Unique to the Galapagos

90% of the reptiles 46% of the insects 50% of the birds

But the wildlife is in serious danger following an oil spill close to the island of San Cristobal, on the eastern side of the islands.

Colonies of sea lions, marine iguanas, birds such as blue-footed boobies and all forms of sea life are under threat.

Darwin's visit

The volcanic islands, famous for their giant tortoises, provided material for some of Charles Darwin's key research in formulating his theory of evolution.

Giant tortoises gave the archipelago its name

In 1835, he spent five weeks on the the islands, making extensive collections of plants and animals and observations of their natural history. For example, he found 13 different types of finches whose beaks were modified to different sub-environments on the islands.

Mr Darwin's 1845 book, Voyage of the Beagle, includes a detailed chapter on the Galapagos.

Now a world heritage site, the remote archipelago is managed by the Galapagos National Park Service, with help and advice from the Charles Darwin Foundation. The islands are administered by Ecuador, which in 1959 declared 97% of the total land area a national park.

The national park is fiercely protected, because it is the last place on Earth that preserves the unique biodiversity of a complete archipelago. Almost all its native flora and fauna remains intact.

Danger

But following the oil spill, Galapagos National Park officials said long-term danger would come from the fuel sinking to the ocean floor, destroying algae that is vital to the food chain, threatening marine iguanas, sharks, birds and other species.

The oil spill has already harmed birds

They said the current was pushing the spill south, and that within days it could reach Spain Island, where large colonies of sea lions and other marine animals congregate.

Sea lions were already in need of extra protection before the tanker Jessica ran aground. They live in large colonies on the beaches, feeding on fish and squid, but have come under threat in recent years from the increase in human activity.

Naturally sociable creatures, they like being close to humans - but it means they can be injured and killed by fishing hooks and nets.

The archipelago is about 1,000km from Ecuador

Since 1997, the Charles Darwin Research Station has been monitoring the sea lion population, and it will be anxious to prevent the oil spill spreading any further.

"It is clear that an incident of this kind could have a deep and lasting impact," said Peter Kramer, Network Relations Director of the conservation organisation WWF, and a former president of the Darwin Foundation.

"For example, two unique coastal species found only on the Galapagos could be particularly at risk: the Galapagos penguin and the flightless cormorant."

The penguins are the only penguins to be found on the equator. Like the islands' marine iguanas, they can survive because two of the three major ocean currents supply the archipelago with cold, nutrient-rich waters.

WWF campaign

Charles Darwin's Galapagos research led to his theory of evolution

The penguins and iguanas are already under pressure following severe weather blamed on the El Ni�o climate phenomenon.

During El Ni�o, which happens every two to seven years, warm waters in the Pacific move further east than is usual and nutrient-rich upwellings are disrupted.

This results in heavier rainfall in the Galapagos archipelago and the destruction of the red and green algal blooms that are the favoured food of marine iguanas.

The WWF is putting pressure on the Ecuadorian Government and the international shipping community to consider designating the waters around the islands as a particularly sensitive sea area.

"Such measures would help to ensure a much higher level of protection for this unique area of the world," said Sian Pullen, WWF's International Shipping Expert.

The island chain is most famous for its 9,000 rare giant tortoises.

The tortoises, first studied by Charles Darwin, measure up to 1.5m (4.9 feet) long by 1m high, and live more than 100 years.