The problem of introduced species

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The French islands of the Southern oceans are subject to French legislation –in particular to the law of 10/07/1976 concerning nature conservation. They harbour the highest species diversity of invertebrates and plants of all the subantarctic islands. The wild vertebrates living there make up Earth’s richest biomasses. Their plants and animals show some quite original adaptations. These developed in complete isolation over several million years of evolution, in the middle of the Southern Ocean, thousands of kilometres from any continent.

On the whole, absence of industrial activity, controlled management of scientific research and precautions taken in research stations’ operations help protect the natural ecosystems.

However, the introduction of allochthonous species in former times has sometimes had disastrous consequences for local ecosystems. Programmes for the reduction or even eradication of introduced species are conducted aiming to restore degraded natural habitats. Other measures, like a ban on the introduction of domestic animals or specimens of non-indigenous animal or plant species, declared by 2 TAAF orders in 2001, have also been taken.

Until relatively recently a herd of feral cattle was still living on Amsterdam Island. They amounted to 2000 head in 1987. They were descendants of a group from a breeding project initiated in 1871 by a farmer from Réunion, Heurtin. Many escaped, eventually occupying all parts of the island, severely altering the natural balance of the local environment. This especially affected the flora but also threatened the survival of some bird populations. Consequently after several culling campaigns to control numbers, the TAAF decided to eliminate the whole herd, after consultation and agreement by scientists.
Other terrestrial mammals have been introduced:

Mice which appear to have adopted an exclusively herbivorous diet.
Rats which feed on eggs and young chicks in summer but switch to a plant diet in winter.
Cats which feed mainly to the detriment of birds, mice and rats.

The only bird concerned is the common waxbill (Estrilda astrild), a passerine from La Réunion introduced in 1977. Around 100 individuals nest near the Martin-de-Viviès base.

Sub-Amsterdam-Faune-RomainBAZIRE-2014-IPEV-5As for the flora, the confinement of Phylica (Phylica arborea) (island Cape myrtle) to areas inaccessible to cattle suggests that those animals exerted strong pressure on trees and shrubs. Another effect is severe soil degradation in areas formerly under pasture, leaving the rock bare in places.

Impact on the fauna is perhaps signalled by numerous subfossil bone deposits of the Amsterdam albatros over the pasture area. This suggests that disturbance by the cattle (compaction and trampling of the biotope, nests, etc.) forced these birds to keep to high altitude on the Plateau des Tourbières area. A rehabilitation programme for Amsterdam Island has been under way since 1987.

The introduction of rabbits but especially of rats, which probably arrived with the first fishing boats in the 18th Century, severely destabilized the ecosystem of Saint-Paul Island. Bird nesting declined considerably. At the end of the 1990s the rat population was estimated at between 50 000 and 100 000 (only a large offshore rock, La Quille, separated from the isle by a narrow channel, was spared). European funds allowed complete eradication of the rats in 1999. Since then the sea-bird population has steadily built up again, partly from colonies that had taken refuge on La Quille.

The introduction of allochthonous species (mice and rats, then cats used to hunt them) in the Crozet islands caused serious damage to the original ecosystem. Predation on eggs and small species of petrels had a particularly strong impact. Pigs introduced earlier onto Pig Island and goats brought onto Possession Island –both as a source of food– have been eliminated. The plant species introduced, whether on purpose or otherwise, such as dandelion or pearlwort, often spread widely and crowd out local species.

Since the Alfred Faure station was established, the number of plant species introduced has never ceased to rise. There are about 60 to date, but only 10 species have managed to colonize areas outside the base: 2 mouse-ears (Cerastium fontanum and C. glomeratum), a small rush species (Juncus bufonius), 2 grasses (Poa annua and P. pratensis), sheep’s sorrel (Rumex acetosella) and a small Caryophyllaceae (Sagina procumbens) have spread widely over the whole island. Dandelions (Taraxacum erythrospermum and T. officinale) Sub-Crozet-Science-AnaelleATAMANIUK-2012-IPEV-248and a stitchwort (Stellaria alsine) are still confined to a small perimeter on the margins of the station. This stitchwort is the most aggressive of the plants introduced. It forms monospecific patches that keep out autochthonous plants. The other plants introduced pose lower risks for the autochthonous flora because their priority is to survive under the prevailing climatic conditions. Nevertheless, climate warming of just a few degrees would enable these plants to complete their reproduction cycle and produce more abundant seeds. They would then have a stronger spreading capacity and could become a problem.

Many land mammals have been introduced into Kerguelen: sheep, mouflons, reindeer, rabbits (introduced in 1874 from South Africa), rats and cats. A tentative introduction of mink was a failure, fortunately. The brown trout was introduced 20 or 30 years ago in the lakes and it is also encountered in brackish water, around river mouths.

Rabbits introduced quickly multiplied in the Kerguelen Islands causing the destruction of the existing carpets of Azorella which were crowded out by a monospecific meadow of Acaena which replaced that species. Now the only sites with carpets of Azorella are on rabbit-free islands and islets. The Kerguelen cabbage almost suffered the same fate. Other populations of introduced mammals impact on the vegetation, including mice eating seeds of that species, or reindeer feeding on lichens.

A flock of Bizet sheep surviving on Île Longue is the largest in the world of that race, originally from Cantal -a paradox because in France it is under threat! A couple of Corsican mouflons coming from the zoo at Vincennes was introduced to Île Haute in the 1950s. Ten reindeer from Sweden were introduced in 1955-56 on Île Haute. They escaped in 1981, by swimming over to Grande Terre. Now their population is an estimated 4000 head.

Sub-Kerguelen-Science-FabienEGAL-2014-IPEV-3The cats were introduced in 1950 to stave off the spread of rat populations which themselves were introduced unintentionally by whalers in the 19th Century. Unfortunately, petrels are easier to hunt than rats. In spite of the difficulties encountered for acclimatizing, thanks to these birds some cats have managed to settle definitively and are now feral. Some hunters have made attempts to eliminate them, but in vain. The felines started to multiply and the petrel population dramatically declined. When petrel numbers were no longer high enough to sustain the cats, the cats ate the rabbits. A new balance appeared between the cat and rabbit populations, to the detriment of the petrel species. Fortunately, as cats cannot swim, the petrels were able to survive on the nearby islets.

The Kerguelen cat population is the subject of an advanced study project. The pressures of selection and adaptation to cold they underwent when introduced and their present confinement hold a wealth of information for biologists.