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Antarctic islands in the southern Indian Ocean

This ecoregion encompasses five island groups in the southern Indian Ocean: Prince Edward, Crozet, Kerguelen, Heard, and McDonald Islands. Although the growing conditions are harsh, a variety of plant communities have adapted to survive in this sub-Antarctic botanic zone. Included are some endemic species such as the famous Kerguelen cabbage (Pringlea ascorbutica) that sailors once ate to prevent scurvy. A number of birds that breed in the ecoregion have been classified as "Vulnerable", such as the endemic Eaton’s pintail (Anas eatoni). This ecoregion is also the only place where all six species of Antarctic seals have been found, and it is an important breeding ground for the Antarctic and sub-Antarctic fur seal, leopard seal, and southern elephant seal. As on many islands, introduced species threaten the native wildlife. A variety of plants have been introduced in the region, and mice and cats put significant pressure on breeding birds such as petrels.

  • Scientific Code
  • Ecoregion Category
  • Size
    3,200 square miles
  • Status
  • Habitats

Location and General Description
The westernmost landmass of the ecoregion is represented by the Prince Edward Island group (hereafter the PEI). The two islands in this group, Marion and Prince Edward, are found at 46º54’S; 37º45’E and 46º38’S; 37º57’E, and are about 290km2 and 45km2 in area respectively. The PEI is part of a shield volcano, located near the center of the West Indian Ocean Ridge. These two islands are the youngest in the ecoregion, with ages estimated at 270,000 years for Marion Island and 215,000 years for Prince Edward Island (Hänel and Chown 1998). An oceanic climate prevails, with a mean annual temperature variation of about 4ºC (including recorded maximum temperatures of above 20ºC and minimum temperatures of –6.8ºC, with a wind chill factor capable of reducing the experienced effect to -20ºC). High humidity (83%), precipitation (2500mm/year) and wind speeds, as well as cloud cover, are routinely experienced (ibid.). The area was thought to be volcanically quiescent, but a small lava outpouring on Marion Island occurred during 1980 (Verwoerd et al. 1981). There are two main types of lava on the islands, the older gray lava (270,000-48,000 years old) and the younger black lava (circa 15,000 years old) (Hänel and Chown 1998). Very few beaches occur on Marion Island, it being surrounded for the most part by cliffs. Peat soils occur at the lower altitudes. Marion Island reaches a height of 1230m and Prince Edward Island 675m. The soils on Marion Island that developed during the Holocene did not form through the chemical weathering of basaltic lavas, but rather from falls of volcanic ash (Gribnitz et al.1986). The soils are poor in nitrogen, which is not caused by heterotrophic nitrogen fixation (Smith 1985). Marion Island has intermittent lotic waters for the most part (Grobbelaar 1978), with most of the rainfall draining into the ocean through the ground (Hänel and Chown 1998).

The PEI and the Crozet Island group, the latter found between 46º00’S to 46º30’S; 50º00’E to 52º30E, are closer to each other than either is to any other land mass. The total landmass exposed by the Crozet archipelago is 500km2. It is divided into two groups of islands that are bisected by the 2km deep Indivat basin, east of which there is an abrupt thinning of the crust (Recq et al.1998). There are five main volcanic islands, the western group comprising Île aux Cochons, Îles des Pingouins and Îlots des Apotres, and the eastern group comprising Île de la Possession and Île de l’Est. The two eastern islands are the oldest among the Crozet islands, at approximately 8 million years old. Of the western islands, Île aux Cochons is 0.4 million years old, Îles des Pingouins is 1.1 million years old and Îles des Apotres is 5.5 million years old (Chown 1990). Île de la Possession is the largest island in this archipelago, with an area of 130km2. It reaches a height of 934m (Girod and Nougier 1972). Île de l’Est is the second largest island, and is 120km2 in area. The sheer cliffs on this island reach 1100m high. This deeply dissected shield volcano is made up of two basalt units, one basal and one upper, the two separated by a pyroclastic horizon. Île aux Cochons is the third largest island, at 70km2. This island is a 600m high volcanic cone, and its open crater consists of alternations of pyroclastic rocks and lava flows (Girod and Nougier 1972). Îles des Pingouins and Îlots des Apotres are 3km2 and 2km2 in area, and reach heights of 420m and 289m, respectively. The archipelago experiences a cold, oceanic climate, with an annual mean temperature of 5ºC, monthly mean temperature ranges between 2.9ºC and 7.9ºC in winter and summer respectively, an annual rainfall of 2400mm per year, and strong winds (Frenot et al. 1989). The climate of the Crozet Islands is more similar to that of Marion Island than to that of the Kerguelen Islands (Bellair-Roche 1972).

