The flora of the Pitcairn Islands: a review

vascular plant flora of the Pitcairn Islands, south-central Pacific Ocean, is described based on extensive new collections made in 1991 and previously published records. Two vascular plants occur on Ducie Atoll; one (Pemphis acidula) is a new record. Sixty-three native vascular plants occur on Henderson, of which nine are endemic; Canavalia rosea, Operculina lurpethum, Psilotum nudum and Solanum americanum are new records for the island. Oeno Atoll has 16 native vascular plants; the single endemic (Bidens hendersonensis var. oenoensis) was not found in 1991 despite careful searches. Triumfetta procumbens was new for Oeno. Sixty-six native vascular plants have now been recorded from Pitcairn Island, there are two endemic ferns and seven endemic angiosperms in this number. A number of non-native taxa were new to Pitcairn. Some of the previously described taxa could not be found on Pitcairn, probably because they are very rare and only a small amount of time was spent collecting on Pitcairn. Many of the Pitcairn taxa are threatened by the spread of introduced species, especially Syzygium jambos.     

The Pitcairn Islands, South Pacific Ocean: plate tectonic and climatic contexts

The remote Pitcairn Group in the South Pacific Ocean comprises a volcanic island (Pitcairn Island), two low coral atolls (Oeno, Ducie) and a raised coralline island (Henderson Island). The geological history of these islands, on anomalously thin oceanic lithosphere, is related to the development of two subparallel island chains (Oeno-Henderson-Ducie; Pitcairn) associated with intra-Pacific plate 'hotspot' activity; the surface manifestation of this activity has been partly determined by structural lineations in the plate inherited from past plate history. The climate of the Pitcairn Islands is determined by the position of the subtropical high pressure system and the South Pacific Convergence Zone. Variations in the strength of this atmospheric circulation system, measured by changes in the Southern Oscillation index of pressure difference, provide a partial explanation of the long-term variability of mean annual rainfall at Pitcairn Island. Knowledge of past climates in the Pitcairn Group remains speculative. Maps of the Pitcairn Islands and a report of climate at Henderson Island (2/91-1/92) are included in the paper.     

The phytogeographical affinities of the Pitcairn Islands – a model for south‐eastern Polynesia?

Aim To identify how the Pitcairn group relates biogeographically to the south‐eastern Polynesian region and if, as a subset of the regions flora, it can then be used as a model for biogeographical analyses. Location The Pitcairn group (25°4′ S, 130°06′ W) comprises four islands: Pitcairn, a relatively young, high volcanic Island; Henderson, an uplifted atoll, the uplift caused by the eruption of Pitcairn; and two atolls, Ducie and Oeno. The remote location, young age and range of island types found in the Pitcairn Island group makes the group ideal for the study of island biogeography and evolution. Methods A detailed literature survey was carried out and several data sets were compiled. Dispersal method, propagule number and range data were collected for each of the 114 species that occurs in the Pitcairn group, and environmental data was also gathered for islands in Polynesia. Analyses were carried out using non‐metric multidimensional scaling and clustering techniques. Results The flora of the Pitcairn Islands is derived from the flora of other island groups in the south‐eastern Polynesian region, notably those of the Austral, Society and Cook Islands. Species with a Pacific‐wide distribution dominate the overall Pitcairn group flora. However, each of the islands show different patterns; Pitcairn is dominated by species with Pacific, Polynesian and endemic distributions, with anemochory as the dominant dispersal method (39.5%); Henderson is also dominated by species with Pacific, Polynesian and endemic distributions, but zoochory is the dominant dispersal method (59.4); Oeno and Ducie are dominated by Pantropic species with hydrochory as the most common dispersal method (52.9% and 100%, respectively). Main conclusions ? Habitat availability is the most significant factor determining the composition and size of the flora. ? South‐east Polynesia is a valid biogeographical unit, and should include the Cook, Austral, Society, Marquesas, Gambier, Tuamotu and Pitcairn Islands with Rapa, but should exclude Easter Island, Tonga and Samoa. ? Regionalization schemes should take island type into consideration. ? The Pitcairn Island group can serve as a useful model for Pacific biogeographical analyses.     

