Chagos Archipelago - Ecology

Ecology

The biodiversity of the Chagos archipelago and its surrounding waters is one of the main reasons it is so special. But this incredible diversity is under threat as at least 60 species that call Chagos home are already on the IUCN Red List of Threatened Species.

Coral

The reefs host 220 species of coral including the endemic brain coral Ctenella chagius. The coral cover is dense and healthy even in deep water on the steep outer reef slopes. Thick stands of branching staghorn coral (Acropora sp) protect the low lying islands from wave erosion. Despite the loss of much of the coral in a bleaching event in 1998 the recovery in the Chagos has been remarkable and overall coral cover increases year on year.

Fish

The reefs are also home to at least 784 species of fish that stay near to the shores of the islands including the endemic Chagos clownfish (Amphiprion chagosensis) and many of the larger wrasse and grouper that have already been lost from over-fishing in other reefs in the region.

As well as the healthy communities of reef fish there are significant populations of pelagic fish such as manta rays (Manta birostris), sharks and tuna. Shark numbers have dramatically declined as a result of illegal fishing boats that seek to remove their fins and also as accidental by-catch in the two tuna fisheries that used to operate seasonally in the Chagos.

Birds

Seventeen species of breeding seabirds can be found nesting in huge colonies on many of the islands in the archipelago, and 10 of the islands have received formal designation as Important Bird Areas, by Birdlife International. This means that Chagos has the most diverse breeding seabird community within this tropical region. Of particular interest are the large colonies of sooty terns (Sterna fuscata), brown and lesser noddies (Anous stolidus and Anous tenuirostris) wedge-tailed shearwaters (Puffinus pacificus) and red-footed boobies (Sula sula).

Turtles

The remote islands make perfect undisturbed nursery sites for nests of green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) turtles. The populations of both species in Chagos are of global significance given the "Critically Endangered" status of hawksbills and the "Endangered" status of green turtles on the IUCN Red List. Chagos turtles were heavily exploited during the previous two centuries, but they and their habitats are now well protected by the administration of the British Indian Ocean Territory and are recovering well.

Crabs

The coconut crab (Birgus latro) is the world’s largest terrestrial arthropod, reaching over one metre in leg span and 3.5-4 kilos in weight. As a juvenile it behaves like a hermit crab and uses empty coconut shells as protection but as an adult this giant crab climbs trees and can crack through a coconut with its massive claws. Despite its wide global distribution, it is rare in most of the areas it is found. It is primarily threatened by over-collection for food, but also as ornaments for sale to tourists and as bait for fish traps. Demand for coconut crabs as souvenirs is strong, and other threats include habitat destruction and predation from introduced species such as rats. The coconut crabs on Chagos constitute one of the most undisturbed populations in the world. An important part of their biology is the long distances their young can travel as larvae. This means the Chagos coconut crabs are a vital source for replenishing other over-exploited populations in the Indian Ocean region.

Insects

A total of 113 species of insect have been recorded from the Chagos Islands.

Plants

The Chagos Islands have been colonised by plants since there was sufficient soil to support them – probably less than 4,000 years. Seeds and spores arrived on the emerging islands by wind and sea, or from passing sea birds. The native flora of the Chagos Islands is thought to comprise forty-one species of flowering plants and four ferns as well as a wide variety of mosses, liverworts, fungi and cyanobacteria.

Today, the status of the Chagos Islands’ native flora depends very much on past exploitation of particular islands. About 280 species of flowering plants and ferns have now been recorded on the islands, but this increase reflects the introduction of non-native plants by humans, either accidentally or deliberately. Because some of these non-native species have become invasive and pose a threat to the native ecosystems, plans are being developed to control them. On some islands, native forests were felled to plant coconut palms for the production of copra oil. Other islands remain unspoiled and support a wide range of habitats, including unique Pisonia forests and large clumps of the gigantic fish poison tree (Barringtonia asiatica). Unspoiled islands provide us with the biological information that we need in order to re-establish the native plant communities on heavily altered islands. These efforts will ultimately help to improve the biodiversity of the Chagos Islands.