The Kerguelen Islands (48º27’S to 50º00’S; 60º27’E to 70º35’E) are located 1500km east of the Crozet Islands and reach a maximum altitude of 1850m. The entire exposed land area is 7000km2. The group is basically one large island, Grand Terre, of about 6600km2, and about 300 other islets. The climate is oceanic, with a low mean average temperature of 4.5ºC, 70% to 80% humidity, heavy cloud cover, and strong westerly winds. Rainfall exceeds 1100mm per year. There is a permanent ice cap covering an area of 50km by 20km in the west. The archipelago is approximately 39 million years old, with 85% of its surface area covered by flood basalts (Frey et al. 2000).

Heard Island is at 53º06’S; 73º30E and has an area of 380km2. It consists of an active main volcano, 21km in diameter and 2750m high, as well as a small, secondary volcanic center (de la Rüe 1929, cited by Girod and Nougier 1972). The volcano is covered by an ice cap (Girod and Nougier 1972). Very little soil has been developed, and the island is primarily basaltic. Black volcanic sands are found to the east and west (Perrin 1995). The island experiences a cool, maritime climate, with a mean annual temperature of 1ºC, rainfall of 1400mm per year, and strong westerly winds.

About 40km west of Heard Island are the McDonald Islands at 50º03’S; 72º36’E, comprising McDonald Island (sensu stricto), Flat Island and Meyer Rock (Perrin 1995). The main island is McDonald Island, which is divided into two sections joined by a narrow isthmus. The island is surrounded by steep cliffs, and consists of basaltic lava and tuffaceous material. These islands are very rarely visited. The climate is probably very similar to that of Heard Island. The islands’ area is about 2.6km2, and they have a maximum altitude of 230m.

Most of the vascular flora of the islands of the Kerguelen Province are spread throughout the temperate region of the Southern Ocean. Endemic vascular plants include Poa cookii and Pringlea ascorbutica (both found on all the islands in the ecoregion). P. ascorbutica is the famous Kerguelen cabbage that sailors used to eat to prevent scurvy in days past. Other endemic vascular plants include Polystichum marionense (only found on the Crozet and Prince Edward Islands), Ranunculus moseleyi (only found on the Prince Edward, Crozet and Kerguelen Islands), Lyallia kerguelensis (only found on the Kerguelen Islands), Elaphoglossum randii (only found on Marion Island), Poa kerguelensis (only found on Heard and the Crozet Islands) and Colobanthus kerguelensis (possibly on all the islands). Other non-endemic, but notable, plant species include Crassula moshata, Aceana magellanica, the cushion-shaped Azorella selago, the feathery Cotula plumosa, and the grass Agrostis magellanica. Among the non-flowering plants, the fern Blechnum penna-marina should be noted.

A number of lichen species are endemic to the ecoregion, but many lichens in this ecoregion are bipolar. Many liverworts are endemic to the ecoregion. Plant distribution patterns indicate a strong relationship between the ecoregion islands, a relationship that has been found in some invertebrate groups as well.

Biodiversity Features
The first thing a casual observer notices about this ecoregion is the overlap of normally Subantarctic and normally Antarctic species, from plankton, to fish, to higher predators. One therefore finds on these islands species that may be best suited to one or other of the two environmental regimes, but not necessarily their combination. For instance, the breeding success of the gentoo penguin (Pygoscelis papua) is lowest at its northernmost breeding localities (the Prince Edward and Crozet Islands). This may be because at these relatively northern latitudes the species has to hunt less suitable Subantarctic marine prey (Williams 1980).

Three of the most obvious sympatric congeneric species in the ecoregion are the two sooty albatrosses (Phoebetria spp.), the two giant petrels (Macronectes spp.) and the two fur seals (Arctocephalus spp.). The sooty albatross (Phoebetria fusca) and the light-mantled sooty albatross (P. palpebrata) breed sympatrically only within this ecoregion i.e. on Marion, the Crozet and the Kerguelen Islands (Marchant and Higgins 1990, Berutti 1979). P. palpebrata, the longest-lived of all birds (Weimerskirch et al. 1987 cited in Croxall and Gales 1997), normally has a more southerly, Antarctic range than does P. fusca. P. palpebrata from Marion Island, for instance, must therefore fly further south than its congeneric to forage. This places more stress on this species at that locale (Berruti 1979). In contrast to Marion Island, where its population is much larger than that of P. palpebrata, P. fusca is outnumbered by its congeneric on the Kerguelen Islands. The Kerguelen Islands are, in turn, the most southerly breeding point of the more northern species. Interspecific competition for food and breeding sites between these two congenerics is low or non-existent where they breed sympatrically (Weimerskirch et al. 1986, Berruti 1979), and there are no recorded attempts at interbreeding.

However, at times sympatric congenerics do attempt to interbreed. The Northern Giant petrel Macronectes halli and Southern Giant petrel M. giganteus breed sympatrically within the ecoregion, although their sympatric breeding area spreads over a wider range outside of it (Johnstone et al. 1976). Burger (1978) and Johnstone (1978) reported failed breeding attempts between these two species on Marion and Macquarie Islands respectively (the latter island outside the ecoregion). Although Johnstone et al. (1976) remarked that, on islands where sympatric breeding occurs, there is genetic isolation between the two species, they point out that on Gough Island (also outside the ecoregion), Macronectus spp. individuals show a combination of both species’ characteristics. This suggests the ability to interbreed at some sites, although the reasons behind this may be only locally relevant (ibid.).

