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The Prince Edward Islands

Date de soumission : 08/07/2009
Critères: (vii)(viii)(ix)(x)
Catégorie : Naturel
Soumis par :
Department of Water & Environmental Affairs
Etat, province ou région :
Southern Ocean. For administrative purposes, the Prince Edward Islands are regarded as part of the Cape Town Magisterial District of the Western Cape Province.
Ref.: 5457
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Description

Marion Island extends from S46 49 30 to S46 58 30 and from E37 52 50 to E38 00 45. Prince Edward Island is located approximately 19km northeast of Marion Island.

Topography

Marion Island consists of a central highland area that reaches 1,249m above sea level (ASL) at its highest point. There is a 4-5 km wide coastal plain (up to 300m ASL) on the northern and eastern sides of the Island that slopes gently up to the highlands. The coastal plain on the western and southern sides of the islands is only about 100m in altitude and irregular due to extensive abundant conical cones of scoria (volcanic cinder).

Prince Edward Island rises to 672m ASL at its highest point (Van Zinderen Bakker Peak) and consists of a central highland that slopes gently to the east and drops to the western lowland in the form of a 400m high escarpment.

The coastlines of both islands consist mostly of coastal cliffs rising abruptly from the sea, interspersed by small pebble and boulder beaches in protected bays.

Geology and geomorphology

The island group is located near the centre of the West Indian Ocean Ridge and represents summits of a volcano rising more that 3500m from the ocean floor. The age of the oldest lava flows on Marion Island are estimated at 450,000 years. Marion Island may be regarded as an active volcano, since several small eruptions, mainly of a minor nature, have occurred on the island since 1980. This makes Marion Island one of only three sub-Antarctic Islands to have erupted volcanically in recorded history. It is thought that Prince Edward Island is a remnant of a closely associated shield volcano, of which four fifths have since subsided below sea level.

Two stages of volcano activity can easily be recognized on both islands: older grey lava and younger black lava flows. The grey basalt lavas, which are between 270,000 and 48,000 years old, occur manly as elevated ridges with a smooth topography and bear extensive marks of glaciation in the form of deep striations, unsorted rocky material and large solitary boulders. Glaciation occurred between 12,000 and 16,000 years ago. Prince Edward Island does not show signs of glaciation, possibly because the island has never been covered by an ice sheet or because the glaciated sections have since eroded away.

With the retreat of glaciers about 16 000 years ago, Marion Island was subjected to a second wave of lave flows that formed the black basalt lavas. The black lavas form very ragged flows between and over the grey lavas and mainly occupy intervening valleys between the ridges. As they have never been subjected to glaciation, their topography is very uneven. These younger flows are associated with approximately 130 scoria cones on Marion Island. Scoria cones are distinctive features of the geomorphology of both islands.

There is a stationary glacier or "ice plateau" in the central highlands of Marion Island - the only glacier on South African territory. The glacier is static and consists mostly of hard blue ice that is partially hidden by large moraines.

Soils

Most of the rock on the island has not weathered sufficiently to form deep, well developed soils. Many of the higher lying "feldmark" or wind desert areas are characterized by desolate wind-swept surfaces covered by loose stones and volcanic ash. Most of the soils consist almost entirely of slowly decomposing organic matter (peat) from plants and fine volcanic ash. Generally, the islands soils are characterized by immaturity, negligible influence of parent material on the soil profiles and a marked effect of slight variations in topography and wind exposure.

Deeper peal soils occur along the waterlogged coastal plain and in valleys that are protected from wind. Peat slips are common features of these soils, especially on slopes where they have been disturbed by human trampling or by seals. Soils of low-altitude and vegetated areas are usually pea, containing volcanic ash in varying amounts.

Climate

The island group has a cool climate with an annual mean temperature of 5.9°C. The absolute maximum and minimum temperatures ever recorded at the base are 23°C and -6.8°C respectively. Absolute minimum temperatures are below zero every month of the year, but even in the winter temperatures rarely fall below -4°C because of the moderating influence of the ocean.

