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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.


Map of British Indian Ocean Territory

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.


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 coconut crab Birgus latro.

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’.

Invasive species and biosecurity

A total of 262 non-native species (marine and terrestrial) was recorded for BIOT by the RSPB Stocktake (Churchyard et al 2014).


Total species

Non-native species










Problems with invasive non-native species

Over half of the islands in the archipelago have been degraded because of invasive species. Most of the islands in the group are rarely visited but there are black rats (Rattus rattus) on at least 30 of the islands and feral cats and donkeys are present on Diego Garcia.

House geckoes (the common house gecko Hemidactylus frenatus and Mourning gecko Lepidodactylus lugubrus) have been present on Diego Garcia for many years, probably arriving in the first ships. The agamid lizard Calotes versicolor was first seen in 2001 and is spreading over the island.

Indications are that seabird numbers are significantly reduced on rat infested islands. Marine Toads (Rhinella marina) are also present on Diego Garcia and are regarded as an introduced species of conservation concern. Coconut Palms have dominated or replaced native vegetation on a number of islands.

Read stories and information on the introduced fauna of Diego Garcia (external link)


Introduced agamid lizard Calotes versicolor.

Priority invasive non-native species and actions

There is an ongoing programme of cat eradication on Diego Garcia.

A rat eradication project on Eagle Islands in 2006 failed. Black rats were successfully eradicated from Ile Vache Marine in 2014, with the island declared rat-free in 2017. 


In 2017 a biosecurity gap analysis (PDF) was completed. BIOT does not have dedicated biosecurity officers. There is a customs team who inspect luggage and personnel for contraband items (drugs etc), who receive biosecurity training on checking for plant and animal material in both containers and in personal luggage. The customs team look at all containers coming onto the island for infestations. If they see any evidence of this, they seal the containers and get pest control in to treat the affected container. Imported materials (large granite rocks) for coastal defence are inspected for the presence of soils etc. These are stored on a concreted area for a period of c.28 days before being utilized. Public awareness is good, with regular monthly training covering the introduction of invasive species, pests, and twice a year training on Brown Tree Snake and other invasive species awareness. An on island review of procedures during all stages of container arrival and departure is being carried out during the beginning of 2020 to address gaps in knowledge. 

In 2018 a pathway analysis was completed. Pathways of introduction are relatively few. There is a military airport and harbour on the main island, Diego Garcia. There are no commercial flights and permits are only issued to yachts in safe passage through the archipelago. Between 2007 and 2017 an annual average of 50 yachts were issued licenses to visit BIOT, varying from 16 to 105.

Cargo arrives by both ship and plane. Within the region, cargo is flown in from Bahrain and Singapore and by cargo ship from Singapore and Guam. Military passenger planes arrive from Bahrain and Singapore, each approximately two to three times a week. Other military vessels (including submarines) can come from anywhere and berth in Diego Garcia, while other military flights can come from Okinawa (Japan), Australia and other points of origin. These are generally on short stop-overs. A patrol vessel travels between Diego Garcia and the outer islands, which may take shelter in the lagoon of outer islands. Smaller rib boats are used for landing and passengers swim ashore. Military ships also move within the archipelago, berthing alongside the wharf in Diego Garcia and anchored off shore in the outer islands.

Horizon scanning was carried out in 2019, identifying a total of 27 new invasive species of concern which have the potential to arrive within the next 5 – 10 years; read a report on the horizon scanning (PDF).  


Following the successful eradication of black rats from Ile Vache Marine, there are plans to eradicate rats from further islands in the archipelago.

Drawing on commitments made under the Environment Charter, Overseas Territories Environment Programme(OTEP) nine OTEP funded projects (external link) were delivered in total between 2004 and 2012.