The only two eared seals (otarids) in the region, the antarctic fur seal (Arctocephalus gazella) and the subantarctic fur seal (A. tropicalis), breed sympatrically only on the Prince Edward, Crozet and Heard Islands within the ecoregion, and on Macquarie Island outside of it (Goldsworthy and Shauhnessy 1989). Only a small percentage of the worldwide population of A. gazella is found in this ecoregion, the antarctic fur seal being otherwise exclusively confined to south of the Antarctic Convergence. Outside the ecoregion, A. tropicalis breeds at Amsterdam, Saint Paul, Gough and Tristan da Cunha Islands, all to the north. Interbreeding between A. gazella, which normally breeds south of the convergence, and A. tropicalis, which normally breeds north of it, has been reported from islands where they breed sympatrically (e.g. Hofmeyr et al. 1997; Kerley 1989). The possibility that the antarctic fur seal may be limited by its Antarctic adaptations on the more temperate climate of Marion Island has been mooted (Bester and Bartlett 1990). Bester and van Jaarsveld (1994) have also reported that the subantarctic fur seal is smaller on Marion Island than it is at more northern latitudes. There are no indigenous land mammals throughout the terrestrial Subantarctic (Smith 1987).

A number of birds that breed in the ecoregion have been classified as "Vulnerable". These are Eaton’s pintail (Anas eatoni) (an endemic), wandering albatross (Diomedea exulans), sooty albatross (Phoebetria fusca), Indian yellow-nosed albatross (Thalassarche carteri), gray-headed albatross (Thalassarche chrysostoma), rockhopper penguin (Eudyptes chrysocome), macaroni penguin (Eudyptes chrysolophus) (see Woehler and Gilbert 1990 for reports of limited, but natural hybridisation between these two congeneric crested penguins at Heard and Marion Islands), southern giant petrel (Macronectus giganteus) and white-chinned petrel (Procellaria aequinoctialis). Macronectus giganteus and Eudyptes chrysocome may possibly qualify for "Endangered" status, whilst Phoebetria fusca may even qualify as "Critically Endangered" (IUCN 2000).

Lower risk, but "near threatened" bird species found in the ecoregion are Kerguelen tern (Sterna virgata) (an endemic), northern giant petrel (Macronectus halli), the gray petrel (Procellaria cinerea) (one of the winter breeding petrels - Newton and Fugler 1989), gentoo penguin (Pygoscelis papua), light-mantled albatross (Phoebetria palpebrata) and black-browed albatross (Thassalarche melanophrys) (ibid.). All albatrosses breeding in this ecoregion have been accorded "vulnerable" or "near threatened " status. Albatrosses have "the highest proportion of threatened species in any bird family that comprises more than a single species" (Croxall and Gales 1997).

Other than those species already mentioned above, the following birds also breed in the ecoregion: subantarctic skua (Catharacta skua); kelp gull (Larus dominicanus); fulmar prion (Pachyptila crassirostris), Antarctic prion (Pachyptila desolata), thin-billed prion (Pachyptila belcheri), fairy prion (Pachyptila turtur), Antarctic tern (Sterna vittata); cape petrel (Daption capense), black-bellied storm petrel (Fregetta tropica), gray-backed storm petrel (Garrodia nereis), blue petrel (Halobaena caerulea), Wilson’s storm petrel (Oceanites oceanicus), common diving petrel (Pelecanoides urinatrix), South Georgian diving petrel (Pelecanoides georgicus), white-headed petrel (Pterodroma lessonii) and soft-plumaged petrel (Pterodroma mollis). There is a possibility that the chinstrap penguin (Pygoscelis antarctica) breeds on Heard Island (Marchant and Higgins 1990).

Only one group of oceanic islands, the Crozet group, supports a greater number of breeding seabirds than the Prince Edward Islands (Williams et al. 1979). All six species of albatrosses that breed in the ecoregion breed sympatrically on the Crozet Islands (Weimerskirch et al. 1986). 80% of the world’s population of Salvin’s prion (Pachyptila salvini salvini) is found on Île de l’Est of the Crozet group. Greater than 10% of the world’s breeding populations of sooty (P. fusca), yellow-nosed, gray-headed and wandering albatrosses, as well as macaroni penguin, are found at the Prince Edward Islands alone (Williams et al. 1979). Certain wandering albatross (Diomedea exulans) populations outside the ecoregion may not be of the same subspecies as those within the ecoregion (Prince et al. 1997, Marchant and Higgins 1990), which, if confirmed, will mean that the breeding range of by far the majority of the ecoregion’s subspecies is confined to that area. 31% of the world’s population of the circumpolar king penguin (Aptenodytes patagonicus) are found on Marion Island alone (Du Plessis et al. 1994). More than half the world’s population of king penguin is found on the Crozet archipelago (Cherel et al. 1993). The world’s largest colony of this highly specialized mesopelagic predator (Duhamel 1998) is found on Île aux Cochons of the Crozet group, with 300,000 breeding pairs (Jouventin and Micol 1995). 173,000 breeding pairs of A. patagonicus are also found on the Kerguelen Islands (Duhamel 1998).

