Where Is Aldabra?

 

 

   The Aldabra Atoll, home of the Aldabra Giant Tortoise is in the Indian Ocean, off the east coast of Africa and a place reminiscent of when dinosaurs ruled the world. Millions of years ago giant tortoises roamed almost everywhere, mainly because the continents were joined. However, by the end of the last ice age competition and predation from mammal and humans had made them almost extinct.

 

   In 1509 Aldabra was documented for the first time, on a Portuguese map of the area around Madagascar, however, a group of Arabs is actually thought to be the first known people to discover Aldabra during an exploratory trip in the Indian Ocean, nearly 600 years earlier in 916 AD. The name Aldabra is reported to have come from the Arabic word ‘Al-Khadra’ which means ‘the green’, in reference to the lagoon’s green reflection on the clouds above it. An alternative suggestion is that the name was derived from the star ‘Aldabaran’ which was used by early Arab sailors to chart their sailing course.

 

   In 1742, as the Islands of Seychelles were seized by the French, when Captain Lazard Picault was sent to Seychelles and to Aldabra to assess their potential, this was the first recorded visit to Aldabra. On Aldabra, the crew were astonished by the size of the giant tortoises and valued them as a source of meat, but they found the granitic islands of the Seychelles more useful and Aldabra was considered “not good for anything”. Aldabra’s remote location and harsh environment deterred permanent human settlement for many years. This is not to say however, that it did not escape exploitation despite Picault’s early derogatory assessment of the atoll.

 

   It is 285 miles from Madagascar to the Aldabra Atoll, which is its closest inhabited neighbour and 700 km east of Tanzania, on mainland Africa. Aldabra is the most south westerly Atoll amongst the Islands known as The Seychelles.

 

   Being located nearer the equator, the weather is opposite to the UK. There the Average temperatures range between 22-31 degrees Celsius and it is cooler in August and warmer in December.

 

   Aldabra is only 8 metres (26ft) at its highest elevation above sea level, and so is at great risk from rising water levels caused by climate change. The island is the worlds 2^nd largest coral atoll and is 34km (21miles) long by 13km (8.1miles) wide, which is just a little thinner but about the size of the Isle of Wight.

 

   An Atoll is a ring shaped Island, including a coral reef, which encircles a lagoon. The largest coral atoll in the world is The Great Chagos Bank in the South China Sea, near the Philippines.

 

   Unusually the Aldabra Giant Tortoise is top of the food chain there, but as there are no apex predators on the Island this large reptile has become the most influential animal on the Atoll. They perform a similar function as to an Elephant on the mainland. They are used by many plants as a valuable method of pollination because they browse food, grasses, trees and bushes and then distribute the propagated seeds, via their poo, after they pass through its digestive tract, across the island. Such is their influence that their intense grazing actions have put pressure on natural flowers to alter the way they pollinate, so the tortoises can't take flower heads off too soon, making them directly responsible for plant evolution taking place.

 

   Also, as their big bodies lumber about they also forge pathways and corridors, through the undergrowth allowing smaller species ease of movement too. A bit like The Highways Agency putting in new road systems! This is known as being a Keystone Species.

 

   Aldabra is a harsh place to live, with low amounts of food available to them and even less access to regular fresh water, for that they have to wait for the monsoons to arrive. Monsoon season is November to March. However with 300 miles of open water surrounding them there is no where else for them to easily go.

 

 

 

A Layout Of Aldabra Atoll Showing The Types Of Vegetation That Grow There. Most Of The Giant Tortoise Population Live On The South Island of Grand Terre.

 

 

   The Atoll of Aldabra consists of 13 limestone islands, which are the result of ancient volcanic or seismic activity. It was formed about 125, 000 years ago when the volcano first erupted and material was deposited on the sea bed, the eruptions continued to grow until the cool larva finally it broke the ocean surface. Coral grew around the island at sea level and when the old volcano subsided beneath the sea, a coral atoll was formed.