Systematic review of the land snails of the Pitcairn Islands

The land snails (and semi-terrestrial molluscs) of the four islands that comprise the Pitcairn group are reviewed and the indigenous species illustrated. The strictly terrestrial molluscan faunas from the two atolls (Oeno and Ducie) are poor, like many other atolls in the Pacific. Each supports less than six species with wide geographical ranges. In contrast, the terrestrial molluscan fauna from Henderson Island, an uplifted atoll, is more diverse with at least 16 species belonging to seven families. Over half these taxa appear to be endemic, at least at the level of sub-species. Two species of semi-terrestrial molluscs have also been found on Henderson. Analyses of archaeological deposits in caves near the North Beach have revealed that at least a further six species of land snail formerly occurred on Henderson. The volcanic island of Pitcairn, the only island in the group still inhabited, supports the greatest number of terrestrial molluscs. Twenty-six species of land snail (and one semi-terrestrial pulmonate) were found living there during the recent expedition and a further three taxa were recognized amongst museum material. At least seven of these species are thought to be recent adventives and a further three are likely to have been prehistoric introductions. One Henderson (Georissa hendersoni) and three Pitcairn endemics (Pacificella filica, Sinployea pitcaimensis and Diastole tenuistriata) are formally described as new species. Some of the Pitcairn endemics occur in very restricted areas (less than a hectare) and it is important that measures should be taken to prevent the spread of invasive plants, such as rose-apple, that would threaten their survival.     

Initial checklist of fishes from Henderson Island, Pitcairn Group

Notes are given on the presence and distribution of reef fishes at Henderson Island (24^o21'S, 128^o19'W) from two separate visits in 1971 and 1991. Limited collections were made in 1971, whilst 1991 records are based on confirmed sightings and in situ photographs. 183 species from 49 families are listed, with museum numbers and lengths being given for collected specimens. This is thought to represent 80–90% of species present. The family with the greatest representation is the Labridae with 21 species. Only 31% of species appear to be shared with neighbouring Ducie Atoll, 390 km to the east, from where 138 species have been recorded, though this low percentage may simply reflect the greater recording effort undertaken at Henderson. Levels of endemism in the Pitcairn Group as a whole appear low (<2%).     

The breeding biology of the gadfly petrels Pterodroma spp. of the Pitcairn Islands: characteristics, population sizes and controls

This paper reports the breeding biology and nesting seasons of the gadfly petrels which nest on the four islands of the Pitcairn group, Pitcairn, Henderson, Oeno and Ducie. The species currently breeding are Murphy's petrel Pterodroma ultima , Kermadec petrel P. neglecta , Herald petrel P. heraldica and Henderson petrel P. atrata. Of these, Murphy's petrel is the most numerous; an estimated 250000 pairs bred on Ducie, which is probably the major breeding station of the species. Novel basic breeding data for Murphy's petrel are presented. Incubation spells, averaging 19.3 days, are exceptionally long for a petrel. Phoenix petrel P. alba appears to have ceased to breed on the Pitcairn Islands since the 1922 surveys of the Whitney Expedition. Nesting success was low on Henderson Island during the study. For all four breeding species, less than 20% of eggs laid yielded fledglings. Failure occurred at the early chick stage and observations indicated that it was due to predation by Pacific rats Rattus exulans. Although rats are present on Ducie, predation was apparently less severe there. The situation on Oeno may be intermediate. I consider how the populations of Henderson are maintained in the face of this intense predation. The Murphy's petrel population may be sustained by immigration from Ducie while the Herald and Henderson petrel populations could be undergoing a long-term decline on Henderson. It is not clear how the Kermadec petrel population is maintained. The conservation implications of these findings are discussed.     

The composition and relationships of the marine molluscan fauna of the Pitcairn Islands

The recent expedition has doubled the number of marine molluscs known from the Pitcairn Islands. Over 80 taxa are recorded from Ducie and Pitcairn (both still poorly known), 240 from Oeno and 320 from Henderson. A total exceeding 400 taxa is now known from the group as a whole. Most of this increase results from a vastly improved knowledge of the small taxa that had been neglected in earlier surveys. Important faunal differences exist between the four islands in the group which are related to the different character of each island. These totals still underestimate the true diversities, but they provide data that are comparable in quality to those from adjacent regions. In this context the molluscan fauna from the Pitcairn Islands is seen to be impoverished when compared to those from islands further west. Most of the fauna is composed of widespread Indo-West Pacific species, but there are several taxa that have more restricted ranges centred on S.E. Polynesia. A few, including some undescribed taxa discovered on the expedition, appear to be endemic. Significantly, many of these are characterized by non-planktotrophic larval development. Comparison of the modern fauna from Henderson with the fossil fauna from the Pleistocene reef on its plateau reveals important differences. About 25% of the fossil molluscs are currently unknown from the group and about 5% appear to be undescribed. These high rates of turnover demonstrate that the faunas are temporally unstable.     

