No species of plant, bird, amphibian, reptile, mollusc, crustacean or mammal is
endemic on Diego Garcia or the surrounding waters, though some species of fish and aquatic invertebrates are endemic. All plants, wildlife, and aquatic species are protected to some degree. Much of the lagoon and other waters of Diego Garcia are protected wetlands, following an application by the UK in 2004 to obtain
Ramsar site wetlands conservation status, and large parts of the island are nature preserves.
Geography Diego Garcia is the largest land mass in the Chagos Archipelago (which includes
Peros Banhos, the
Salomon Islands, the
Three Brothers, the
Egmont Islands, and the
Great Chagos Bank), being an atoll occupying approximately , of which is dry land. The continuous portion of the atoll rim stretches from one end to the other, enclosing a lagoon long and up to wide, with a pass opening at the north. Three small islands are located in the pass. The total area of the atoll is about . The lagoon area is roughly with depths ranging down to about . The total land area (excluding peripheral reefs) is around . The coral reef surrounding the seaward side of the atoll is generally broad, flat, and shallow around below mean sea level in most locations and varying from in width. This fringing seaward reef shelf comprises an area around . At the outer edge of the reef shelf, the bottom slopes very steeply into deep water, at some locations dropping to more than within of the shore. Diego Garcia is frequently subject to earthquakes caused by
tectonic plate movement along the
Carlsberg Ridge located just to the west of the island. One was recorded in 1812; one measuring 7.6 on the Richter Scale hit on 30 November 1983, at 11:46 pm local time and lasted 72 seconds, resulting in minor damage including wave damage to a stretch of the southern end of the island, and another on 2 December 2002, an earthquake measuring 4.6 on the Richter scale struck the island at 12:21 am.
Oceanography Diego Garcia lies within the influence of the
South Equatorial Current year-round. The surface currents of the Indian Ocean also have a monsoonal regimen associated with the Asian Monsoonal wind regimen. Sea surface temperatures are in the range of year-round.
Fresh water supply Diego Garcia is the above-water rim of a coral atoll composed of
Holocene coral rubble and sand to the depth of about , overlaying
Pleistocene limestone deposited at the then-sea level on top of a
seamount rising about from the floor of the Indian Ocean. The Holocene sediments are porous and completely saturated with sea water. Any rain falling on the above-water rim quickly percolates through the surface sand and encounters the salt water underneath. Diego Garcia is of sufficient width to minimise tidal fluctuations in the
aquifer, and the rainfall (in excess of per year on average) is sufficient in amount and periodicity for the fresh water to form a series of convex, freshwater,
Ghyben-Herzberg lenses floating on the heavier salt water in the saturated sediments. The horizontal structure of each lens is influenced by variations in the type and porosity of the subsurface deposits, which on Diego Garcia are minor. At depth, the lens is globular; near the surface, it generally conforms to the shape of the island. When a Ghyben-Herzberg lens is fully formed, its floating nature will push a
freshwater head above
mean sea level, and if the island is wide enough, the depth of the lens below mean sea level will be 40 times the height of the water table above sea level. On Diego Garcia, this equates to a maximum depth of . However, the actual size and depth of each lens is dependent on the width and shape of the island at that point, the permeability of the aquifer, and the equilibrium between recharging rainfall and losses to evaporation to the atmosphere, transpiration by plants, tidal advection, and human use. In the plantation period, shallow wells, supplemented by rainwater collected in
cisterns, provided sufficient water for the pastoral lifestyle of the small population. On Diego Garcia today, the military base uses over 100 shallow "horizontal" wells to produce over per day from the "Cantonment" lens on the northwest arm of the island—sufficient water for western-style usage for a population of 3,500. This lens holds an estimated of fresh water and has an average daily recharge from rainfall over , of which 40% remains in the lens and 60% is lost through
evapotranspiration. Extracting fresh water from a lens for human consumption requires careful calculation of the sustainable yield of the lens by season because each lens is susceptible to corruption by
saltwater intrusion caused by overuse or drought. In addition, overwash by tsunamis and tropical storms has corrupted lenses in the Maldives and several Pacific islands. Vertical wells can cause salt upcoming into the lens, and overextraction will reduce freshwater pressure resulting in lateral intrusion by seawater. Because the porosity of the surface soil results in virtually zero runoff, lenses are easily polluted by fecal waste, burials, and chemical spills. Corruption of a lens can take years to "flush out" and reform, depending on the ratio of recharge to losses. Two are of significance to island wildlife and to recharge their respective freshwater lenses. One of these is centred on the northwest point of the atoll; another is found near the Point Marianne Cemetery on the southeast end of the airfield. Other, smaller freshwater wetlands are found along the east side of the runway, and in the vicinity of the receiver antenna field on the northwest arm of the atoll. Also, several man-made freshwater ponds resulted from excavations made during construction of the airfield and road on the western half of the atoll rim. These fill from rainfall and from extending into the Ghyben-Herzberg lenses found on this island.