 

  The land contains varied habitats consisting of large mangrove swamps, which provide vital shade for the tortoises in the hot temperatures and these mainly border the lagoon which they love to wallow in through hot parts of the day. However, the tidal water of the lagoon is also teeming with Black Tipped sharks and Manta Rays, with millions of tonnes of water rushing in and out twice every day. Scrub forest peters down to grassland and then to the coastal dunes edging the largely inaccessible Atoll thanks to the coral reef which goes around most of the island preventing boats from getting too close. It is these reefs that helped to protect the Islands inhabitants from the plundering sailing ships of the 18^th and 19^th century.

 

   In 1811 the French lost Mauritius and Mahé to the British, and Aldabra became a British dependency governed from Mauritius. Then in 1874, the authorities on Mauritius announced their intention to begin commercial activities on Aldabra with a mangrove wood cutting and timber industry. However, Charles Darwin, along with other outstanding British naturalists of that time (including Thomas H. Huxley, Joseph Hooker, J. Kirk, Alfred Newton and Richard Owen), stated their grave concern about the effect of such an industry on the giant tortoise population of Aldabra and as a result the Governor of Mauritius abandoned the timber industry proposal.

 

   In 1879 an attempt was made to settle the atoll by a party of 27 adults and 13 children, all Norwegians from Bergen, who arrived via Madagascar to found a community on communistic principles. This scheme failed because the group disbanded before even reaching Aldabra; when they learned the truth of the harsh environment on Picard, they changed their destination.

 

   The first settlement was finally built on Ile Magnan in 1888 at the west channel of Aldabra by the lessee Jules Cauvin. He came to exploit the atoll for mangrove timber, green turtles and fish and also tried to plant coconuts. As lessees changed, other commercial exploits, such as cotton and sisal production, were also attempted unsuccessfully. These settlers also released domestic animals such as pigs and goats on the atoll. The mainstays of successful human exploitation through this time were fishing, timber and sadly the harvesting of green turtles and giant tortoises for their meat, which dramatically reduced their numbers.

 

   The lagoon connects with the ocean in only 4 places, where there is a gap in the coral reef, so there is very limited access to the internal areas. This water area is so big that Mahé, the largest inhabited island of Seychelles, would actually fit inside this lagoon.

 

   Aldabra Island has 4 main Islands, North or Middle Island also known as Malabar. Grand Terre, which is the South Island and is the largest, Polymnie and a small settlement island called Ille Picard or West Island, where the research station is located. Tortoises are most plentiful on South Island but a few live on North and West. Researchers have found that  tortoises on Malabar and Piccard average twice the size of the ones on Grand Terre and also lay 3-4 times as many eggs per clutch.

 

   The coastline is approx 50 miles long with only about 1% of it being beach, which is so vitally important for the Green Sea Turtles. The rest is nearly impossible to cross on foot and because of this it is quite difficult to even get around the atoll, it has remained largely uninhabited and the giant tortoises have been free to colonise it and become the largest occupants.

 

    It is not just the Giant Tortoises that are of unique importance to the area. Reportedly there are 307 different species of plant and animals on Aldabra including one of the largest populations of nesting Green Sea Turtles in the Western Indian Ocean and these lay their eggs on the small beaches. In 2011 a paper published on the nesting green turtle showed an increase of 500-800% in last 40 years, which can definitely be heralded as a success for conservation and we can only hope these levels continue in future years

 

   Several invasive species have been introduced to Aldabra since man first began to step ashore, some of it was accidental some more deliberate. These alien species prey on the native wildlife and fauna affecting the biodiversity of the area. An example is feral goats which would compete with the tortoises for foliage and grass, these were finally eradicated in 2012 after an intensive period of tracking, and elimination, completing an eradication programme that lasted for more than a quarter of a century. Seychelles Island Foundation, who are now in charge of Aldabra Atoll, is researching method to eradicate more of these threats and some progress has been made.

