Description
Location and General Description
Rakiura, also known as Stewart Island, lies about 25 km south of the South Island of New Zealand. The 1,700 km2 island is separated from the mainland by the shallow Foveaux Strait, but has been connected to the South Island during past glacial periods, most recently about 14,000 years ago. Stewart Island is hilly and wet, with little flat ground. Mean annual rainfall varies from 1,000 mm to 3,000 mm depending on altitude, and rainless periods of more than 2 weeks are uncommon (McGlone and Wilson 1996). The highest peak, Mt. Anglem, rises to 975 m above sea level. The island is composed primarily of diorite gneiss and granite, which decays rapidly to clay (Cockayne 1909). The clay soil is not very fertile, but the wet conditions, mild temperatures, and humus from decaying vegetation allow forest to thrive across the island. The main factors limiting the forest cover on Rakiura Island are wind and salt spray. This ecoregion also includes the Snares Islands, a 3.3 km2 island group located 105 km southwest of Rakiura. The Snares are volcanic in origin, and consist of one main island and several rocky islets. Climatic conditions are similar to Rakiura.
Podocarp-hardwood forests dominate the lowlands of Rakiura Island, but give way to sub-alpine manuka (Leptospermum scoparium) scrub above 300 m (Cockayne 1909). The dominant lowland trees are rimu (Dacrydium cupressinum), kamahi (Weinmannia racemosa), and southern rata or ironwood (Metrosideros umbellata). There are more than 50 other tree and shrub species in the forest, including Senecio spp., daisy-trees (Olearia spp.), and Coprosma spp. Cushion plants, liverworts, mosses, and the fern Blechnum discolor are all common in the understory. The tree trunks are draped with orchids (e.g. Dendrobium cunninghamii) and ferns such as Hymenophyllum dilatatum, Polystichum adiantiforme, Polypodium diversifolium, and Asplenium flaccidum. Due to its small size, only 20 plant taxa are native to the Snares Islands (Molloy 1994). On the main island of the Snares, Hebe elliptica grows around coastal fringes, while Olearia lyallii forms central stands reaching up to 9 m in height (Wardle 1991).
Biodiversity Features
Rakiura Island has a lush and distinctive vegetation. The forest here is largely undisturbed by humans, and provides an interesting perspective on what forests on the New Zealand mainland might once have been like (Molloy 1994). There are few endemic plant species on Rakiura, and the island is more notable for the species that are not found here despite being common in similar habitats on mainland New Zealand. Strikingly, there are no Nothofagus spp., Libocedrus bidwillii, or Phyllocladus alpinus present on the island (McGlone and Wilson 1996). The podocarps and hardwoods are thought to have been reestablished at the end of the last ice age by birds, but southern beech Nothofagus spp. rely on freshwater stream dispersal, which would have been prevented by the rising Foveaux Strait (Molloy 1994).
Rakiura Island is well known for its rich birdlife. The island become a haven for mainland species, and some unique subspecies, in the wake of European colonization of New Zealand. European settlers brought a whole range of introduced predators to New Zealand, including cats (Felis silvestris), rats (Rattus exulans, R. norvegicus, R. rattus), stoats (Mustela erminea), ferrets (M. furo), weasels (M. nivalis), and hedgehogs (Erinaceus europaeus), which together drove many New Zealand birds to the brink of extinction. Fortunately for the avifauna of Rakiura Island, ferrets, weasels, and stoats were never introduced there, and populations of cats and rats were largely confined to the small settled area near Half Moon Bay.
As a result, bird populations on Rakiura fared much better than those on the neighbouring South Island, and flightless birds like the Stewart Island kiwi (Apteryx australis lawryi) are still found here. The kakapo (Strigops habroptilus), the world’s only flightless, nocturnal parrot, was extirpated from the mainland by introduced predators but survived on Stewart Island until 1997, at which point the last few individuals were translocated to smaller, predator-free islands for more intensive management (Clout and Merton 1998). The kakapo is now listed as "extinct in the wild" by the IUCN (Hilton-Taylor 2000), but approximately 56 individuals survive on government-owned offshore islands. Other species such as the endemic laughing owl (Sceloglaux albifacies EX) are probably now extinct. The Stewart Island shag (Phalacrocorax chalconotus VU) is found on Rakiura and the southeastern region of South Island (Stattersfield et al. 1998). Extant endemics on Rakiura and its offshore islets include subspecies of southern robin (Petroica australis rakiura), weka (Gallirallus australis scotti), and fernbird (Bowdleria punctata stewartiana), as well as a leaf-veined slug, a Paryphanta spp., and the harlequin gecko (Hoplodactylus nebulosis). The harlequin gecko is commonly found among subalpine cushion plants and is one of the world’s southernmost lizard species (Molloy 1994).
