The Prince Edward Islands house many species that do not occur anywhere else or only on one or two similar islands. The introduction of alien species is the single largest conservation concern at the islands as their isolation has prevented many common continental species from reaching them. Introduced species can have a catastrophic impacts on the islands. Unfortunately this scenario has already happened on Marion island, luckily Prince Edward Island has been more fortunate to date.

Impact of Feral Cats
Impact of Mice
Impact of Introduced Plants and Insects
Impacts Offshore
What is Being Done?

Impact of Feral Cats

The introduction, and consequent impact on the avifauna, of cats to Marion Island is considered to be one of the biggest conservation issues on Marion Island. The intial idea surrounding cats being brought to the Island was due to the plague of mice that early teams were exposed to. These mice were originally introduced with the early sealers and since their introduction have constantly plagued the early Marion teams. This problem was tackled by one of the earlier teams whereby they introduced five cats to Marion base. Of these five cats one was a castrated male tabby and a female (introduced in January 1949) followed by a litter of three kittens brought to the Island in August of that year. The result was that cat numbers multiplied and became feral (first wild cat seen in 1951 approx. 12km West of the Base station). By 1975 there were more than 2000 cats which were feeding on burrowing petrels around the island (a far easier prey than the mice at the Marion base). In consequence these cats killed approximately 500 000 birds in 1975 alone. The effect was devastating, the common Diving Petrel (Pelecanoides urinatrix) was exterminated and three other species were virtually exterminated - namely the Greatwinged Petrel (Pterodrama macroptera), the Softplumaged Petrel (P. mollis) and the Grey Petrel (Procellaria cinerea). Fortunately the problem surrounding the impacts of the feral cats was addressed and in consequence a control programme using a viral disease, hunting, trapping and ultimately poisining was enforced and by 1991 more than 3000 cats had been killed. Since 1991 not a single cat has been seen on Marion Island making this eradication programme one of the most successful eradication programmes ever.

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Impact of Mice

Mice feed off the small insect species (esspecially the larvae of the Marion flightless moths) and in consequence may effectively be closing off the nutrient bottleneck at Marion island which in due course will severly affect ecosystem functioning. It has also been noted that mice are capable of changing the plant communities and other ecosystem processes by removing and eating the seeds of certain plants (thus preventing natural propagation) and by burrowing into vegetation (this often causes plants such as Azorella selago cushions to die off). This impact generated by the mice has been more pronounced in recent years, an attribute associated to a visible increase in the numbers and activity of mice which has been associated with the general warming of the island (induced by global climate change).The predation by the mice have been considered to be a contributing factor to not only the declines in the adults flightless moth (Pringleophaga marioni) and small insect species, but also the decline in sheathbill numbers at Marion Island over the past 20 years. The large difference in sheathbill numbers between Marion Island and Prince Edward Island are considered to be indirectly due to the impacts of mice. Due to the fact that sheathbills feed on invertebrates and are relient on this food source to survive over winter, any reduction in invertebrate numbers as a result of mice predation would have a negative influence on sheathbills. Unfortunately, mice still pose a problem and are still present in large numbers.

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Impacts of Introduced Plants and Insects

The terrestrial ecosystem on Marion Island is also affected by various introduce plant and insect species. Two species in particular have had a major affect namely, Agrostis stolonifera (an introduced grass species) and Plutella xylostella (the introduced Diamond-backed moth). Agrostis stolonifera, a common, pale yellow, longish grass species on the east coast of the island was introduced with sheep fodder. This species is an aggressive species that is invading habitats previously dominated by the dwarf-shrub Aceana magellanica. Wherever Agrostis stolonifera occurs, it outcompetes most of the indigenous plant species and forms a monoculture, compared to the diverse communities it replaces. Plutella xylostella feeds voraciously on Kerguelen cabbages and in conjunction with an introduced fungus it is posing a significant threat to this unique cabbage species.

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Impacts Offshore

The conservation issue surrounding Marion Island and Prince Edward Island is not restricted to the islands themselves. A more recent and equally important conservation issue is the fishing of Patagonian toothfish around the islands. The issue surrounding the fishing is the fact that many of the fishing vessels use methods (long lines with thousands of large, baited hooks) which are resulting in many birds being caught. The species which are the most severly affected are the small albatrosses (Grey-heade and Yellow-nosed Albatrosses) as well as the Whitechinned Petrels. Those vessels that are licenced take care in reducing the incidental mortality of seabirds as far as possible, however there are many vessels which are not licenced to fish in this area by the South African government and who pay scant regard to the regulations.

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What is Being Done?

Major care is taken to ensure that visitors and cargo destined for the island are "alien" free. Essentially this involves that all visitors to the island are required to inspect all their clothing for seeds etc., for boots to be washed and for for all cargo to be rodent free. This forms the major reason why so much care is taken to ensure that goods taken to the island is as clean as possible and why any visits to Prince Edward Island require that new clothig are issued and that helicopters are scanned and cleaned before visiting the island. This is to ensure that Prince Edward Island remains free of alien species that occur only on Marion (i.e. mice, cabbage moth and the majority of alien plants). The prevention of alien introductions to Marion Island is of high priority, and where introductions have ocurred attempts are being made to restrict these species so that they do not spread around the island. With all introductions come monitoring procedures of the species and attempts to understand their biology and physiology so that predictions can be made as to what potential they have to spread, what the result of the interactions of the alien and indigenous species are (i.e are there negative consequences as a result of this interaction), and to what effect the changing climate will have on these interactions. Alien invasions are of great concern and hence much effort is placed on research to try and give answers to these questions (see Research Projects).

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