The islands of the Tristan group-Tristan da Cunha (96 km²), Inaccessible (13 km²) and Nightingale (4 km²)-are of volcanic origin and lie within 40 km of each other in the mid South Atlantic Ocean, c.2,800 km from South Africa and 3,200 km from South America. The group is a dependency of St Helena (some 2,000 km to the north, Secondary Area s038), which is itself a UK dependent territory. The island of Gough, about 350 km to the south-east, is also politically part of the same group, but is treated separately here (as EBA 080). Tristan is the largest and highest of the islands in the EBA, and is the only one to be inhabited.

On Tristan, the vegetation consists of tussock grassland on the coast, fern-bush and island tree Phylica arborea thickets at low elevations, wet heath above 750 m, moor and 'feldmark' vegetation (an assemblage of dwarf, cushion-forming and crevice plants) on higher slopes, and alpine tundra above 1,500 m. On Inaccesible, the vegetation includes tussock grassland, fern-bush and freshwater bogs, while on Nightingale the predominant vegetation is dense tussock grassland (see Wace and Holdgate 1976, Clark and Dingwall 1985).

This tiny EBA is exceptional in having three endemic genera-Atlantisia, Nesocichla and Nesospiza. The Nesospiza buntings are of particular interest because, like the famous Darwin's finches from the Galápagos Islands (see EBA 031), they have undergone remarkable speciation, with the two species (N. acunhae and N. wilkinsi) differing markedly in size and co-occurring without interbreeding on Nightingale. On Inaccessible, where they also co-occur, there are two altitudinally segregated colour morphs of N. acunhae, as well as a hybrid complex involving acunhae and wilkinsi (Ryan et al. 1994). Distinct subspecies are recognized of the thrush N. eremita from each of the three main islands.

All of the EBA's restricted-range species are found in a wide variety of habitats, but are more abundant in some than in others (e.g. Atlantisia rogersi is most common in coastal tussock grassland away from the cliffs). All the extant species occur on Inaccessible, and A. rogersi is confined to it. On Tristan there is a small population of Gough Moorhen Gallinula comeri introduced from Gough (see EBA 080).

As well as its restricted-range landbirds, there are several seabird subspecies and species which are largely confined to these islands and to Gough when breeding, including Tristan (Wandering) Albatross Diomedea exulans dabbenena (Inaccessible and Gough; treated as a good species by Robertson and Nunn in press), White-chinned Petrel Procellaria aequinoctialis conspicillata (Inaccessible only; probably a good and hence endemic species: P. Ryan in litt. 1993), Great Shearwater Puffinus gravis (mainly Tristan-Gough group), and Atlantic Petrel Pterodroma incerta (Tristan-Gough group only).

On Tristan, most of the vegetation has been considerably modified as a result of grazing by livestock, and the coastal tussock grassland has been largely destroyed. This loss of habitat is likely to have contributed to the extinction of Gallinula nesiotis and Nesospiza acunhae, although introduced predators (cats and rats) must also have played an important part (Fraser and Briggs 1992, P. Ryan in litt. 1993). Nesocichla eremita is the only native landbird surviving on Tristan, although the population has decreased markedly since the island has been colonized by man and his commensals (Fraser . 1994).

On Inaccessible and Nightingale, habitat destruction (through the deliberate introduction of livestock) is a potential problem, but by far the greatest threat would come from the accidental introduction of alien species, especially predatory mammals (P. Ryan in litt. 1993). Thus, despite being numerically strong, all the restricted-range birds are considered threatened or Near Threatened. Atlantisia rogersi is arguably the most vulnerable of the threatened species, being flightless (indeed the world's smallest flightless bird), although it lives at high density (probably at carrying capacity) and numbers an estimated 8,400 birds (Fraser et al. 1992, M. W. Fraser in litt. 1993). Gallinula comeri (see 'Restricted-range species', above), also considered threatened (Vulnerable), numbers c.250 pairs on Tristan.

In addition to their importance for endemic landbirds, the Tristan islands are internationally important for their colonies of some 20 species of seabirds (Richardson 1984, Williams 1984, Fraser et al. 1988). Threatened and Near Threatened species include Diomedea exulans (Vulnerable, at its northernmost breeding locality on Inaccessible, but only a few pairs; see 'Restricted-range species'), Pterodroma incerta (Vulnerable, some hundreds of pairs; see 'Restricted-range species') and Sooty Albatross Phoebetria fusca (Near Threatened).

Inaccessible was declared a nature reserve in 1994, and although Tristan islanders still retain the right to collect driftwood and guano from this uninhabited island, other access is restricted and all living resources are protected.

Stattersfield, A. J., Crosby, M. J., Long, A. J. and Wege, D. C. (1998) Endemic Bird Areas of the World. Priorities for biodiversity conservation. BirdLife Conservation Series 7. Cambridge, UK: BirdLife International.

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Tristan Islands. Downloaded from https://datazone.birdlife.org/eba/factsheet/86 on 27/12/2024.