 

   In 1962 the British government commissioned a secret military survey of Aldabra and other western Indian Ocean islands. In April 1965, The Times of London newspaper reported that Aldabra, Diego Garcia and Cocos-Keeling were being considered as sites for a future Anglo- American military base. Conservationists in Seychelles and in the international community, spearheaded by the Royal Society of London were outraged at this proposal an it became known as the ‘Aldabra Affair’. Led by Professor David Stoddart, they successfully campaigned to save Aldabra’s unique fauna and flora from the encroachment of military technology which would have likely resulted in their decline and extinction. In 1967 the plans for a military staging post were abandoned much to the relief of conservationists, scientists and supporters around the world.

 

 

 

 

   After this the scientific interest in Aldabra increased and its value was recognized by the Royal Society, who planned and carried out a series of exploratory visits. In 1971, members of the Royal Society established a scientific research station on Aldabra near to the old settlement on Picard Island. The next few years were a time of intense scientific research and discovery with many scientific papers published on the unique flora and fauna of the Atoll.

 

   In 1979 the Seychelles Islands Foundation (SIF) was established as a public trust by presidential decree and was mandated to manage and conserve Aldabra. SIF manages and protects the UNESCO World Heritage Sites of Aldabra Atoll and the Vallée de Mai and has the President of Seychelles, Wavel Ramkalawan, as Patron.

 

   In 1996 SIF initiated the complete renovation of the Aldabra research station and field camps around the atoll. In the past 30 years considerable scientific research has been undertaken at Aldabra with many scientific papers published, some following on from the earlier initial research of the 1970’s. SIF has also developed and sponsors an education and outreach programme making Aldabra accessible to visits from Seychellois schoolchildren so they too can be inspired by this incredible place. After all it is the locals who are going to be looking after it in the future and they need to be inspired by its unique magic, to care and fight to keep it undeveloped.

 

   Whilst the Giant Tortoise may be the most famous resident of Aldabra there are other native species who are being reintroduced, studied and brought back from the brink of extinction. One success story began in 1999, when 18 flightless Aldabra Rail were reintroduced and after only 12 years there was an estimated population of 2765 of these birds.

 

   Sadly, in 2007 the Banded Aldabra Snail was declared extinct on the Atoll, However, to the scientists amazement and delight, in 2014 it was rediscovered on the northern island of Malabar.

 

  Aldabra is situated close to one of the busiest shipping routes in the Indian Ocean. In 1982, was inscribed as a UNESCO Marine World Heritage Site and The International Maritime Organisation has listed the atoll as an area to be avoided by all ships. Recently the Seychelles Island Foundation has extended outer edge of the marine protection zone, in the sea, to give the Atoll even greater protection. However, there is still a real possibility that if an oil tanker sustained any damage in this area, the subsequent oil spill could have devastating consequences for Aldabra’s wildlife.

 

   Aldabra is a site of global scientific interest but its remote location makes operational management a major logistical challenge. In 2008, SIF started to investigate ways to increase energy efficiency, and develop a renewable energy system, instead of using the traditional diesel generators which were costly to run; aiming both to reduce operational costs and their environmental impact. A 25 kWp hybrid photovoltaic-diesel energy system was installed in 2012. In the first year of operation, 94% (38,171 kWh) of the station’s electricity demand was generated by the new solar power system. This contributed to a significant reduction in CO2 emissions (a total of 97,523 kg of CO2 per year were avoided, of which 59% resulted from investments into energy efficiency measures and 41% was contributed by the PV system) and subsequently the research station’s carbon footprint was largely reduced. Since implementation of the photovoltaic system, their diesel demand has decreased by 97% which will lead to a projected saving of €68,000, resulting in a system payback time of only three years.

 

   For anyone wishing to visit Aldabra, it is important to realise that the atoll has no airstrip, no harbour or jetty and no helipad. There is no hotel or guest house and Aldabra is not set up to accommodate general visitors. All visitors arrive on live-aboard vessels but the activities of such vessels in this region have been severely compromised by piracy in recent years

Modern day piracy is a big problem in the Western Indian Ocean and their increased threat in the area, for the last 5 years has reduced tourism in the outer Seychelles islands and this means there is less money coming onto the area. The cruise ships and dive vessels are keeping well clear which means the research station on Aldabra receives less funding from the tourism dependent area.

