British Indian Ocean Territory

About the territory

The British Indian Ocean Territory (BIOT) consists of the 55 islands of the Chagos Archipelago, 86 sea mounts and 243 deep sea-knolls, covering about 640,000 km2of ocean. The land area of the islands is around 60 km2and they lie between 4° and 8° South and 70° and 75° East. There are also 4,000 km2of near-surface coral reefs. The nearest land is the Maldive Islands 500 km to the North. There are five atolls: The Great Chagos Bank, Peros Banhos, Salomon, Egmont and Diego Garcia, and ten reefs and submerged shoals. Annual rainfall is between 250 and 400cm. Average temperatures range between 24°C and 31°C.

The Islands were uninhabited when first discovered in the 16th century and the French assumed sovereignty over them in the late 18th century. At the Treaty of Paris 1814 the French ceded Mauritius and its dependencies, which included the Chagos Archipelago, to the United Kingdom. In 1965 the islands were detached from the colony of Mauritius in order to form the British Indian Ocean Territory. The population was relocated to Mauritius and the Seychelles so that the islands could be set aside for UK and US defence purposes. Since then UK and US military personnel and civilian contract employees have been the only inhabitants. In 2018 these numbered approximately 3,000 people (external link), all on Diego Garcia.

Coconut crab climbing a tree

The coconut crab Birgus latro.


The Territory has the greatest marine biodiversity in the UK and its Overseas Territories, as well as some of the cleanest seas and healthiest reef systems in the world, protected by a no-take Marine Protected Area (MPA). The MPA was declared on 1 April 2010, and is one of the largest in the world.

BIOT is home to the world’s biggest arthropod, the coconut crab Birgus latro, which can reach up to one metre across, with densities on Diego Garcia amongst the highest globally. The outer islands and atolls are colonised by internationally important numbers of seabirds, with many thousands of pairs of sooty terns, brown boobies and red-footed boobies regularly breeding there.

The biodiversity of BIOT has been fairly extensively studied as a result of a number of scientific expeditions. There are very few endemics but they include one marine alga, one brain coral and a gastropod. According to the Kew database there are about 280 species of flowering plants and ferns. However, the original native flora is considered to comprise only 41 species of flowering plants and four species of ferns, the remainder having been introduced. There is also a wide variety of mosses, liverworts, fungi and cyanobacteria. There are no endemic plant species.

There are over 384 species of mollusc and the 1996 Expedition identified 95 insect species. Two endemic species of butterfly, (Jamonia villida) and (Hypolimnus bolina), and an endemic subspecies of the hawkmoth (Macroglossum corythus) were also recorded along with six species of damselflies and dragonflies.

Three species of fish are endemic: an anemone fish (Amphiprionchagosensis), a goby (Trimmatom offucius) and a worm fish (Paragunellichthys fehlmani). An undescribed razorfish may also be endemic.

There are no terrestrial amphibians or reptiles.

Policy and legislation

BIOT has an Environment Charter signed jointly with the UK Government. Guiding Principle 7 is to safeguard and restore native species, habitats and landscape features, and control or eradicate invasive species. Under the associated commitment 2, BIOT Administration will ensure the protection and restoration of key habitats, species and landscape features through legislation and appropriate management structures and mechanisms, including a protected area policy, and attempt the control and eradication of invasive species. Implementation progress was reviewed in 2007, 2010 and 2016, read the 2016 review report (external link).

A comprehensive BIOT Administration conservation priorities document drafted, which provides both policy and a strategy for biosecurity and invasive species management across the archipelago, incorporating the results of the horizon scanning and pathway action planning workshops held under the CSSF project in August 2018.Eleven conservation and environmental priorities, two of which concern invasive species:

  • Eradicating invasive rats which threaten native seabird populations, and impact the delicate balance of BIOT’s ecosystem;
  • Protecting BIOT from invasive flora and fauna.

There is no specific legislation regarding invasive species. The Prohibited Imports and Exports Order 2009, made under the Imports and Exports (Control) Ordinance 2009, includes the prohibition of importing into the Territory any fill material which contains plant or animal material not originating in the Territory. It also prohibits the exportation without written permission of any wildlife (including seashells, corals, eggs etc.) whether alive or dead. Under The Visitors and Visiting Vessels Ordinance 2006, Guidance for Visitors includes the following: Fauna and flora from outside the Chagos Archipelago must not be introduced into the Territory. In particular, pets are not to be landed.

A review of the existing legislation with regards biosecurity provisions was carried out in 2019 under the project ‘Tackling invasive non-native species in the UK Overseas Territories’.

Thin strip of image show tree trunk and bark