Climate Diego Garcia has an equatorial
tropical rainforest climate (
Köppen Af). The surrounding sea surface temperature is the primary climatic control, and temperatures are generally uniform throughout the year, with an average maximum of by day during March and April, and from July to September. Diurnal variation is roughly , falling to the low by night. Diego Garcia is at minimum risk from
tropical cyclones due to its proximity to the
equator where the
coriolis parameter required to organise circulation of the
upper atmosphere is minimal. Low-intensity storms have hit the island, including one in 1901, which blew over 1,500 coconut trees; one on 16 September 1944, which caused the wreck of a Royal Air Force PBY Catalina; one in September 1990 which demolished the tent city then being constructed for United States Air Force bomber crews during Operation Desert Storm; and one on 22 July 2007, when winds exceeded and over of rain fell in 24 hours. A biological survey conducted in early 2005 indicated erosional effects of the tsunami wave on Diego Garcia and other islands of the Chagos Archipelago. One stretch of shoreline was found to have been breached by the tsunami wave, representing about 10% of the eastern arm. A biological survey by the Chagos Conservation Trust reported that the resulting inundation additionally washed away shoreline shrubs and small to medium-sized coconut palms. }} }}
Vegetation The first
botanical observations of the island were made by Hume in 1883, when the coconut plantations had been in operation for a full century. Subsequent studies and collections during the plantation era were made in 1885, 1905, 1939, and 1967. Thus, very little of the nature of the precontact vegetation is known. The 1967 survey, published by the
Smithsonian is used as the most authoritative baseline for more recent research. These studies indicate the vegetation of the island may be changing rapidly. For example, J. M. W. Topp collected data annually between 1993 and 2003 and found that on the average three new plant species arrived each year, mainly on Diego Garcia. His research added fully a third more species to Stoddart. Topp and Martin Hamilton of
Kew Gardens compiled the most recent checklist of vegetation in 2009. In 1967, Stoddart described the land area of Diego Garcia as having a
littoral hedge of
Scaevola taccada, while inland,
Cocos nucifera (coconut) was the most dominant tree, covering most of the island. The substory was either managed and park-like, with understory less than 0.5 m in height, or consisted of what he called "Cocos Bon-Dieu" – an intermediate story of juvenile trees and a luxuriant ground layer of self-sown seedlings – causing those areas to be relatively impenetrable. Also, areas of remnant tropical hardwood forest are at the sites of the plantation-era villages, as well as
Casuarina equisetifolia (iron wood pines) woodlands. None of these was
endemic, and another survey in 2005 identified just 36 species as "native", meaning arriving without the assistance of humans, and found elsewhere in the world. No
terrestrial plant species are of any conservation-related concern at present. Of the 36 native vascular plants on Diego Garcia, 12 are trees, five are
shrubs, seven are
dicotyledon herbs, three are grasses, four are
vines, and five are
ferns. The 12 tree species are:
Barringtonia asiatica (fish-poison tree),
Calophyllum inophyllum (Alexandrian laurel),
Cocos nucifera,
Cordia subcordata,
Guettarda speciosa,
Intsia bijuga,
Hernandia sonora,
Morinda citrifolia,
Neisosperma oppositifolium,
Pisonia grandis,
Terminalia catappa, and
Heliotropium foertherianum. Another three tree species are common, and may be native, but they may also have been introduced by humans:
Casuarina equisetifolia,
Hibiscus tiliaceus, and
Pipturus argenteus. The five native shrubs are:
Caesalpinia bonduc,
Pemphis acidula,
Premna serratifolia,
Scaevola taccada (often mispronounced "Scaveola"), and
Suriana maritima. Also, 134 species of plants are classified as "weedy" or "naturalised alien species", being those unintentionally introduced by man, or intentionally introduced as ornamentals or crop plants which have now "gone native", including 32 new species recorded since 1995, indicating a very rapid rate of introduction. The remainder of the species list consists of cultivated food or ornamental species, grown in restricted environments such as a planter's pot. In 2004, 10
plant communities were recognised on the atoll rim: • Mixed native forest, with no dominant canopy species • Marshes are divided into three different types: cattail (
Typha domingensis), wetland, and mixed species. Cattail marshes contained almost entirely cattails. These areas are often man-made reservoirs or drainages that have been almost entirely monotypic. Wetlands were based upon vegetation that occurred in the area with fresh water. Mixed-species marshes were highly variable and usually had no standing water.