The mean surface air temperature has increased by 0.93°C from 1951 to 1988. This is ascribed to changing oceanic and atmospheric circulation patterns at sea level.

The island group experiences high precipitation (an average of 2,500 mm per annum), mainly in the form of rain, which is distributed fairly every throughout the year. Most of the rain falls as light showers. Heavy falls of over 25mm/day occur about twice a month on average. The island group experiences an average of 25 days with precipitation a month and 303 days with precipitation per annum.

Snow is frequent in winter, particularly from July to September, and sometimes covers the whole of Marion Island, but in low-lying areas it usually melts within a few days. The base experiences an average of 95 days of snow and 46 days of fog per annum.

Marion Island has a high level of cloudiness (annual sunshine ca 30% of the maximum possible. On average, no days with more than 90% of possible sunshine are encountered. An annual average of 130 days with a cloud base below 300m above sea level is encountered. Average annual cloud cover is 79%.

The island group is situated in the "Roaring Forties". Thus the islands are subjected to westerly to northwesterly winds approximately 60% of the time. Gale force (>55km/h) winds lasting at least one hour are experienced for an average of 107 days per annum. Gales usually exceed this speed and duration, however, and can reach speeds of up to 200km/h. Winds exceeding 70km/h often continue unabated for more than 24 hours.

General geology

The features of the sub-Antarctic islands that have combined to produce their particular ecosystem are geographic isolation, wind exposure, temperature, high rainfall, and the strong influence of the marine ecosystem (e.g. manuring by birds and seals).

Two factors in particular have contributed to a relatively low floral and faunal diversity on the sub-Antarctic islands in general and on this island group in particular. The first factor is that the island group is geologically very young. The second factor is the remoteness of the islands from continents. A slow process of colonization has established biota on small "pinpricks" of land across vast expanses of ocean. 

Thus, there is a low number of species of indigenous flora. Many of these indigenous species have wide ecological amplitudes and occur over a range of habitats. Five percent of the plant species are endemic to the island group and 23% of the plant species are restricted to the South Indian Ocean Biogeographically Province.

Indigenous species that play a major role in continental ecosystems (e.g. herbivorous and carnivorous land mammals) are absent from the natural terrestrial ecosystem of the island group. Combined with the very high primary production of many plant communities, this has a significant effect on ecosystem structure and function. It means that detritivores like fungi and bacteria, rather than herbivores, control energy flow. Arthropods and other invertebrates play very important roles as detritivores and invertebrates are by far the most dominant herbivores in the ecosystem.

Nutrient cycling

The ecosystem of the island group can be regarded as semi-closed systems with significant interaction between the terrestrial and oceanic systems. The ecosystem is characteristic by a significant level of nutrient transfer between the terrestrial environment and the ocean.  Seabirds and seals bring nutrients to the islands, mainly in the form of guano. These nutrients support the growth of specific plant communities, particularly in the vicinity of penguin and seal colonies, but also farther inland. An example is the way that tussock grasslands replace fernbrake communities where burrowing petrels and prions establish their burrows. These nutrients are further spread through the soils by invertebrates.

Nutrients are returned to the oceans when they are washed off the island by rainfall. The nutrients are absorbed by plankton, and are then cycled higher up the food chain and eventually to inshore-foraging seabirds such as Gentoo penguins and cormorants and to seals. 

Vegetation

The vegetation of Marion Island is relatively poor in species. This is typical of sub-Antartic islands, due to the isolation from other landmasses and rigorous climate. The island group has 22 indigenous vascular plant species and 21 alien plant species, either naturalized or transient. Mosses (79 species), liverworts (36 species) and lichens (ca 50 species) are important components of the vegetation.

Most of the islands' vegetation has a very slow growth rate due to the extreme climate. This, combined with slow reproduction, makes the vegetation very sensitive to external disturbances.