BIOT has received three Darwin Initiative funded projects, listed below. For more information read the project reports (external link):

  • Ile Vache marine restoration project (DPLIUS011)
  • Creating a Terrestrial Action Plan for the Chagos Archipelago (DPLUS041)
  • Reducing the impacts of plastic on the BIOT natural environment (DPLUS090)

The project ‘Tackling invasive non-native species in the UK Overseas Territories’ was implemented between 2016 and 2020 to strengthen biosecurity in the OTs, funded by the UK government Foreign and Commonwealth Office’s Conflict, Stability and Security Fund (CSSF). BIOT benefitted from a number of activities to strengthen its biosecurity, including: pathway analysis, horizon scanning, pathway action planning, legislative support for reviewing the biosecurity legislation, access to online learning, and technical support. Read further details.

The Blue Belt Programme seeks to enhance long-term sustainable marine protection strategies for the UKOTs, and provides £20 million of funding over 4 years (2016 to 2020), also funded by CSSF and delivered by the Centre for Environment, Fisheries and Aquaculture Science (Cefas). The Programme assisted the BIOT Administration on a number of activities, including the development of a Conservation Management Plan, and a marine survey. Read project details and reports (external link).

Useful information

Information for visitors

Before travelling please ensure all items are clean, free from organic materials such as mud, faeces, seeds and invertebrates:

  • Check your footwear, outdoor clothing and day packs to make sure they are clean and free from weed seeds, mud, invertebrates and plant material. If possible we recommend that you travel with new outdoor clothing and equipment. Shake out or vacuum all the compartments and pockets before you pack.
  • If you have been camping, check that your tent and other equipment is clean, dry and free of dirt and invertebrates such as ants and spiders. Shake it out before you pack it up for travel and ensure no soil remains on tent pegs.
  • If you have been hiking, visiting a wilderness area, farm or zoo, make sure your footwear and clothes are clean and free from seeds, mud and faeces. Check boot soles for mud between the treads, Velcro fastenings for seeds and plant material, and shake out or vacuum pockets to remove any dirt and plant material.
  • If you are carrying golf, fishing or other sports and outdoor equipment with you, make sure they are clean, dry and free from dirt and any live creatures.

Advice for vessels in transit (external link) 


BIOT Administration
Overseas Territories Directorate
Foreign and Commonwealth Office
King Charles Street

Fax: 020 7008 1589

Email: BIOTAdmin@fco.gov.uk


Barnsley, S., Cary, E., Pienkowski, M. and Wensink, C.(Eds) (2016). Review of performance by 2016 of UK Overseas Territories and Crown Dependencies in implementing the 2001 Environment Charters or their equivalents and moving towards the Aichi Targets and Sustainable Development Targets. UK Overseas Territories Conservation Forum, April 2016.

Carr, P. (2011). A guide to the birds of the British Indian Ocean Territory. Pisces Publications for the RSPB.

Churchyard, T., Eaton, M., Hall, J., Millett, J., Farr, A., Cuthbert, R. and Stringer, C. (2014). The UK’s wildlife overseas: a stocktake of nature in our Overseas Territories. Sandy, UK: RSPB.

Edgar, P. (2010). The Amphibians and reptiles of the UK Overseas Territories, Crown Dependencies and Sovereign Base Areas: Species Inventory and Overview of Conservation and Research Priorities. Bournemouth, UK: Amphibian and Reptile Conservation.

Harper, G.A., Carr, P. and Pitman, H. (2019). Eradicating black rats from the Chagos – working towards the whole archipelago. In: C.R. Veitch, M.N. Clout, A.R. Martin, J.C. Russell and C.J. West (eds.) (2019). Island invasives: scaling up to meet the challenge, pp. 26–30. Occasional Paper SSC no. 62. Gland, Switzerland: IUCN.

Proctor, D. and Fleming, L. V., eds. (1999). Biodiversity: the UK Overseas Territories. Peterborough, UK: JNCC.

Sanders, S. M., ed. (2006). Important Bird Areas in the United Kingdom Overseas Territories. Sandy, UK: RSPB.

Varnham, K. (2006). Non-native species in UK Overseas Territories: a review. Peterborough, UK: JNCC Report No. 372.