The king penguin (Aptenodytes patagonicus) is unique among penguins, in that it has a breeding cycle that takes longer than a year to complete (Du Plessis et al. 1994). The two major predators of the king penguin are the Subantarctic skua (Catharacta skua), which takes eggs and small chicks, and the southern giant petrel (Macronectes giganteus), which can take up to 10% of the chicks (Hunter 1991). The two sibling petrel species, the northern giant petrel (Macronectes halli) and southern giant petrel (M. giganteus) (Hunter 1985) are the largest predator/scavengers in the Southern Ocean ecosystem (Hunter and Brooke 1992). Apart from their normal prey of penguins, burrowing petrels, seal carrion, cephalopods, crustaceans and fish, Punta and Herrera (1995) have observed them preying on adult imperial cormorants in Argentina, and they have been observed to seasonally access crustacean prey items from the stomachs of their fish and penguins kills at Kerguelen and Crozet Islands, leaving the rest of the original prey item untouched (Arnaud 1972). There is even a recorded instance of a M. giganteus individual killing and partly eating an immature black-browed albatross (Thalassarche melanophrys) off Newfoundland Head (outside the ecoregion - Cox 1978).

Three seals breed in the ecoregion, the southern elephant seal (Mirounga leonina) and the two fur seals, Arctocephalus gazella and A. tropicalis. In winter, large numbers of the leopard seal (Hydrurga leptonyx), a highly opportunistic predator, haul out on Heard Island (Bonner and Walton 1985). The leopard seal at Heard Island is thus often mentioned in the literature. However, despite the fact that one stillborn H. leptonyx birth was recorded on Heard Island from a female that had been confined in a cage for a month (Bonner and Walton 1985), it does not breed in the ecoregion (Clarke and Dingwall 1985). A seasonal appearance of H. leptonyx juveniles was recorded off the Kerguelen Islands between 1982 and 1986, at a locality where a small number of adults are normally present throughout the year (Borsa 1990).

There may be competition for food between the subantarctic fur seal (Arctocephalus tropicalis) and the southern elephant seal (Mirounga leonina) at certain sites within the ecoregion, for example Marion Island. Both fur seals are increasing in numbers throughout their ranges since sealing ceased (Bester 1984), and at Crozet it is reported that both species’ populations are reaching their maximum possible growth rate (Guinet et al. 1994). These seals were once driven to the brink of extinction by hunting (Kerley 1984). Interestingly, there exists a large breeding colony of the antarctic fur seal (A. gazella) on Îles Leygues of the Kerguelen group that was never harvested, which recolonized the main island after exploitation had ended (Jouventin and Micol 1995).

The Kerguelen-Heard breeding population of Mirounga leonina is one of the three main breeding populations world-wide (the other two being outside the ecoregion – Green and Burton 1993). M. leonina individuals from this breeding population have been observed moving between Vestfold Hills, and Amsterdam, Heard and the Kerguelen Islands. M. leonina individuals also move between the Crozet and Prince Edward Islands, but no movements between the Crozet and Kerguelen Islands have been recorded, suggesting that the two populations are isolated from one other (Guinet et al. 1992). Previous steep population declines of M. leonina at Marion, Prince Edward, Crozet, Kerguelen and Heard Islands (Guinet et al. 1999, Bester and Miller 1987) appear to have now ended (Guinet et al. 1999, Slip and Burton 1999).

Among other suggestions, the previous population decline of Mirounga leonina at the Crozet Islands was linked to killer whale (Orcinus orca) predation (Guinet et al. 1999). A particular pod of O. orca at the Crozet Islands has developed an interesting hunting technique whereby individuals intentionally strand themselves on the shore in order to capture prey. Juveniles are taught to strand themselves by specific, often unrelated members of the pod that specialize in such teaching duties. Once the juveniles have mastered this skill, they must still be helped back to the sea with their prey by older members for their first few hunts (Guinet and Bouvier 1995, Guinet 1991). Of the other cetaceans associated with this ecoregion, the sei whale and fin whale populations around the Prince Edward, Crozet and Kerguelen Islands have been judged to be from two single stocks (Michalev 1978)

Current Status
The most striking characteristic of this ecoregion is its relatively unspoiled nature (e.g. Smith 1987). This alone makes it worthy of intensive conservation effort. Indeed, the relative lack of data from some of the sites indicate the scarcity of visits there even by scientific staff. However, the only major Subantarctic island that is totally free from introductions of non-indigenous flora and fauna, as far as is known, is McDonald Island (Perrin 1995, Hughes 1987). Despite assertions to the contrary in the literature, Heard Island is not totally pristine, as the alien grass Poa annua has been recorded there (WCMC 1997)