Six main plant communities can be distinguished. Vegetation distribution is mainly affected by factors such as the soil-water regime, the influence of salt spray, mechanical damage (e.g. due to trampling) and enrichment by guano deposition.

Invertebrates

Some 147 species of indigenous and introduced invertebrates are known from Marion Island. This includes 19 alien species that have become naturalized and 13 introduced species that have not, as yet, established themselves. 39 species of soil ciliates have been found on Marion Island.

Seven endemic invertebrates' species have been identified. The endemic species include two springtails (Isotoma marionensis and Katianna n sp.), three beetles (Bothrometous elongates, Ectemnorhinus marioni and E. similis) and two moths (Pringleophaga marioni and P. kerguelensis).

The distribution and density of many invertebrates is strongly influenced by manuring by birds and seals. High densities and biomass of invertebrates accompany high plant densities, soil nutrient content and plant nutrient content in heavily manured areas.

Mammals

There are three seal species on the land group. Their numbers are indicated in parentheses: the Southern elephant seal, Mirounga leonine (2,000), Antarctica fur seal Arctocephalus gazella (330), and sub-Antarctic fur seal, A. tropicalis (44,800). Leopard seals, Hydrurga leptonyx, and Weddell seals, Leptonychotes weddellii, are occasional non-breeding vagrants.

There are no indigenous land mammals on the island group. Introduced house mice are present on Marion Island, but do not occur on Prince Edward Island. The introduced feral cat was exterminated in the early 1990s.

Birds

The island group supports 29 species of breeding birds as well as 22 species of vagrant seabirds and 28 species of non-marine vagrant species. Although the breeding bird species include only one endemic taxon (the Lesser sheathbill, Chionis minor marionenis) most species have a very limited breeding area that is restricted to a handful of sub-Antarctic islands. Furthermore, the large distances between breeding sites and the high philopatry (natal site fidelity) characteristics of these species have led to limited genetic interchange and hence considerable geographical variation within species.

Most of these species are wholly or predominatly dependant on the marine environment for their energy needs and are capable of foraging great distances sway from the island group. They only use the island group as a platform for breeding and moulting (in the case of penguins). Outside their breeding seasons they disperse away from the islands to more productive foraging areas. The large numbers of seabirds that breed on the island group are an important vehicle for importing nutrients from the marine environment to the terrestrial island environment, primarily in the form of guano. The seabirds on the island group are generally long lived. They only breed after a prolonged juvenile stage and breed very slowly. Almost all species breed only once a year and only lay one or two eggs. The chick-rearing period is prolonged with moderate breeding success. Some albatross species only lay one egg every second year. Just over half the eggs successfully fledge a chick in a given season. This means that populations are extremely sensitive to adult mortality, and will take a long time to recover from a perturbation to the population demographics (e.g. a decrease in adult survival).

Four orders of seabirds are present on the island group: Sphenisciformes (penguins), Procellariiformes (albatrosses and petrels), Charadriiformes (skuas, gulls and terns) and Pelecaniformes (cormorant).

Freshwater systems

Marion Islands has three perennial streams, but it is not known whether Prince Edward Island has any perennial watercourses due to the infrequent visitation of this island. Apart from the flowing water types, there is a range of lentic waters on the island group, including shallow lakes, lava lakelets (primarily on black lava flows), crater lakes (in the craters of scoria cones) and wallows formed by the activity of animals.

There are no indigenous fish in the freshwater environments (the introduced brown and rainbow trout are now extinct), and zooplankton, therefore, represents the highest level in the freshwater food chain. Two species of copepods, Pseudoboeckella volucris and Daphniopsus studeri, dominate fresh waters and there are also a number of species of mites and a common freshwater midge, Linophyes minimus.