Marine Ostracoda from Pitcairn, Oeno and Henderson Islands

All of the samples yielded podocopid ostracods. Most of the faunas from Pitcairn and Oeno were poorly preserved and sparse; those from Henderson were much more abundant, more diverse and better preserved. Only 33 species are recorded from the joint fauna and this may reflect the limited number of environments sampled or the remote geographical situation of the islands. At least 20 species (and a new genus) are restricted to the islands. Other species are known from the Solomon Islands, Australia and Indonesia. Of a considerable number of Indo-Pacific pandemic species, only three Kotoracythere inconspicua (Brady), Tenedocythere transoceanica (Teeter) and Triebelina sertata (Triebel) have been identified to date. The fauna is not, however, nearly so endemic as that of Easter Island on which the two authors are currently working.     

Biodiversity and origin of the non-flying terrestrial arthropods of Henderson Island

Henderson Island, a raised coral island on the extreme south-easterly edge of the Indo-Pacific plate, is of great importance as one of the few examples of a Pacific island with intact lowland forest. It is also of biogeographic interest as it is practically the final island in a series of island chains, along which the fauna of Polynesia has colonized, reaching back to New Guinea. New collections of the non-flying terrestrial arthropod fauna were made on Henderson Island in 1991. In excess of 100 taxa are now known. There is a rich mite fauna (especially oribatids), with many apparently endemic, some 26 species of spider, and nine species of isopod (including three endemic to Henderson or nearby Ducie Atoll). In addition Diplopoda, Chilopoda, Amphipoda, Pseudoscorpiones, Diplura, Protura and Collembola are represented. The majority of the fauna is derived from the west, as expected, though many taxa appear to be introduced, some of them from the neotropics (e.g. Frigga crocuta and perhaps Hoplophorella stilifer).     

Rapid, independent evolution of flightlessness in four species of Pacific Island rails (Rallidae): an analysis based on mitochondrial sequence data

Flightless rails were once ubiquitous in the avifauna of Pacific oceanic islands. Most species have become extinct since human colonization of islands began about 2000 years ago. In this study, we use mitochondrial sequence data to estimate the phylogenetic relationships and ages of four species of flightless insular rails in the genus Porzana : palmeri , from Laysan Island in the Hawaiian archipelago; sandwichensis , from the island of Hawaii; monasa , from Kosrae Island in Micronesia; and atra , from Henderson Island in the Pitcairn group. Although all four species survived into historic times, all but atra are now extinct. The optimal trees show that palmeri is descended from Porzana pusilla , a volant crake distributed widely throughout the Old World. Porzana sandwichensis , P. monasa , and P. atra are each descended from the lineage leading to P. tabuensis , a volant rail widespread in northern and eastern Australia and on islands north to Micronesia and the Philippines and east through Polynesia. Loss of flight appears to have evolved rapidly in these insular rails, based on both sequence divergence values and data on the ages of the islands. In the case of the Laysan Rail ( palmeri ), divergences including loss of flight probably evolved in less than 125,000 years .     

The modern avifauna of the Pitcairn Islands

Excluding the gadfly petrels Pterodroma spp. and the resident landbirds, this paper details the present status of bird species seen during an expedition to the Pitcairn Islands (Ducie, Henderson, Oeno and Pitcairn) in 1991 and early 1992. Ten species were recorded in the Islands for the first time. Several Southern Ocean petrels were recorded, most in the midwinter period of June and July. The populations of Christmas Island shearwaters breeding on Oeno and Ducie laid in synchrony every 9–10 months. The species is one of few with such a sub-annual, synchronized regime. The majority of other seabirds had an annual breeding cycle, laying between May and October. Around one percent of the world population of the bristle-thighed curlew passes the non-breeding season in the Pitcairn Islands.     

Seasonality and numbers of green turtles Chelonia mydas nesting on the Pitcairn Islands

During 1991 and 1992 the first regular field observations of green turtles, Chelonia mydas , on the Pitcairn Islands (South Pacific) revealed that nesting occurs only on Henderson where about 10 females laid eggs (c. 1% of the French Polynesian population). Substrates on Pitcairn and Ducie Atoll are unsuitable for nesting and no signs of nesting were found on Oeno Atoll where the terrain appears suitable.     

Henderson Island prehistory: colonization and extinction on a remote Polynesian island

Situated at the extreme margin of the Indo-West Pacific biotic province, the four islands of the isolated Pitcairn Group hold interest for biogeographers and archaeologists alike. Human settlement may have been as early as the 8th century AD for the uplifted limestone island of Henderson, the most pristine island of its kind. An archaeological survey of the Pitcairn Islands is provided, while Henderson is examined in detail. Recent extensive excavations provide a record of change during 600 years of human occupation. Adaptation to the ecologically-marginal conditions is documented by artefacts, more than 150000 vertebrate bones, molluscs and subfossil plant remains recovered from stratigraphic contexts. The effects of prehistoric human occupation on the pristine environment are revealed by Polynesian plant and animal introductions, bird extinctions and range reductions, possible over-predation of marine molluscs, exploitation of sea turtles, and large-scale burning for swidden agriculture. The origin of human colonists is documented by analysing imported artefacts by geochemical characterization (x-ray fluorescence analysis). The human abandonment of Henderson, by the seventeenth century, is viewed in the context of prehistoric regional dynamics.     