Vulnerability of the islands to alien invasives
Subantarctic islands are characterised by low diversity among their indigenous biota (Smith 1987). Furthermore, especially in the case of the youngest of the islands, one can expect to find relatively simple food webs (Blankley and Grindley 1985). This means fewer species at each trophic level. Individual species may turn out to be ‘key species’, more than usually important in the ability of the ecosystem to transfer energy up the food chain. Examples of this can be found in many components of the island ecosystem. A marine example is the most widespread and abundant crustacean at the Prince Edward Islands, the partially protandric caridean, Nauticaris marionis, which seems to be vital in making oceanic primary production available to the island’s top predators, particularly some of the penguin species (see Branch et al. 1991, Perissinotto and McQuaid 1990). A littoral example is the limpet (Nacella delesserti), which has an unspecialized diet. This central prey species is the only large invertebrate grazer on Marion Island’s shores (Blankley and Branch 1985). A terrestrial example is the Marion flightless moth (Pringleophaga marioni), which is important in the release of nutrients from dead plant material, and which is currently being threatened by an invasive mammal (Hänel and Chown 1998).

On these islands, all available niches may not be filled because of the isolation in which they evolved, as well as their relative youth (Smith and Lewis-Smith 1987). Indigenous biota that do occupy niches may not be best adapted to them (Smith 1987). Such generalist biota on islands are particularly vulnerable to competition from more specialised species that evolved in more competitive continental systems (ibid.).

Types and Severity of Threats
Plant and/or animal aliens have invaded all the islands in the ecoregion, apart from McDonald Island (as far as is known). Some of these invasions have had serious effects.

Non-mammalian invasives (see Clark and Dingwall 1985 and Gremmen 1981).

Some of the most prominent alien plants on one or more of the islands of the ecoregion are Agrostis stolonifera, Poa annua, Poa pratensis, Cerastium fontanum, Rumex acetosella, Sagina apetala and Stellaria medea. Alien plants can, and do, have different effects. On Marion, some alien plants simply died out naturally after being introduced e.g. Pinus pinaster. Some aliens do not spread even though they persist e.g. Dactylis glomerata, Lolium perenne, Festuca pratensis and Poa pratensis (the first three form meadows, after having been specifically introduced to sustain the alien sheep population on the Kerguelen Islands – see Chapuis et al. 1994). Poa annua, on the other hand, is an alien plant that has colonized most Subantarctic islands including Prince Edward Island (sensu stricto) and Heard Island (WCMC 1997). It colonizes areas on Marion where there are no indigenous plant competitors, in strongly compacted soils. Its invasion of the Kerguelen Islands has shown similar trends. It’s ability to colonize new, unfilled niches was seen on South Georgia (outside the ecoregion), where it replaced the natural vegetation after the latter had first been destroyed by introduced reindeer. Similarly, on Marion, it is a prominent feature of areas that have been affected by animals. P. annua does well on islands were there are introduced herbivores (Watson 1975, cited in Gremmen 1981).

Some alien plants can also displace local plants from certain areas e.g. the aliens Agrostis stolonifera and Sagina apetala. Two floristic community types on Marion Island, which is in the main relatively unspoiled, had to actually be defined by the abundance and dominance of the aliens Poa annua and Agrostis stolinifera, despite it being a stated general rule that that particular classification system avoided the use of alien taxa as far as possible (Gremmen 1981). Although A. stolinifera does not seem to threaten native species on Marion Island, stands where it occurs can show a reduction in the mean number of indigenous plant species (Gremmen et al. 1998).

Alien invertebrates are periodically recorded from the ecoregion, some of which have become naturalized (e.g. Bartsch 1999, Hänel and Davies 1998, Chown and Language 1994). Of these many invertebrate aliens, the diamond-backed moth (Plutella xylostella) deserves special mention. It was recorded for the first time in the ecoregion on Marion Island, and its presence there could have grave effects on the endemic Kerguelen cabbage (Pringlea antiscorbutica) (Crafford and Chown 1987). Entire stands of P. antiscorbutica can be destroyed by this moth (Smith 1987). The destruction of P. antiscorbutica is being furthered by the presence of another alien, a fungus that also attacks the same plant. It is likely that insects are being introduced at the Marion Island base station along with incoming supplies (ibid.), although there is the suggestion that the introduction of P. xylostella was natural, rather than having been mediated by humans (Chown and Avenant 1992, cited in Cooper 1995a).

Attempts have been made to introduce salmonid fish into the rivers of Marion, Crozet, and Kerguelen Islands. The introductions have only been successful on Kerguelen, which has relatively large rivers (Cooper et al. 1992), although the brown trout (Salmo trutta) had to be eradicated from Marion Island (Cooper 1995b). The six acclimatized salmonid species, Salmo trutta, Salmo gairdneri, Salmo salar, Salvelinus fontinalis, Salvelinus namaycush and Oncorhynchus kisutch, have colonized diverse environments on Kerguelen (Davaine and Beall 1982). The rivers there have never supported indigenous fish. On Marion Island it has been shown that zooplankton are at the apex of the limnological food chain (Grobbelaar 1978). There has been a marked affect on the invertebrate fauna at Kerguelen by these introduced salmonids (Clark and Dingwall 1985). Notwithstanding this fact, feasibility studies have been done on the prospects of farming the coho salmon (Oncorhynchus kisutch) at the Kerguelen archipelago. Eggs are imported from the USA and reared for about ten months before release into local rivers. Since 1987, imported eggs and locally collected progeny have been alternately reared and released (Davaine 1991).