Wildlife s are protected on Diego Garcia. All the terrestrial and aquatic fauna of Diego Garcia are protected, with the exception of certain game fish, rats, and cats; hefty fines are levied against violators.
Crustaceans The island is a haven for several types of
crustacean; "warrior crabs" (
Cardisoma carnifex) overrun the jungle at night. The extremely large
coconut crab or robber crab (
Birgus latro) is found here in large numbers. Because of the protections provided the species on this atoll, and the isolation of the east rim of the atoll, the species is recorded in greater densities there than anywhere else in its range (339 crabs/ha).
Mammals No
mammal species are native on Diego Garcia, with no record of bats. Other than rats (
Rattus rattus), all "wild" mammal species are feral descendants of domesticated species. During the plantation era, Diego Garcia was home to large herds of Sicilian donkeys (
Equus asinus), dozens of horses (
Equus caballus), hundreds of dogs (
Canis familiaris), and house cats (
Felis catus). In 1971, the BIOT Commissioner ordered the extermination of
feral dogs following the departure of the last plantation workers, and the program continued through 1975, when the last feral dog was observed and shot. Donkeys, which numbered over 400 in 1972, were down to just 20 individuals in 2005. The last horse was observed in 1995, The 680-hectare
Barton Point Nature Reserve was identified as an
Important Bird Area for its large breeding colony of red-footed boobies.
Introduced birds The island hosts introduced bird species from many regions, including cattle egrets (
Bubulcus ibis), Indian barred ground dove, also called the zebra dove (
Geopelia striata), turtle dove (
Nesoenas picturata), Indian mynah (
Acridotheres tristis), Madagascar fody (
Foudia madagascariensis), and chickens (
Gallus gallus).
Terrestrial reptiles and freshwater amphibians Currently, three lizards and one toad are known to inhabit Diego Garcia, and possibly one snake. All are believed to have been introduced by human activity. The house gecko (
Hemidactylus frenatus), the mourning gecko (
Lepidodactylus lugubris), the garden lizard (an agamid) (
Calotes versicolor), and the cane toad (
Bufo marinus). A viable population of a type of blind snake from the family Typhlopidae may be present, probably the brahminy blind snake (
Ramphotyphlops braminus). This snake feeds on the larvae, eggs, and pupae of
ants and
termites, and is about the size of a large earthworm.
Sea turtles Diego Garcia provides suitable foraging and nesting habitat for both the hawksbill turtle (
Eretmochelys imbricata) and the green turtle (
Chelonia mydas). Juvenile hawksbills are quite common in the lagoon and at Barachois Sylvane (also known as Turtle Cove) in the southern part of the lagoon. Adult hawksbills and greens are common in the surrounding seas and nest regularly on the ocean-side beaches of the atoll. Hawksbills have been observed nesting during June and July, and from November to March. Greens have been observed nesting in every month; the average female lays three clutches per season, each having an average clutch size of 113 eggs.
Diurnal nesting is common in both species. An estimated 300–700 hawksbills and 400–800 greens nest in the Chagos.
Endangered species Four reptiles and six cetaceans are endangered and may or may not be found on or around Diego Garcia: Hawksbill turtle (
Eretmocheyls imbricata) – known; leatherback turtle (
Dermochelys coriacea) – possible; green turtle (
Chelonia mydas) – known; olive ridley turtle (
Lepidochelys oliveacea) – possible; sperm whale (
Physeter macrocephalus) – possible; sei whale (
Balaeonoptera borealis) – possible; finback whale (
Balaeonoptera physalus) – possible; Bryde's whale (
Balaeonoptera edeni) – possible; blue whale (
Balaeonoptera musculus) – possible; humpback whale (
Megaptera novaeangliae) – possible; southern right whale (
Eubalaena australis) – possible. ==United Kingdom military activities==