Alien invaders

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From the cute and seemingly harmless grey squirrel shredding Britain's woodlands, to the chick-eating mouse threatening the bird population, Michael McCarthy hunts out the predators that wouldn't be there without us

House mouse

Arrived Originally from across Eurasia. The second most populous mammal species after man, mice (Mus musculus) have been transported all over the world by humans whenever they have moved.

Damage assessment Mice do considerable damage by destroying crops and consuming or contaminating human food supplies. But this week came news of particularly horrific effect of mice as alien species: on remote Gough Island, in the South Atlantic, mice, which probably came ashore from ships in the 19th century, are eating alive the large but defenceless chicks of big seabirds such as petrels and albatrosses.

Eradication efforts A constant battle. Build a better mousetrap, and the world will certainly beat a path to your door.

Zebra mussel

Arrived A small mollusc native to the Caspian and Black seas, the zebra mussel (Dreissena polymorpha) reached the US and Canada via ocean-going vessels that emptied their ballast tanks in the ports of the Great Lakes. They were first discovered in the US in 1988, and have also been found in Western European waters.

Damage assessment The exploding population of zebra mussels has carpeted some parts of the Mississippi river bed with thousands of mussels per square yard, and is devastating native American mussel populations. They also cause great economic damage by blocking the water intakes of power stations.

Eradication efforts The US authorities are still scratching their heads as they search for an effective way of ridding waterways of this fast-breeding species.

Signal crayfish

Arrived Pacifastacus leniusculus is a North American freshwater species that was brought into Britain and other European countries by crayfish farmers in the 1970s. It has since escaped into the wild.

Damage assessment Implicated in the steep decline of Britain's native white-clawed crayfish (Austropotamobius pallipes), which has disappeared from many rivers. The American species is larger, reproduces more rapidly, is more resistant to disease and generally out-competes its rival. It also upsets the ecosystem by eating more invertebrates and fish eggs.

Eradication efforts Restrictions on crayfish farming have been brought in to keep the invaders out of unaffected areas, but it seems unlikely that they can be removed from Britain. Trapping is being trialled.

Muntjac deer

Arrived A native of China and South-east Asia, the Muntjac (Muntiacus reevesi) was introduced into Woburn Park in 1900 by the 11th Duke of Bedford, and also released into the area near Whipsnade Zoo in 1921 by persons unknown.

Damage assessment This is our smallest deer and is now probably our most successful in breeding terms. Its numbers have soared and it is doing widespread damage by eating the "understorey" of woodlands - the bushes, shrubs and flowers under the main woodland trees.

Eradication efforts In some woodlands, muntjac numbers are kept under control by shooting.

Grey squirrel

Arrived A native of North America, animal fanciers and collectors brought the grey squirrel (Sciurus carolinensis) to Britain in the mid-19th century, probably initially in 1867 at Woburn in Bedfordshire.

Damage assessment The grey squirrel has driven the native British red squirrel to the brink of extinction in most parts of England by out-competing it for winter food and by carrying the parapox virus, which is harmless to the grey but kills the red. Its mushrooming numbers are doing huge damage to woodlands (the males strip bark off young trees), and it is thought to prey on eggs and young birds.

Eradication efforts Regular culling goes on in many parks, ornamental woodlands and forests, but grey squirrels keep bouncing back. The possibility of using contraceptive drugs is being investigated.

Cane toad

Arrived A large amphibian native to Central and South America, Bufo marinus was introduced to Australia in 1935 by sugar cane farmerse in an attempt to control two beetle pests. The experiment failed and the toad has now spread in its millions throughout the north-east of the country.

Damage assessment Populations of some native Australian amphibians have been seriously affected by competition from cane toads, which will eat almost any living thing, including mammals and birds. They are poisonous at all stages of their development and larger animals preying on them are likely to die.

Eradication efforts As yet no official eradication programmes have been implemented, but Australian scientists are now looking at the possibility of biological control.