A non-mammalian vertebrate alien, the mallard (Anas platyrhynchos), also occurs on the Kerguelen Islands.

Mammalian invasives (see Dingwall 1995, Jouventin and Micol 1995, Chapuis et al. 1994)

Mammalian alien invasions outside the French islands have been largely minimal. The Australian islands are not affected, and neither is Prince Edward Island (sensu stricto). A number of mammals have been introduced to Marion Island over the years, of which only the house mouse (Mus musculus) remains. The domestic cat (Felis catus) was only recently eradicated, an achievement of note that has been examined by other Subantarctic island authorities with interest (Chapuis et al. 1994). Felis catus also probably caused the most ecological damage of all the invaders on Marion (e.g. van Rensburg and Bester 1988). This damage is still evident, and all but the most recent literature on the island habitually refers to its presence. The cat on Marion Island therefore deserves special mention in this section on persistent mammalian invasives.

Felis catus was introduced to Marion Island in 1949 in an attempt to control mice at the island’s permanent meteorological station. A rapid increase in cat numbers, and the fact that the preferred prey of cats were birds, not mice, drove one indigenous bird species (the common diving petrel (Pelecanoides urinatrix)) to local extinction, and three other bird species (the great-winged petrel (Pterodroma macroptera), the soft-plumaged petrel (Pterodroma mollis) and the gray petrel (Procellaria cinerea)) to near local extinction. Mice were only consumed in greater quantities at times of the year when birds were less abundant (Bloomer and Bester 1990). A highly successful campaign was conducted by the South African authorities, during which the cats were alternatively trapped, subjected to a viral disease, poisoned and shot, and which finally saw the first successful eradication of cats from a Subantarctic island ever achieved (e.g. Bester and Buchner 2000, Cooper et al. 1995). Marion Island is also the largest island in the world where this has ever been done (Bloomer and Bester 1992, cited in Cooper 1995a). The last time a cat was seen on the island was in 1991, and burrowing petrels, which were severely affected by the cats, have now shown signs of recovery (Cooper et al. 1995, Newton and Fugler 1989). Pelecanoides urinatrix is now also breeding on Marion Island again (Hänel and Chown 1998).

The only remaining invasive on Marion, the omnivorous house mouse (Mus musculus), was probably introduced during the early 1800s. Importantly, it eats macro-invertebrates. The island’s macro-invertebrates, such as moths, weevils, earthworms, snails and slugs, are important in the release of inorganic nutrients contained in plant litter (Smith and Steenkamp 1992). The effects of Mus musculus presence could be numerous. M. musculus numbers may be increasing through a combination of climatic warming and the removal of Felis catus. This in turn may be placing additional pressure on the remaining macro-invertebrate prey of Chionis minor, affecting the bird negatively (Huyser et al. 2000). Already, fewer adult Pringleophaga marioni moths are being found on Marion Island (Hänel and Chown 1998). Because macro-invertebrates are vital for nutrient recycling on the island, fewer available macro-invertebrates may also affect plant communities. Mice also physically damage plants themselves e.g. Azorella selago.

On the French Islands, many more mammalian invaders are present, five of which were deliberately introduced (cats, rabbits, reindeer, sheep and the Corsican mouflon).

The European rabbit (Oryctolagus cuniculus) occurs on both French archipelagos, to which it was introduced in the 1800s. It destroys indigenous vegetation (e.g. Pringlea antiscorbutica and Azorella selago), as well as the reproduction sites of burrowing petrels. The main effect of the O. cuniculus is indirect, though, as it provides a food source for cats during winter. This allows the cat population to remain relatively high outside the bird breeding season, resulting in a higher impact on the avifauna the next year.

The two rodents, the house mouse (Mus musculus) and the black rat (Rattus rattus) were both introduced to the Crozet archipelago in the 1800s. Mus musculus was introduced to the Kerguelen archipelago at around the same time as it was at Crozet, but Rattus rattus only found its way to the Kerguelen archipelago in the 1900s. On Crozet, Rattus rattus preys on the chicks and eggs of small petrels, and may have been responsible for the disappearance of the blue petrel (Halobaena caerulea), the gray-backed storm petrel (Garrodia nereis) and the Kerguelen diving petrel (Lugensa brevirostris) on Île de la Possession, to which island the rat is confined. Other petrel species have shown reduced survival on this island. However, those species that are able to breed above 400m at Île de la Possession, can do so in large numbers. On Kerguelen, it has been noted that rats are mostly concentrated near the human settlement, and therefore cause little damage to the avifauna there (Jouventin and Micol 1995).