The Snares group is known for its thriving seabird populations, with 23 different species sighted here. No predatory land mammals have been introduced to the Snares Islands and the total seabird population is estimated to equal that of Great Britain and Ireland, with 2.75 million breeding pairs of sooty shearwaters (Puffinus griseus) alone (Heather and Robertson 1997, Molloy 1994). The Snares crested penguin (Eudyptes robustus VU) is restricted to these islands, as is a subspecies of New Zealand snipe (Coenocorypha aucklandica huegeli). C.a. iredalei was endemic to Stewart Island, but died out when weka (Gallirallus australis) and rats (Rattus rattus) were introduced (Stattersfield et al. 1998).
Current Status
Almost 93 percent of Rakiura Island is owned by the New Zealand government, and over half is designated as either a nature reserve or scenic reserve. The level of protection on the island is excellent, and all of the major habitats are included in the current reserve system. The New Zealand government is currently considering a proposal to turn much of Rakiura into a National Park, to unify the present mosaic of land management activities and give Rakiura a higher public profile. Human settlements on the island have always been small, and have had a very limited impact on the vegetation of the island (McGlone and Wilson 1996). Although there was some logging activity on Rakiura between 1860 and 1930, it was mostly confined to the area around Half Moon Bay and Paterson Inlet so that the majority of Rakiura Island still supports intact native forest. Some of the smaller islands surrounding Rakiura are also protected and serve as refuges for highly endangered species such as the kakapo and the South Island subspecies of saddleback (Philesturnus carunculatus carunculatus), which was nearly extirpated from Rakiura by the arrival of rats. The Snares are uninhabited and wholly protected as a National Park. Landings are restricted by a permit system (Stattersfield et al. 1998).
Types and Severity of Threats
The main conservation concern on Rakiura Island, as in much of New Zealand, is the impact of introduced species. Introduced deer (Odocoileus virginianus) and brush-tailed possums (Trichosurus vulpecula) browse heavily on native vegetation, and have significantly affected both the understory and the canopy of Rakiura’s forests (Powlesland et al. 1992). Introduced cats have had a devastating effect on the native birds and were instrumental in the decline of the kakapo on Rakiura Island. They continue to pose a threat to the Stewart Island kiwi. Introduced rats are also a problem, but fortunately no ferrets or stoats have ever been released on the island. There are currently no introduced predatory land mammals on the Snares, and preservation of the unique fauna found here is contingent upon the total restriction of alien species introductions.
Justification of Ecoregion Delineation
These islands off the southern coast of New Zealand are considered botanically connected to the mainland (Wardle 1991). However, they are designated a separate ecoregion due to their long isolation from the mainland, endemic flora, and very different history of human settlement in comparison to the mainland.
References
Clout, M.N. and D.V. Merton. 1998. Saving the Kakapo: the conservation of the world’s most peculiar parrot. Bird Conservation International 8: 281-296.
Cockayne, L. 1909. Report on a botanical survey of Stewart Island. Department of Lands, New Zealand.
Heather, B. D. and H. A. Robertson. 1997. The Field Guide to the Birds of New Zealand. Oxford University Press, Oxford, United Kingdom.
Hilton-Taylor, C. 2000. 1998. The IUCN 2000 Red List of Threatened Species. IUCN, Gland, Switzerland and Cambridge, United Kingdom.
McGlone, M.S. and H.D. Wilson. 1996. Holocene vegetation and climate of Stewart Island, New Zealand. New Zealand Journal of Botany 34: 369-388.
Molloy, L. 1994. Wild New Zealand. New Zealand Department of Conservation/MIT Press, Cambridge, Massachusetts
Powlesland, R.G., B. D. Lloyd, H. A. Best, and D. V. Merton. 1992. Breeding Biology of the Kakapo, Strigops habroptilus, on Stewart Island, New Zealand. Ibis 134: 361-373.
Wardle, P. 1991. Vegetation of New Zealand. Cambridge University Press, Cambridge, United Kingdom.
Prepared by: Winnie Roberts
Reviewed by: