The site comprises the whole of Tristan Island, as described in the ‘General introduction’.
See Box and Table 2 for key species. Although as many as 56 bird taxa have been recorded, there are now only 13 known species of breeding seabirds and two species of resident landbirds. The seabirds include Eudyptes chrysocome moseleyi, Diomedea chlororhynchos, Phoebetria fusca, Pterodroma macroptera, P. mollis, P. incerta, Pachyptila vittata, Procellaria cinerea, Puffinus gravis, P. griseus, Catharacta antarctica, Sterna vittata and Anous stolidus. Tristan is the only known breeding site within the group for Pterodroma incerta and of Puffinus griseus, while numbers of Diomedea chlororhynchos are the highest for any island in the Dependency. Pterodroma brevirostris and Puffinus assimilis may also breed. Pterodroma macroptera, P. incerta and Procellaria cinerea have not been proven to breed elsewhere in the Tristan group, possibly because they are winter breeders. There are currently an estimated 40,000 breeding pairs of seabirds, most known from the south-eastern quadrant, which has suffered least from human disturbance. The estimated breeding density is only 500 pairs per km². The terrestrial species include the restricted-range Gallinula comeri, introduced from Gough Island (SH008) in 1956 (3,000 pairs, 1993 estimate), and Nesocichla e. eremita, confined to this island and numbering 40–60 pairs in 1974.There are more records of non-breeding visitors and vagrants on Tristan than from the other islands of the group. This is probably due to the island’s larger size and permanent human presence. Also, due to persecution, there are fewer Catharacta to prey upon them as they arrive. Most records are from the settlement area. Seabirds include Diomedea exulans, D. melanophris, Macronectes giganteus, M. halli, Fulmarus glacialoides, Daption capense, Procellaria a. aequinoctialis, P. a. conspicillata, Puffinus gravis, Oceanites oceanicus, Pelagodroma marina and Larus dominicanus. Shorebirds and landbirds include Casmerodius albus, Egretta thula, Bubulcus ibis, Porphyrula martinica, Calidris fuscicollis and Hirundo rustica. The strong westerlies create favourable conditions for a crossing from South America, and this would explain the presence of gallinules and other non-breeding landbirds that have reached Tristan from that continent. The site requires much further field study, especially the southern side.
Non-bird biodiversity: There are no endemic mammals. The only breeding native mammal is the fur seal Arctocephalus tropicalis, of which there is a small colony at Cave Point on the southern side of the island. Elephant seals Mirounga leonina haul out regularly on Tristan beaches and breed sporadically. Eubalaena glacialis (EN) occurs in offshore waters between September and November, but in very low numbers. Of 62 native terrestrial invertebrates recorded, only four are endemic.
Pressure/threats to key biodiversity
Protection of the birds of the Tristan group is provided for by the Tristan da Cunha Conservation Ordinance of 1976 (amended 1984 and 1986). Tristan itself, as the only inhabited island, has incurred the greatest effects of human activity. These include overgrazing by sheep, tree-felling, fire and, in particular, predation by introduced mammals.Before the arrival of man, the island may have supported 19 seabird species and four landbird species. Diomedea exulans dabbenena became extinct as a breeder between 1880 and 1907 due to excessive culling, and Macronectes (probably M. giganteus) became similarly extinct around 1870, due to disturbance and a decrease in its food-supply, and is now only a non-breeding visitor to the Tristan group. Catharacta antarctica may soon also disappear as a breeding species due to persecution. Nesospiza acunhae became extinct on Tristan between 1852 and 1873, probably due to the destruction of the low-lying tussock. Gallinula nesiotis is thought to have become extinct between 1873 and 1906.The south-eastern sector, which remains the largest refuge for Nesocichla and seabirds and is rarely visited, should remain a wilderness area, a yardstick against which to assess the human impact on the avifauna elsewhere on Tristan.On Tristan, Nesocichla eremita has decreased markedly, due to overgrazing, introductions of alien plants, predation by cats and nest-predation by rats. The current population is restricted largely to ‘gulches’ on the Base plateau. There are no accurate data on population trends, but a decrease is suggested by reports that the species no longer inhabits ‘gulches’ near the Hillpiece (Settlement Plain), nor visits the settlement itself, although in the last 25 years birds have been seen in Phylica, above the new volcano. The genetic identity of the population is threatened by introgression from birds (other subspecies) brought over from Inaccessible and Nightingale Islands.
BirdLife International (2019) Important Bird Areas factsheet: Tristan Island. Downloaded from
http://www.birdlife.org on 16/06/2019.