum or C. abbotti buchenorum) - only occurs here and on nearby CosmoledoCosmoledoCosmoledo is an atoll of the Aldabra Group and belongs to the Outer Islands of the Seychelles. The atoll is 14.5 km long east-west, and 11.5 km north-south. The total land area is about 5.2 km², while the lagoon measures 145 km² in area . It is located at . The closest island is Astove Island,...
- Madagascar CisticolaMadagascar CisticolaThe Madagascar Cisticola is a species of bird in the cisticola family . It forms a superspecies with the closely related Zitting Cisticola and the Socotra Cisticola....
(Cisticola cherina) - Pied CrowPied CrowThe Pied Crow is a widely distributed African bird species in the crow genus.Structurally, the Pied Crow is better thought of as a small crow-sized Raven, especially as it can hybridise with the Somali Crow where their ranges meet in the Horn of Africa...
(Corvus albus) - Common Madagascar White-eye (Zosterops maderaspatanus maderaspatanus)
There are few seabirds, probably due to the presence of rat
Rat
Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus...
s and feral pigs, left behind when the settlement was abandoned. Unlike the other two atolls of the group, (Aldabra
Aldabra
Aldabra, the world's second largest coral atoll, is in the Aldabra Group of islands in the Indian Ocean that form part of the Seychelles. Uninhabited and extremely isolated, Aldabra is virtually untouched by humans, has distinctive island fauna including the Aldabra Giant Tortoise, and is...
and Cosmoledo), there are no predator-free islets except for a few small sandbanks close to the lagoon entrance. At the lagoon, Caspian Tern
Caspian Tern
The Caspian Tern is a species of tern, with a subcosmopolitan but scattered distribution. Despite its extensive range, it is monotypic of its genus, and has no subspecies accepted either...
s (Hydroprogne caspia) will often be seen but it is not known whether or not they ever breed on the sandbanks. Indopacific Sooty Tern
Sooty Tern
The Sooty Tern, Onychoprion fuscatus , is a seabird of the tern family . It is a bird of the tropical oceans, breeding on islands throughout the equatorial zone. Colloquially, it is known as the Wideawake Tern or just wideawake...
s (Onychoprion fuscatus nubilosus) and Western Black-naped Tern
Black-naped Tern
The Black-naped Tern is an oceanic tern mostly found in tropical and subtropical areas of the Pacific and Indian Oceans. It is rarely found inland....
s (Sterna sumatrana mathewsi) have also been reported. Green turtles (Chelonia mydas) remain very common here despite years of exploitation and remarkably high numbers will be seen on dives or from tender trips to the lagoon entrance close to high tide.