Coral atoll where giant tortoises outnumber man 10,000:1

March 25th, 2013 in Biology / Ecology
Aldabra giant tortoises are pictured at a botanic garden in Mahe, Seychelles on March 5, 2012. The reptiles, whose scientific name is Aldabrachelys giganteaone, are among the world's largest tortoises and reportedly live for more than 200 years.


Aldabra giant tortoises are pictured at a botanic garden in Mahe, Seychelles on March 5, 2012. The reptiles, whose scientific name is Aldabrachelys giganteaone, are among the world's largest tortoises and reportedly live for more than 200 years.

It is perhaps not surprising that there are only a handful of humans on one of the most remote islands on Earth, coral atolls far out in the turquoise seas of the Indian Ocean.

What is unexpected are the 100,000 —more than are found on the world famous —with some weighing a staggering 250 kilogrammes (550 pounds) and with shells more than a metre (yard) across.

On Aldabra atoll, far flung coral outcrops belonging to the Seychelles archipelago, the lumbering far outnumber the handful of human inhabitants—scientists and who are there to try to protect them.

The reptiles, whose scientific name is Aldabrachelys giganteaone, are among the world's largest and reportedly live for more than 200 years.

They were once threatened by including goats and rats but efforts to eliminate those animals and preserve the last remaining habitat of this unique reptile have been paying off.

"The population of tortoises has remained stable at 100,000," said Frauke Fleischer-Dogley, head of the Foundation (SIF), which helps protect the United Nations cultural agency (UNESCO) listed .

"That shows that efforts to preserve the species are bearing fruit," she said, citing records from the 1980s and a study from 2012 that had thrown up the same numbers.

The UN Environment Programme (UNEP) lists the cluster of four as "one of the most isolated eco-regions on Earth".

The inner lagoon is dotted with limestone pinnacles topped with coral mushroom-like heads where mangroves grow and seabirds roost or nest.

The tortoise population has remained stable for the past two decades while the humans on this archipelago—1,100 kilometres (700 miles) west from the main island of the Seychelles—has never grown much beyond ten.

Aldabra giant tortoises are pictured at a botanic garden in Mahe, Seychelles on March 5, 2012. Efforts to eliminate invasive species and preserve the last remaining habitat of the giant tortoise in the Seychelles have been paying off.

With no permanent freshwater source, and more than 400 kilometres (250 miles) away from Madagascar, the nearest major landmass, Aldabra was completely uninhabited for a long time.

The isolation of Aldabra—four islands surrounding a lagoon—means that ten percent of the wildlife here is found nowhere else on Earth.

Aldabra is the only place in the world where a reptile is the dominant herbivore, according to UNESCO, with shrubs and grasses that have evolved to take advantage of the manner in which tortoise eat.

The only threat to the tortoises are goats introduced to the isles more than a century ago, and that conservationists have tried to eradicate because they compete with the reptiles for the low growing vegetation.

But they are not the only large reptile: Aldabra's turtles are on the increase.

"In the 1960s, and even before that, turtles were harvested for commercial purposes, but since that stopped we've been seeing an increase in the population," said Janske van de Crommenacker, scientific coordinator on Aldabra.

The breeding season for turtles is December through March: they come to lay their eggs on the white sand beaches of the four islands that make up Aldabra: Grande Terre, Picard, Malabar and Polymnie.

"Every day we see the tracks of 30 to 40 green turtles that have come ashore to lay their eggs on Picard," the island where the conservationists are based, she added.

In order to better understand the movements of female turtles SIF has fitted six of them with satellite transmitters.

"That enables us to know at any given moment where they are. For the moment only six adult females have had transmitters fitted because it's extremely expensive," van de Crommenacker explained.

"We've seen that once they have laid their eggs on Aldabra the females move in different directions: some go towards the African coast, towards Kenya, one went via the Comoros to Madagascar, and another one is still in the Seychelles."

But while the conservationists efforts are now protecting Aldabra, the coral outcrop had a lucky escape: during the late 1960s it was mooted as a possible site for a US and British military base, but the project was shelved after fierce opposition from environmental groups.

The Indian Ocean atoll of Diego Garcia in the Chagos archipelago was chosen instead, some 3,000 kilometres to the east, and is still a major military and naval base today.

Few come to disturb this near pristine wilderness: tourists are allowed to visit but must abide by strict rules.

The number of visitors is limited by the atoll's geographical situation and the cost of getting there, given the absence of any direct scheduled air or sea link.

"We used to have cruise ships that regularly brought tourists to visit the island, but ever since there was a pirate attack in the area in 2008 the number of visitors has gone down considerably," SIF chairman Maurice Loustau-Lalanne.

"Numbers are picking up again, but it's not what it was before 2008," he said.

(c) 2013 AFP

"Coral atoll where giant tortoises outnumber man 10,000:1." March 25th, 2013. http://phys.org/news/2013-03-coral-atoll-giant-tortoises-outnumber.html