Pitcairn is a rugged volcanic island, 5 km2in area, that reaches a maximum height of 347 m. Much of the coastline is either cliff or extremely steep slope. There is no fringing reef.Lying towards the south of the tradewind belt, the island enjoys a mild climate with easterly winds predominating, markedly so in the austral summer. The mean annual temperature (measured at a weather station at 264 m) is 21.2oC, with about a ten degree difference between the warmest and coldest months. The mean annual rainfall is 1716 mm, with no marked seasonal variation.Pitcairn is the only inhabited island in the wider Pitcairn group. It has been inhabited almost continuously since the Bounty mutineers and their Polynesian consorts settled in 1790. There was an earlier period of Polynesian settlement, but this had ended before the arrival of the mutineers and there is little knowledge of its effect on the bird population. The last half century has seen a steady dwindling of the human population which currently (2003) numbers about 50.Both tropical and temperate fruits and vegetables thrive in the fertile volcanic soil, and extensive areas are given over to their cultivation, mostly for local consumption. Other areas of the island are dominated by introduced plants, notably Lantana camara and rose-apple Syzygium jambos, both of which invade other communities and thereby threaten native plants. Guava Psidium cattleianum is also widespread. There are also conspicuous areas where all topsoil has been lost, and heavy erosion is evident. As a result of these several processes - horticulture, alien species and erosion - native vegetation is now restricted to small pockets which are principally situated close to the island's summit ridge or in steep valleys on the south side of Pitcairn.
Pitcairn Island qualifies as an IBA because it is the only nesting locality of the globally vulnerable Pitcairn Reed-warbler. Although now separated as a full species, the warbler has in the past been considered conspecific with the Henderson Reed-warbler and the Rimatara Reed-warbler. Pitcairn's warbler has not been subject to any detailed study, but it appears to be distributed throughout the island in all habitats vegetated with shrubs or trees. If its density is similar to that of the Henderson Reed-warbler, then its population may be around 1,500. There are no other breeding landbird species while, because of the presence of feral cats and Pacific rats, seabirds breed only on inaccessible cliffs in small numbers.
Non-bird biodiversity: The flora of Pitcairn includes 80 species of native vascular plants, of which ten, two ferns and eight angiosperms, are endemic. Fifty one of the native vascular plants are threatened. Particular concern attaches to the endemic Coprosma benefica, known from only 11 individuals, and the endemic fern Angiopteris chauliodonta, restricted to small and fragmented populations. Other species (e.g. Cyclophyllum barbatum, Psydrax odoratum) are becoming rare as they are utilised by the Islanders. Their populations could be enhanced by nursery propagation (see conservation issues). Several species are poorly dispersed on Pitcairn (e.g. Coprosma, Psydrax, Xylosma suaveolens) due to the lack of a frugivorous bird to disseminate fruit.Eight of Pitcairn's 26 species of extant land snail are endemic; three survive only in small remnants of native vegetation around one hectare in extent. If rose-apple or Lantana were to invade these remnants and create an understory inimical to these taxa, they would probably become extinct.
Site-account by M. de L. Brooke.
BirdLife International (2022) Important Bird Areas factsheet: Pitcairn Island. Downloaded from http://www.birdlife.org on 01/12/2022.