Marine environment

The island group is in the path of one of the world widest current systems, the Antarctic Circumpolar Current (ACC), which flows clockwise around the Antarctic continent at a surface speed of 0.5-2km/h. As a result, the island group effectively has an upstream westerly and downstream (easterly) side. This is important because all the island group's land-based vertebrate predators depend on the sea for food. The availability of food is controlled by oceanographic conditions. The ACC carries food, in the form of plankton, to the island group from the west. The island group also lies close to two major oceanic frontal systems. These fronts separate major water bodies with different chemical and physical properties and act as strong biogeographical boundaries with different suites of marine species to the north and south of each front. In addition, the fronts are areas of enhanced biological activity. They consequently from important feeding grounds for land-based predators.

Both islands in the island group have relatively unstable and hostile littoral environments, which results in a generally low biodiversity and low density of littoral organisms. All the shores around the islands are exposed (those with a westerly aspect severely so), due to the predominantly westerly winds. Large swells and unstable substrates (e.g. boulders) that result in abrasion also contribute to unfavorable conditions.

The island group forms the highest point of a shallow plateau, approximately 200 to 500m deep, that drops off very rapidly into much deeper water (ca. 3000 m). This plateau supports a rich seabed community of approximately 550 species, dominated by filter feeders. These are largely supported by local phytoplankton production.  The swimming prawn, Nauticaris marionis, links this community to the seabirds. Adult prawns feed on the seabed community and are important in the diets of birds with short foraging ranges, especially the Gentoo, Macaroni and Rockhopper penguins and the Imperial cormorant.

33 species of fish from 13 families are known from the oceans around the island group.

Land Ownership: South Africa 

Management Structure: The Prince Edward Islands are currently managed by the Department of Water and Environmental Affairs, with direction from the Prince Edward Islands Management Committee.

Budgetary Matters: The research and infrastructure on Marion Island is funded by government.

Site Readiness: The process of moving towards re-nomination is in the initial stages.  Preliminary meetings have been held with the relevant role players and a strategy needs to be developed for re-nomination by UNESCO's 1 February 2011 deadline, after the new base on Marion Island has been completed and the new Environmental Management Plan has been approved.

Justification de la Valeur Universelle Exceptionelle

The island group is one of the most important and well-conserved examples of sub-Antarctic islands and is one of only six groups of islands that represent this ecosystem on earth.

The island group harbors a significant percentage of the world population of breeding seabirds - in the case of some species up to 30% and even 40% of the world population. In view of the very limited breeding habitat available for these species and their small global population sizes, their populations on the island group are of universal value to biodiversity conservation. This is particularly true for the species that are poorly represented or not represented at all on existing World Heritage Sites.

The variety of landforms and terrain types are outstanding in that they are of sufficient size and diversity to support the full range of sub-Antarctic vegetation in communities. Six main vegetation assemblages and several variations within these occur on the islands. This includes the best-conserved and most extensive examples of the sub-Antarctic fernbrake communities in the sub-Antarctic

The island group (especially Prince Edward Island) has some of the least disturbed sub-Antarctic floral communities. There is a single introduced land mammal (only on Marion Islands) that does not seem to have a significant effect on the indigenous biota. None of the land mammals that are responsible for ecosystem modifications at sub-Antarctic islands are present on the island group.

The island group supports one endemic subspecies of bird, as well as seven known endemic invertebrate species. These endemic invertebrates demonstrate evolutionary adaptations to the islands adverse climatic conditions.

The island group also contains exceptional examples of volcanic processes and is an outstanding example of a shield volcano with a range of landforms of volcanic and glacial origin.

The isolation of the islands from direct human influences and the pristine nature of Prince Edward Island (compared to Marion Island) not influenced by exotic fauna and flora, provides an outstanding opportunity for research into the ecological effects of climatic change and colonization by alien species.

Criterion (vii): The natural beauty and unspoiled wilderness quality of the landscapes of the island group and the concentrations of biota (especially seabirds) in spectacularly large colonies are features of exceptional aesthetic importance.