The fossil birds of Henderson Island, Pitcairn Group: natural turnover and human impact, a synopsis

Recent studies of island biotas have suggested that the impact of man on indigenous flora and fauna is much greater than previously suspected. This impact resulted in the introduction of many new species and the extinction of many unique life-forms. Henderson Island, in the Pitcairn Group, has been found to be an excellent laboratory for the study of natural faunal turnover and the impact of people on the natural environment. This was principally due to the island's remote location and its limestone structure, which resulted in the excellent preservation of fossil remains. During the Sir Peter Scott Commemorative Expedition to the Pitcairn Islands, extensive excavations were undertaken resulting in the collection of 42213 bird bones. It was possible to identify 31%. Of the 31 taxa identified, four seabirds appear to be vagrants, a surprisingly high number illustrating that the uncritical evaluation of fossil bird lists from other islands risks over-estimating the number of indigenous species. As a result of the arrival of Polynesian people during the first half of this millennium, half of Henderson's endemic landbirds became extinct, as did most of the small ground-nesting seabirds. The lower sea level during cold stages creates many temporary limestone 'high' islands. This results in many 'former-atolls' developing geological and ecological similarity to Henderson. Hence lower sea-level greatly facilitates the movement of flora and fauna between currently isolated oceanic 'high' islands.     

Geomorphology of the uplifted Pleistocene atoll at Henderson Island, Pitcairn Group

Henderson Island, in the Pitcairn Group, preserves a Pleistocene atoll physiography with the rim of the raised reef structure, supporting spur and groove topography, enclosing a central lagoon. Excellent preservation of coral reef communities occurs along the ancient atoll rim and within the central lagoon. The previously interpreted depositional nature of the fossil atoll structure is herein corroborated with geomorphologic and stratigraphic evidence from previously un-visited portions of the island. Stratigraphic and lateral facies relationships indicate a physiographic zonation which includes spur and grooves, outer reef flat, lagoon margin, and an interior lagoon with patch reefs. The in situ occurrence and zonation of reef coral communities around the periphery and within the interior of the island appear to reflect the original physiography of the atoll lagoon, with the most pronounced reef development on the SE side of the original atoll. Stratigraphic units which comprise the raised atoll lagoon structure represent different time intervals, so the atoll lagoon structure formed during various sea level fluctuations. The modern atolls of the Pitcairn Group, Oeno and Ducie, provide some comparisons (similarities and differences) with the fossil lagoon on top of Henderson Island.     

Biodiversity and biogeography of Henderson Island's insects

Henderson Island is an example of a raised coral atoll, and as such supports a much more diverse biota than geographically similar low atolls. It is also, due to its remote location, largely undisturbed by the activities of humans. It is therefore unique in the Pacific Ocean, and comparable to Aldabra in the Indian Ocean. Biogeographically it is also interesting as it is amongst the most easterly islands of the Indo-West Pacific region, and has been largely colonized by species dispersing from island to island across the ocean from the west. New and extensive collections of insects were made in 1991. Although not fully identified yet, it is clear that the fauna is depauperate, with only about 180 species known to date. Despite its easterly location almost all species are derived from the west, and much of the fauna is likely to be indigenous. The most diverse orders are the Lepidoptera ( c. 53 spp.), Coleoptera ( c. 38 spp.), Diptera ( c. 37 spp.), Hymenoptera ( c. 21 spp.), Homoptera ( c. 14 spp.) and Heteroptera (4 spp.). Of particular interest are the weevils, Heteroptera and Homoptera which exhibit considerable endemism with perhaps some examples of intra-island radiation. Although depauperate, Henderson supports a much larger insect fauna than that of nearby Ducie Atoll (total known insect fauna =15 spp).     

Recent planktonic foraminiferal distribution in high latitudes of the South Pacific: A multivariate statistical study

Planktonic foraminiferal assemblages from 32 surface sediment samples from high latitudes of the South Pacific Ocean have been subjected to a multivariate statistical classification method termed “principal coordinates analysis”. On the basis of the presence or absence of 18 species of planktonic Foraminifera, and the frequency and coiling direction of the cold-water species Globigerina pachyderma, the samples were clustered into five groups, where one group represents the subtropical, two the subantarctic, and two the Antarctic water mass. This assemblage-grouping method is suitable for investigations of past climatic changes.