Domestic pig

Arrived The domesticated version of the Eurasian wild boar (Sus scrofa domestica) was exported to all parts of the world by farming.

Damage assessment Pigs can cause great harm to crops and native vegetation wherever they are found. But perhaps the most spectacular example of the environmental damage was the role the pig is believed to have played in the extinction of the dodo, the huge flightless pigeon of Mauritius. The bird was first seen in the early 1500s and was gone by 1680; pigs introduced to the island are thought to have succeeded in destroying its nests and eggs by grubbing about in the Mauritian undergrowth.

Eradication efforts It is not difficult to get rid of pigs, but it was too late for the dodo.

American mink

Arrived A native of North America, Mustela vison was originally brought to Britain and much of Western Europe by fur farmers in the 1920s, when a mink coat was every woman's dream. Over the succeeding years, many escaped, and later, animal libbers released them deliberately from fur farms. Breeding in the wild was proven in the 1950s.

Damage assessment The American mink has virtually wiped out the water vole in England, and the island-nesting sea birds from many of the sea lochs of Scotland, because it has filled a niche in Britain for a small aquatic predator. It is an excellent swimmer (unlike stoats and weasels) and voraciously kills animals and birds (unlike otters).

Eradication efforts Targeted trapping in some areas to protect water vole populations. Some conservationists would like a national campaign of eradication, but this seems unlikely on the grounds of practical difficulty and immense cost.

Brown/black rat

Arrived Brown and black rats (Rattus norvegicus and Rattus rattus) were introduced to many islands across the world, from Lundy in the Bristol Channel to the islands of the Pacific, by visiting ships from the 16th century onwards.

Damage assessment The two major rat species between them have spread throughout the world and will feed on almost anything edible. But the worst environmental damage they have done is to wipe out the unique bird species from a series of Pacific islands.

Eradication efforts Rats can be eradicated given time, money and effort, and have been on Lundy, allowing puffins to breed again.

Rabbit

Arrived A native of Western Europe, Oryctolagus cuniculus was exported to Australia by emigrants from Britain who wanted a source of food and sport. The population exploded when Thomas Austin released 24 wild rabbits on to his Victoria property in 1859.

Damage assessment Rabbits have become among the foremost environmental pests in Australia, with numbers reaching "plague proportions" before myxomatosis struck. They have been a chief cause of habitat destruction, native flora and fauna extinction, land degradation and crop destruction.

Eradication efforts Myxomatosis, a benign disease of the American cottontail rabbit transmitted by fleas and mosquitos, is fatal to the European rabbit and was deliberately introduced to Australia as a control measure in the 1950s. In the first two years it reduced the rabbit population from 600 million to less than 100 million, but populations have since risen as resistance has developed. More biological control methods are being considered.

Domestic cat

Arrived Cats (Felis catus) in various forms occur naturally worldwide, except in Australia. The house cat was domesticated in the eastern Mediterranean about 3,000 years ago and introduced to many islands around the world by visiting ships from the 18th and 19th centuries onwards.

Damage assessment As with rats, the introduction of domestic cats has been catastrophic on islands where endemic bird species have developed with no natural predators, especially when the cats become feral, or semi-wild. The kakapo, the flightless parrot of New Zealand, has been brought to the edge of extinction by rats and cats.

Eradication efforts Cats are big and visible enough to be eradicated successfully from island habitats.

Coypu

Arrived This large, semi-aquatic rodent (Myocastor coypus) is a native of South America and was brought to eastern England by fur farmers in the 1930s. The animals later escaped into the wild.

Damage assessment Coypus became established in the Norfolk Broads and the Fens and caused significant damage by burrowing into dykes and river banks and eating root crops.

Eradication efforts A rare example of a completely successful eradication. In 1964 the former Ministry of Agriculture started trapping coypus in East Anglia. In the late 1970s, the campaign was stepped up, and the last individual from a population of more than 75,000 was trapped and shot in 1986.

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