The island group has a wide variety of contrasting landforms, including boulder and pebble beaches, black volcanic sand beaches, high coastal cliffs of more than 100m, ice-covered mountain peaks, isolated coastal stacks, wide coastal plains and volcanic cones. In combination, these landforms make for a unique aesthetic experience. The typically red volcanic scoria cones contrast with deep green vegetation of the coastal plains, while the densely vegetated coastal plains with the windswept desert and exposed rocky mountainous highlands. The numerous contrasts in colour, texture and form, and the natural appearance of the islands impart a sense of wilderness that is very rare on global scale.

Criterion (viii): The shield volcanoes of the island group provide outstanding examples of major stages of the earth's history, including significant ongoing geological processes in the development of landforms and significant geomorphic or physiographic features.

Criterion (ix): The island group (with particular reference to Prince Edward Island, which has only three alien vascular introduced plants) provides an excellent example of biological and ecological processes in an undisturbed ecosystem. Prince Edward Island is pristine compared to other sub-Antarctica islands, with the exception of the Heard and McDonald Island World Heritage Site.

The relatively young age of the island group (less than 500 000 years), geologically speaking, means that the islands have been colonized from other continental and islands sources very recently and that its terrestrial ecosystem has had very little time to develop compared to other sub-Antarctic islands.

Criterion (x): The island group represents a significant part of an extremely rare and threatened ecosystem on a global scale. The island group is located in the Southern Ocean (a zone extending from 40 degrees to 60 degrees South and representing approximately 10% of the earth's surface) where terrestrial habitats are extremely scarce. The avifauna of this huge area is reliant on only 19 island groups for breeding. Of these island groups, only six are considered to be sub-Antarctic (and therefore share a similar suite of species to those breeding on the island group). In this respect alone, the island group is a significant representative of a very rare and unique assemblage of bird species.

The island group supports three marine mammal species, which similar to marine birds of the Southern Ocean, have very restricted global breeding habitats. Southern elephant seal, Antarctic Fur seal and sub-Antarctic Fur seal breed on the island group.

The island group supports three a full range of vegetation assemblages that are characteristic of sub-Antarctic islands.

Scientific research has been conducted at the island group since late 1940s and has been ongoing since the first groundbreaking comprehensive study in the 1960s. This research has allowed an integrated view of the functioning of the island group's ecosystem to emerge. The baseline provided by this research is invaluable for long-term ecological research.

Déclarations d’authenticité et/ou d’intégrité

The Prince Edward Islands remain an authentic largely undisturbed natural landscape.  The management and conservation of the island group by the Prince Edward Islands Management Committee, in accordance with the provisions of the Prince Edward Islands Management Plan, ensures that these islands will not change significantly.  The only intrusion in this island group is the current and new (almost completed) base and 9 field huts on Marion Island. There are no permanent structures on Prince Edward Island.

Comparaison avec d’autres biens similaires

Of all the Southern Ocean islands (including cool-temperate, sub-Antarctic islands) Prince Edward Island emerges as the island with the best combination of biodiversity and state of "pristineness" or absence of human impacts. Although Marion Island is more affected by human-induced disturbance than Prince Edward Island, the size and habitat diversity of Marion Island is extremely valuable in supporting large breeding populations of seabirds, especially penguins and albatrosses.

Habitat diversity of Marion Islands is extremely valuable in supporting large populations of seabirds, especially penguins and albatrosses.

The combination of the quality of breeding habitat on Prince Edward Island and the large number if species on Marion Islands, as well as the significant proportions of the global populations of certain species, makes this island group of Outstanding Universal Value and worthy of World Heritage Status.

In accordance with the World Conservation Union (UICN) guidelines for World Heritage nominations for Southern Ocean Islands the Island Group has been compared to biogeographically related Southern Ocean Islands. The Islands Group has, therefore, been compared to other sub-Antarctic islands, and not to related maritime Antarctic and cool temperate islands that are located in the Southern Ocean.

A preliminary assessment of the World Heritage values of Southern Ocean islands by an IUCN Working Group, prepared according to the Delphi method, ranked the Prince Edward Islands, South Georgia and Heard and MacDonald Islands highest amongst all the sub-Antarctic islands. This assessment found these islands to be "so closely related that it is not practical to distinguish among them as the highest ranked candidates".