Felis catus was introduced in 1956 to the Kerguelen archipelago. As on Marion Island, Felis catus was deliberately introduced to the Kerguelen Islands in an attempt to control rodents. As it did on Marion Island, it preys primarily on burrowing petrels instead, although introduced mice and rabbits do supplement its diet. It has not caused local avifaunal extinctions on the Kerguelen archipelago because bird colonies survive off the main island (Weimerskirch et al. 1988). Île aux Cochons is the only island of the Crozet archipelago to which Felis catus has been introduced. Felis catus was introduced to this island before 1887, and has subsequently caused the local extinction of the white-chinned petrel (Procellaria aequinoctialis), the gray petrel (Procellaria cinerea), and the gadfly petrels. Felis catus has been specifically linked to the "vulnerable" status accorded the endemic Eaton’s pintail (Anas eatoni) (IUCN 2000), which occurs only on these two French archipelagos (see Table 1). It does not enter the burrows of small petrels, as they are too narrow, and must instead rely on capturing this prey when it arrives or leaves the colony. This allows the smaller Salvin’s prion (Pachyptila salvini), fairy prion (Pachyptila turtur) and South Georgian diving petrel (Pelecanoides georgicus) to remain numerous on Île aux Cochons.

The bizet sheep (Ovis aries) (Reale et al. 2000) still occurs on the Kerguelen archipelago, where it was introduced in 1952. It is utilized as food for the scientific station. In 1972, various alien fodder grasses were introduced in order to sustain increased numbers of this species. Ovis aries tramples Azorella selago, which is causing that plant’s disappearance. Another plant that is affected by Ovis aries is Pringlea antiscorbutica, which has already been eliminated in areas where either Ovis aries or another alien herbivore, the Corsican mouflon (Ovis ammon musimon), are found.

The Corsican mouflon (Ovis ammon musimon) was introduced to one small island in the Kerguelen archipelago in 1957. Since the 1970s the mouflon has been regularly hunted for sport. O. ammon musimon has suffered from competition for resources with the introduced reindeer (Rangifer tarandus), as well as hunting, but has benefited from the departure of R. tarandus from its original introduction point, as well as the introduction of the fodder grasses mentioned above. R. tarandus was introduced during the 1950s to two islands of the Kerguelen archipelago, from whence all reindeer gradually migrated to Grande Terre. R. tarandus causes significant damage to Azorella selago, which is its main food source. It virtually eliminated lichens in areas where it was found, but having now left those areas, the lichen populations are recovering. There do not appear to be any serious future plans to control R. tarandus, and as a result of this, more ecological damage from this species can be expected (Jouventin and Micol 1995).

Chapuis et al. (1994) best describe the situation on the French islands. "The three main rules for introductions defined by Pimm (1987) have been broken: (1) introduction in the absence of predators and competitors; (2) introduction of highly polyphagous species (herbivores); (3) introduction in ‘relatively simple communities where the removal of a few plant species will cause the collapse of entire food chains.’" However, this commentary on the modification of the French territories must conclude by noting that some of the smaller islands within these two archipelagos remain unmodified by alien mammals. The environments of many of these smaller islands have also been afforded legal protection and conservation status.

Long-line fishing
There is a rapidly expanding long-line fishing industry directed at the Patagonian toothfish (Dissostichus eleginoides) in the Southern Ocean (Yau et al. 1998), which is marketed as Chilean sea bass (Bateman and Rothwell 1998). This fish, for which there is an extremely lucrative market, was only discovered and named in the early 1990s (Lugten 1997). It has a wide range, which covers a number of political zones (Reilly and Ward 1999). Since 1996, there has been widespread illegal fishing of this resource (Burke 1998). Many fishing boats fly flags of convenience or are unflagged (Bateman and Rothwell 1998). Heavily implicated in this illegal fishery are Chilean operators, in which country there exists a highly developed illegal network to process the fish products (Cardenas and Melillanca 1999).

Concentrated numbers of Dissostichus eleginoides are found around Subantarctic islands (Lugten 1997), including Heard and the Prince Edward, Crozet, Kerguelen Islands within the ecoregion (Zhivov and Krivoruchko 1990). The level of fishing taking place is threatening the sustainability of the resource (Burke 1998). At Heard Island, it has been suggested that the D. eleginoides fishery will most likely affect the southern elephant seal (Mirounga leonina), which has already seen a dramatic population decrease, possibly due to over fishing around the Kerguelen Islands (Green et al. 1998). Over fishing has been linked to M. leonina population declines elsewhere in the ecoregion (Bester and Miller 1987), although it should be mentioned again here that the latest studies suggest that Mirounga leonina populations are stabilizing at all sites (Guinet et al. 1999, Slip and Burton 1999).

