PITCAIRN ISLAND


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Conservation biology on Pitcairn Island South Central Pacific
Noeleen Smyth

Pitcairn Island is the infamous hideout of the Bounty mutineers; it is an extremely isolated island at the southeast of the main group of Polynesian Islands, and roughly situated half way between New Zealand and South America and just south of the tropic of Capricorn. Pitcairn lies within a group of four islands, which are part of the UK Overseas Territories. The group comprises two atolls, Oeno and Ducie (the most southerly atoll on earth), the raised atoll Henderson (a world heritage site), and the high volcanic island Pitcairn (right).

pitcairn island

pitcairn island

Initial investigations involved gathering baseline data on the effects of the invasive species Syzygium jambos (below) on the native forest communities of Pitcairn Island.

syzigium, pitcairn island

These investigations were carried out over a period of three years with annual expeditions ranging from two to three months on Pitcairn Island.

The diversity, vegetation, soil characteristics, and density of Syzygium jambos invaded forest was characterised by means of quadrats and soil samples. Syzygium jambos was found to adversely affect species diversity in forested communities on the island, with diversity in Syzygium jambos invaded forest, less than half that found in native forest communities.

A different suite of species was also found in Syzygium jambos invaded forest. The soil condition of pH within Syzygium jambos invaded woodland was significantly lower than pH values found in native woodland communities. The organic matter content also differed between invaded and native woodland communities. Syzygium jambos was also found to occur at high densities on Pitcairn Island (seedling density 47.7m -², sapling density 6.1m-², and adult density at 2.3m-²). Syzygium jambos negatively affects species diversity and soil chemistry by its presence.

Syzygium jambos profile and general view in Syzygium jambos invaded forest (right).

Developing suitable methods of control for Syzygium jambos was the next step in the process.

Two different physical treatments: cutting, where trees were cut down and removed from the site or frilling, which involved removing the bark from around the base of the tree and leaving the tree standing in-situ and two different chemicals were also used, Roundup ® or Tordon ®, which were applied as a high volume spray to cut and frilled trunks, these treatments were applied over a two year period.

The most suitable and most economic method for Syzygium jambos control on Pitcairn island was found to be frilling using the chemical Roundup ® which gave high Syzygium jambos mortality rates (98.96% in 2005), had cheaper capital and running costs as well as being less labour intensive (66 person hours per 400m-2) and was considered less dangerous on steep slopes by personnel.

syzigium invading forest, pitcairn island

Homalium taypau, pitcairn island A recovery programme to restore elements of the native forest community after Syzygium jambos control was the next challenge.

Experiments into the composition of the soil seed bank in invaded forest was found to be deficient in native species and indicated that weedy species were the likely replacement vegetation to Syzygium jambos in treated sites.

Direct seeding of the native trees, Homalium taypau (left), Meterosideros collina (below) and Hibiscus tiliaceus in treated sites had very limited success.

A plant propagation and growing facility was built on the island in 2003, in order to grow replacement species for the treated sites.

Twenty eight different species were grown, which were representative of species found in native woodland communities as described by Kingston and Waldren (2003) and some rare endemic and native trees and shrubs. Of the 1,400 plants planted in sites during 2003 and 2004, 833 survived.

The mean percentage plant survival in treated sites was 59.5%. Highest survival rates were found for sites where Syzygium jambos was frilled and the site was weeded (80%). Plant survival was lowest in sites in which Syzygium jambos was cut and the site subsequently weeded (42%) though this treatment exhibited the highest plant growth rate (70.8cm in 10 months).

The most common causes of plant mortality were drought (57%), goat browsing (37%), falling debris (3%) from dying frilled Syzygium jambos, and washout (3%).

The final challenge was to conserve some of the remaining populations of threatened rare and endemic plant species on the island.

Meterosideros collina, pitcairn island
Abutilon pitcairnense, pitcairn island

An assessment of the genetic diversity and threats, both demographic and environmental, were made where appropriate for five critically endangered species; Abutilon pitcairnense (left), Myrsine aff. niauensis, Haloragis sp., Coprosma benefica and Lastreopsis cf. pacifica.

Cuttings and seed were successfully obtained of Abutilon pitcairnense. The seedlings obtained from the original genotype were characterised by a molecular fingerprinting technique (AFLP), five distinct genotypes of this species are now conserved in cultivation and genetic diversity remains high (HT0.346).

The sole surviving individual of Myrsine aff. niauensis was successfully propagated by cuttings.

Molecular sequencing of the trnl-f region of the chloroplast genome revealed that the plant on Pitcairn was genetically most similar, to the Henderson Island species, Myrsine hosakae.

Populations of Lastreopsis cf. pacifica and Haloragis sp. were monitored over the three years and the populations on Pitcairn were found to be stable and increasing.

The population of Coprosma benefica, which consisted of 12 dioecious individuals in 1997 (6 female, 1 male and 4 unknown), had decreased to 11 individuals by 2003 (8 female, 1 male and 2 showing both male and female expression).

This knowledge combined provides for recovering the lost native forest habitat on Pitcairn Island and thwarting the exotic invasive species Syzygium jambos, alongside single rare and threatened species conservation and recovery efforts. The holistic approach taken here to both habitat and species recovery was both demanded and required to fulfil the aspirations set out by the term “conservation biology”.

References

Kingston, N. & Waldren, S. 2003. The plant communities and environmental gradients of Pitcairn Island: The significance of invasive species and the need for conservation management. Annals of Botany 92 (1): 31-40.