The Delphi assessment highlighted the island group as one of the islands groups with the greatest value in terms of biodiversity and lack of human impact. The islands group ranked third in terms of biodiversity behind lles Kerguelen and lles Crozet - the islands to which they are most closely related. The island group is classified together with these islands and Heard and McDonald Islands in the South Indian Ocean Biogeographical Province (alternatively named the Kerguelen Biogeographical Province).

The island group (33,500 ha) serves as a breeding site for 29 seabird species. This is less than the number of species on the larger lles Kerguelen (700,00 ha) and dlles Crozet (50.000 ha), which have 36 and 35 species, respectively. However, more bird species occur on the island group than on any of the other three sub-Antarctic islands groups - Macquarie, Heard and McDonald Islands (both existing World Heritage Sites) and South Georgia. The island group had six species more than Macquarie Island, ten more species than Heard and MacDonald Islands and one more species than South Georgia.

In terms of human impact, Dingwall (1995) ranked the island group second (behind Heard and MacDonald Islands), together with the South Sandwich Islands and Bouvetoya. However, the classification of the latter two islands as sub-Antarctic is doubtful. The much colder South Sandwich Islands and Bouvetoya are usually excluded from the sub-Antarctic. If this stricter classification of the sub-Antarctic is accepted, the competitor that is ranked next (South Georgia), is far behind the island group in terms of level of human impact.

The island group is distinguished by the fact that it has only one introduced land mammal species (the House mouse, Mus musculus) compared to the numerous species on many of the other sub-Antarctic islands. The island group, together with Heard and McDonald Islands, is one of only two sub-Antarctic island groups that is presently free of the introduced population of cats, Felis catus, or rats, Rattus spp. Indeed, Prince Edward Island is free and has always been free of any introduced mammals and is widely recognized as one of the most pristine islands in the Southern Ocean.

On South Georgia, which has two introduced land mammals (Norway rat and reindeer), the rats probably have a much greater impact on seabirds than do the mice that are present on Marion Island. Both lles Crozet and lles Kerguelen - the islands to which the island group's ecosystem is most closely related - have several species of introduced land mammals that seriously affect their ecosystems. Both islands have feral cats, house mice and ship rats, whilst the lles Kerguelen also has rabbits, Oryctolagus cuniculus, sheep, Ovis aries, reindeer, Rangifer tarandus, and moufflon, Ovis musiman. The combination of alien land mammals on Kerguelen and Crozet exacts a much higher cumulative impact on their ecosystems than the single introduced land mammal species on Marion Island.

The island group also has fewer introduced land mammals than the existing World Heritage Site of Macquarie Island, which currently has four introduced species. However, it cannot compete with Heard and MacDonald Islands in terms of the number of introduced land mammals, as no land mammals have been introduced to these islands.

Together with neighbouring Marion Island, the near pristine Prince Edward Island is the most temperate of all sub-Antarctic islands, and Marion Island's ecosystem is well understood. The island group provides an unparalleled opportunity to study the effects of climatic change on ecological structure and function. The effects of climate change are particularly clear here. Furthermore, the climate of the island group is unique in a sub-Antarctic context as it can only be compared to the climate of more northerly islands.

The value of Prince Edward Island as undisturbed habitat relatively free from human interference is particularly important. An examination of the relationships between human occupancy and visitation on the one hand and the presence of alien species on the other hand found a significant correlation between the number of human occupants and the  number of alien species introductions. Thus, islands such as Prince Edward Island, which has no permanent human occupation and is subject to not more than a single scientific visit per year, are highly important in the conservation of indigenous sub-Antarctic fauna and flora communication. In this respect, the only sub-Antarctic Islands that can ompare with Prince Edward Island in terms of low visit numbers are Heard and MacDonald Islands, and some of the lles Crozet.