Biologically significant numbers of birds are killed by hooks laid by long-line fisheries (Ryan and Boix-Hinzen 1998). This is especially true of uncontrolled fisheries such as this, where mitigation measures (see below) are obviously not in place. Particularly affected are albatrosses (Croxall and Gales 1997, Gales 1993). Diomedea exulans populations at Crozet and Kerguelen have showed a slow increase since 1986, since the Japanese fishing effort for the southern blue tuna (Thunnus maccoyii) was decreased in this region (Weimerskirch et al. 1997). However, given that younger and more naïve birds would be more affected, there should be a time lag of 5-10 years before a decrease in the wandering albatross’ breeding population is reflected (Moloney et al. 1994). This suggests that perhaps the consequences of the recently developed intense Dissostichus eleginoides fishery must still be fully reflected by breeding populations of Diomedea exulans. There is some evidence that foraging wandering albatrosses follow the known movements of long-line fishing fleets (Tuck et al. 1999).

Diving species such as the gray-headed albatross (Thalassarche chrysostoma) and particularly the white-chinned petrel (Procellaria aequinoctialis) are very vulnerable to the fishery (Cherel et al. 1996). Certainly, P. aequinoctialis forms the bulk of the avian by-catch in long-line fisheries (Weimerskirch et al. 1999). P. aequinoctialis already has a strong natural tendency to follow Orcinus orca pods in order to scavenge from their kills. The black-browed albatross (Thalassarche melanophrys) also does this, but to a lesser extent (Ridoux 1987). Such natural habits make these pelagic birds more vulnerable to humans mimicking O. orca hunting activity. Studies on Crozet Island have shown that P. aequinoctialis forages widely, and that, during incubation, its average foraging range is the longest recorded for any seabird, foraging well beyond the Southern Ocean (Weimerskirch et al. 1999). This makes P. aequinoctialis, along with other birds that forage widely, particularly vulnerable to fishing activity outside protected areas. P. aequinoctialis can also forage at night, reducing the effectiveness of nighttime fishing, which has been used in the legal fishery to reduce fishery-induced albatross mortality.

The population declines of Thalassarche melanophrys at the Kerguelen Islands have also been linked to fishing activity (Dalziell and Poorter 1993), and it too has been recorded as a ship following species (Cherel et al. 1996). The fisheries are also a suggested cause for the decline in Phoebetria fusca and Thalassarche carteri populations (IUCN 2000). Apart from these three albatrosses, it is suspected that the giant petrels (Macronectes spp.) are also being negatively affected by the fishery (Cherel et al. 1996, Dalziell and Poorter 1993).

Other threats to the ecoregion (see Cooper 1995a, Jouventin and Micol 1995)
Pollution: Potentially harmful releases of PCBs into the atmosphere through the burning of plastic waste by island researchers, potential leaks from fuel tanks on island stations, entanglement by and ingestion of fishing debris by the islands’ fauna, and oil pollution (in 1992 about 200 oiled Aptenodytes patagonicus penguins were found on the Crozet archipelago, and in 1980 a Soviet tanker spilt 600 tonnes of petroleum after running aground north of Grande Terre).

Disturbance of King Penguin colonies: Aptenodytes patagonicus are extremely susceptible to noise stress, even showing extreme reactions to thunderstorms (Gauthier et al. 1998). Human induced examples of this include building activities that took place in the middle of a colony of A. patagonicus on Île de la Possession of the Crozet group, which caused the breeding birds to move elsewhere. It has been suggested that the many King Penguins found dead in 1990 on Macquarie Island (outside the ecoregion) after an aircraft had flown by was caused by panic. Similar panic, although no recorded deaths, has been observed on Marion Island (Cooper et al. 1994).

Other human activity: Collection of artefacts by island based personnel and research huts, the latter serving as focal points for the introduction and spread of alien biota.

Global warming: Smith (1991) has warned that the effects of climate change on Marion Island could result in opportunities for new organisms to colonise, and affect the breeding success of birds. The same is true of the other Subantarctic islands. Also possible is the release of current environmental constraints on already introduced aliens (e.g. Mus musculus on Marion Island). It has also been shown that the breeding performances of some seals and birds in the ecoregion are reduced by El Niño Southern Oscillation (ENSO) events (Guinet et al. 1998, Guinet et al. 1994).

Justification of Ecoregion Delineation
Benninghoff (1987) lists three geobotanical regions that comprise the Antarctic Terrestrial Biome. One of these is the Subantarctic Geobotanical Region. The Subantarctic Geobotanical Region is further subdivided into three provinces, one of which is the South Indian Ocean Province (Lewis-Smith 1984, but see also Smith and Lewis-Smith 1987). The South Indian Ocean Province coincides exactly with the Indian Ocean Subantarctic Tundra ecoregion considered here. This group of islands was once known as the Kerguelen Province of the Subantarctic zone (van Zinderen Bakker et al. 1971). The name "Kerguelen Province" is still commonly used in the literature (e.g. Zdzitowiecki and Pisano 1996, Branch et al. 1991), but Knox (1977) lists a number of different definitions of the Kerguelen Province, based on different biological criteria.

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