Pitcairn Island

Year of compilation: 2006

Site description
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.

Key biodiversity
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.

Pressure/threats to key biodiversity
For the obvious reason that it is the only inhabited island in the Pitcairn group, Pitcairn is the most ecologically disturbed. Nonetheless, it remains of significant conservation interest, notably from a botanical point of view. The preservation of the native flora will require a combination of measures, including ex-situ propagation, in-situ protection, and control of rose apple.To aid ex-situ propagation, a small nursery was started in 1998. The critically threatened Coprosma (see above) is a good candidate for propagation, from cuttings and seeds, followed by planting out. Propagation of Angiopteris from spores is likely to be difficult because the gametophyte is mycotrophic, but is worth attempting.

While the rose-apple has not yet invaded the southern flank of Pitcairn, this will probably happen in due time if not prevented. Ideally, the best pockets of native vegetation should be weeded periodically, for example in Faute Valley and Tautama. Funding for the labour involved is required. Consideration should also be given to protecting at least parts of these areas from goats by fencing. Other important pockets of native vegetation (e.g. Brown's Water, home to a high number of endemic and threatened species) should be protected from road encroachment.The large tracts of rose-apple growing over Pitcairn are of little wildlife value. In the long-term, it would be ideal if this introduced invasive species could be replaced by native and, especially, threatened species. To this end, seedlings grown in the nursery could be planted out in small areas where the rose-apple had been cleared. This general methodology will perhaps need refining in the light of experience, but is certainly to be preferred to wholescale clearance of rose-apple. Since little grows underneath rose-apple, such clearance would undoubtedly be accompanied by loss of topsoil and erosion. The ideas are now being tested in a project run by Trinity College, Dublin and funded by the Foreign and Commonwealth office, London.Since the Pitcairn Reed-warbler appears able to co-exist with Pacific rats Rattus exulans and with feral cats, and to live in a variety of altered and disturbed habitats, there appears no immediate cause for concern. This would change were other rat species to reach Pitcairn. In 1997 and 1998, two attempts to eradicate rats were made using hand distribution of poison baits. If successful, the eradication would have contributed to human welfare, allowed the recolonisation of Pitcairn by surface-nesting seabirds, and reduced the risk that Oeno (see that island's account) would be re-invaded by rats. While the attempts certainly reduced the rat population to a very low level, with immediate evident benefits to the Islanders' fruit and vegetable crops, neither attempt was entirely successful. Inevitably, it is impossible to be certain about the reason(s) for failure, but there is wide agreement that, in the event of a third attempt, the period of monitoring by dedicated personnel after the main bait distribution should be lengthened to several months. There is a population of free-ranging goats on Pitcairn. When numbers increase beyond about 100, the situation at the time of writing, their impact on the vegetation and erosion visibly escalate. Regular culling appears justified.

Protected areas
No formal protection to site. Local Government regulations extend general protection to wildlife.

Site-account by M. de L. Brooke.

Recommended citation
BirdLife International (2023) Important Bird Areas factsheet: Pitcairn Island. Downloaded from http://www.birdlife.org on 04/02/2023.