 

   Improved vessel security and enforcement in the region is gradually lessoning the threat and impact but it will be a slow journey to recover its popularity. One of the downsides of the tourism to the area is the increase on rubbish that pollutes the ocean. The Aldabra clean up project was first launched in 2018 and when a second clean up took place in early 2019, they collected over 25 metric tonnes of marine plastic litter, during the five week expedition.

  

More information on Aldabra can be found at the website for Seychelles Islands Foundation (SIF) www.sif.sc

 

 

 

 

Where Did The Tortoises Come From Before They Landed On Aldabra?

 

   Several species of giant tortoise inhabited the Islands of the western Indian Ocean until the middle of the 19^th century. The Mascarene (Mauritius, Rodriguez and Reunion) giant tortoises had been driven to extinction by 1795 whilst the Madagascar-Seychelles genus Dipsochelys still survives. This genus was found on Madagascar, the Comoros, Glorieuse, the Aldabra group (Aldabra, Assumption, Astove and Cosmoledo), Farquhar and the Seychelles group of islands. The only surviving wild population is on Aldabra (the species is known variously as Dipsochelys dussurriieri, Dipsochelys elephantina or Geochelone gigantea).

 

Evolution of Dipsochelys Dussurriieri

   In the past the evolutionary history of these species has remained speculative. The main thoughts are that there are two alternative views; either that the genus evolved on Madagascar and colonised all the other islands from there or that it evolved in Seychelles and colonised Madagascar, which then provided the source for all other populations. Both these scenarios have Madagascar as the source for the Aldabran population due to the proximity of these islands and the fact that Aldabra has been colonised by giant tortoises on at least three occasions (138, 125 and 50 thousand years ago). It would seem probable that repeat colonisation would require a close source population, which Madagascar would provide. There is a major problem with this however, as all the fossil remains from Aldabra are identifiable as D. dussumieri or a very closely related form, while the Madagascan species were both very distinctive and different. Thus colonisation from Madagascar does not fit with the available evidence, which leaves a suggested alternative explanation, more probable.

 

   The Madagascan, Seychelles and Aldabran species are all well known, while remains found from the nearby Islands of Comoros and Glorieuse are too fragmentary to be identifiable beyond the generic 'Dipsochelys species.' The important points in this evolutionary scenario are that the Madagascan species form a group of their own, the Aldabran species is closely related to a Seychelles species and both Madagascan and Aldabran species have evolved from ancestors that probably inhabited Seychelles.

 

   This scenario, that they originated from the Seychelles, agrees with the known palaeobiogeography and the estimated dates of extinction of the different populations but the repeated colonisation of Aldabra from Seychelles does seem unlikely. As all the low lying islands between the granitic Seychelles and Aldabra would have been submerged during the periods when giant tortoises colonised Aldabra, colonists would have had to float over 1000km of open ocean. This is possible and appears to have occurred in many animal and plant species but as a repeat event it seems unlikely to say the least.

 

   Alternately the less well known, extinct, populations may provide an explanation here. Glorieuse is also low lying and would not provide a source population for any colonisation events but the nearby Comoros would have been raised above sea level throughout this period. Their giant tortoises were presumably also of Seychelles origin and may have been very closely related to the first Aldabran colonists as they lie within the same marine currents. These islands are the closest land to Aldabra and with the direction of marine currents would provide an even better source for potential colonists. It can be speculated that all the islands received their first colonists from Seychelles originally, these diverged into the three main southern species - D. abrupta, D. grandidieri and D. dussumieri, the latter possibly with slightly different forms on Aldabra and the Comoros. Sea level rises subsequently eliminated the Aldabran population 138,000 years ago, the re-emergence of the atoll allowed recolonisation from the Comoros 125,000 years ago, followed by repeated inundation, extinction and recolonisation 100,000 and 80,000 years ago. This explains how the same form of giant tortoise could colonise Aldabra whilst appearing to be endemic (naturally restricted) to that atoll and other lower islands nearby. This scenario predicts that when further subfossil material is found in the Comoros it will prove to be very close to and probably indistinguishable from D. dussumieri.

 

   The wider implication of this complex pattern of recolonisation is to note the surprisingly high frequency of recolonisation events. We know that these happened at least three times in the last 150,000 years. Each colonisation could have been achieved by a single gravid female landing on the atoll. Assuming that there is no significant difference in the probability of males and females being washed out to sea and onto Aldabra this would imply that at least 6 tortoises landed on the atoll during those 150,000 years. This represents approximately 1 every 1000 generations. This is a minimum estimate as immigration to established populations would not be detectable. However this does make the point that immigration was likely to have been an important contribution to the Aldabran giant tortoise population. Potential sources of immigrants were Seychelles, Madagascar, Comoros and the nearby low lying islands (Assumption, Astove, Cosmoledo and Farquhar) these last two would have been the most likely sources.

 

Recent history

   All Giant Tortoise populations have declined following contact with humans and in the last 1500 years and a number of them have gone extinct altogether. In the Seychelles there were said to be less than 8,000 animals there in 1787 after 13,000 had been removed, with at least 10,000 of them exported to Mauritius alone by the end if that year. Similar numbers had been exported to India and Africa.

 

   Around 1810 the Giant Tortoises were practically extinct in the Seychelles and the plunderers had moved on to Aldabra by 1822. The wild population on the Seychelles now stands around 300 animals, a pitiful fraction of the thousands that used to live there. On Aldabra densities can reach over 50 per hectare.

 

   Thanks to increase whaling activity in the area from 1823 Aldabra tortoises were regularly exported to the Seychelles, there was one voyage in 1826 who took onboard 2400 tortoises. By 1892 the population on Aldabra had also plummeted to just 1000 animals. It was so drastic that only 13 tortoises were exported in the ten years before the turn of that century, presumably because more could not be located and moved over the inhospitable territory.

 

   Since that time the population seems to have steadily recovered and by 1925 large numbers were reported to be living on the atoll. With such a reduction in the animals living there it means a severe narrowing of the genetic diversity of the tortoises, however this does not appear to be having any significant harmful effects on the resident population.

 

   With the previous recolonisation events of the past they must have gone through similar situation, however in the past animals immigrated to the islands from elsewhere, at least one tortoise per thousand generations but that is not happening now. The Aldabra population now lives in complete isolation, so if new genetic diversity is to be added it may be necessary for man to intervene and specifically transfer animals to the atoll to prevent complications. With so many of the neighbouring islands populations being extinct and so many of the tortoises being taken to Mauritius where the breeding parks now reside, it may be unavoidable that the effects of inbreeding cannot be avoided. As so many of the remaining animals in the area will be related in some form. It may be necessary to reintroduced populations offspring of some of the long-term captive Aldabran tortoises elsewhere around the world.

 

 

 

More information on this subject can be found in Evolution and history of the giant tortoises of the Aldabra Island Group

by  J. GERLACH & K.L. CANNING

 

Most unexpected question ever been asked at a show

   One day, we were at a show, with the tortoises, and one very interested young lad asked me an unusual question, which has always stuck in my mind: -

 

How Many Plane Rides Does It Take To Get To Aldabra?

   I will be honest and say I did not have a clue, so guessed at two. Well, I was right - It turns out that you hop on a plane from either London Gatwick, London Heathrow, Birmingham, Edinburgh, Glasgow or Manchester with the destination of Mahe Island in the Seychelles.

 

   From Mahe you then need to Charter a flight to Assumption, which is another Island in the Aldabra Group and is about 45km from Aldabra itself. As Aldabra does not have and airport, or Helipad, you would need to charter a boat for the last 45km to get you there, it should take about half a day, subject to weather conditions. The boat would also be your sleeping accommodation for your time in the area, as there is no accommodation on the Island either.

 

  Alternately you can charter a boat direct from Mahe Island To Aldabra but the trip, is about 1000km, across this section of the Indian Ocean and will take several days.

 

To that memorable young man, - We hope that you manage to get your trip to Aldabra, one day.