The latest invader is the Chinese mitten crab and the number and size of specimens taken from the Thames in London over the past two months indicate an explosion in the size of its population.
Specialists at the Natural History Museum in south Kensington, London, who are trying to assess the crab's dramatic emergence believe it could pose a serious threat to the native crayfish. Some crabs studied at the museum could straddle a 9in dinner plate.
Named after the "furry" mittens which cover most of its sizeable pincers, the Chinese crab is the latest in a Noah's ark of creatures that have established themselves in Britain since Roman times.
Some, such as the rabbit, are not thought of as alien at all, although it was introduced by the Normans. But the gaudy parakeets which flash green between the tree tops in west Kent look distinctly foreign.
The newcomers are introduced either by accident, as in the case of the mitten crab which is thought to have been discharged into the Thames estuary from a ship's ballast tanks, or on purpose, mainly for food and sport - the rabbit, pheasant, French partridge and carp came by this route.
Escapees are a large and growing group. Some have got in the wild by genuine accident, and probably the parakeets are this category. But others, such as the red-eared terrapins which appeared in strength in ponds as the Ninja Mutant Turtles craze faded, were freed "accidentally on purpose".
Little bigger than a watch-face in their aquariums, the liberated terrapins grew and grew, devouring the native newts and even small water fowl. Such is the risk that makes English Nature, the Government's adviser, extremely wary about the introduction of non-native species.
"It's unnatural," Martin Tither, a spokesman for English Nature, said. "We don't know in advance what the effect on native animals and plants will be. And by the time we do, it can be extremely difficult to restore the balance."
English Nature is one of several bodies trying to safeguard the red squirrel from its more powerful foreign cousin, the grey, which has swept it from most of lowland Britain. The agency is also taking part in trials on culling the ruddy duck, introduced from America to wildfowl reserves in the Fifties. In a kind of feathered sex tourism, the British-based male ruddy ducks are upsetting Spanish conservationists by flying to Iberia and breeding with the threatened white-tailed duck.
Culls have taken place in London parks of the Canada goose, introduced to grace ornamental lakes but now a messy nuisance and aggressive competitor for the grazing of native geese. Canada geese are prodigious defecators, making lakesides smelly and hazardous, and in groups they can frighten small children.
Mitten crabs have been found infrequently in ones and twos in the Thames since 1934. But quite suddenly the river seems infested. Funded by the Environment Agency, the Natural History Museum and Queen Mary and Westfield College, London, have begun a three-month study to assess the extent of the invasion.
In less than a day, researchers caught 200 small crabs at Chelsea. On a second visit their haul was down to 20, but consisted of larger specimens, and then last Thursday 40 to 60 were collected. Again they were larger.
Paul Clark, of the museum, believes that the researchers have seen migrations - the larger crabs going downstream to mate and hatch their larvae in salt water and the smaller ones heading upstream to spend most of their life in fresh water.
It is in the upper reaches of the Thames basin that the crab poses a worrying threat to the native crayfish. Mittens have been found as far up as Thames Ditton and Teddington; in the Roding, Essex, and in the Medway and Darent in Kent.
It is not as if the poor crayfish is not under enough pressure. On top of drought, pollution and water abstraction it has also to contend with four species of larger crayfish which were previously introduced to Britain for eating.
"There are definitely some people spreading these crayfish around on purpose. It's environmental vandalism in the extreme," Mr Clark said. "I have seen samples of the Turkish crayfish in Epping Forest. People try to trap them and sell them to restaurants."
However, not all the aliens are equally unwelcome, and some do not prosper. The Peak District wallabies have been a curiosity since their forebears escaped from a zoo during the Second World War, but the last known sighting was in March - a single wallaby, sadly emaciated.
The Natural History Museum is appealing for the public's help with the mitten crab study. Anyone who finds a crab should telephone the museum before 15 November on 0171 938 9402 or 0171 938 8984.
'Environmental vandalism' of the imported wildlife
Name: North American signal crayfish (Pacifastacus leniusculus).
Origin: Washington state, North America. Introduced in the mid-Seventies for aquaculture purposes.
Distribution: Rivers throughout England and Wales, not yet in Scotland.
Estimated population: Many thousands (accurate figures are not available because of the rapid spread).
Effect: May be spreading "crayfish plague" to English freshwater crayfish (Austropotamobius pallipes; pictured right).
Name: Coypu (Myocastor coypus).
Origin: Native to southern South America, especially Chile. Imported to East Anglia in 1929.
Distribution: Mainly East Anglia.
Estimated population: A disputed peak of 200,000 around 1960 has been suggested; following an eradication campaign, numbers dwindled dramatically. Last documented sighting 1989.
Effect: Damaging; cleared reed swamps and eroded riverbanks with extensive burrowing.
Name: Red-eared terrapins (Trachemys scripta elegans).
Origin: Huge numbers of unwanted North American terrapins released into British waterways by their owners, especially in the wake of the cult children's movie, Teenage Mutant Ninja Turtles.
Distribution: Metropolitan areas have experienced large numbers of dumped terrapins, but it is suspected that they are unable to breed in Britain. Some conservation groups are engaged in recapturing as many as possible.
Estimated population: Several thousand.
Effect: Damaging to native flora and fauna, they have also depleted certain small waterbound species.
Name: Mink (Mustela vison).
Origin: Imported from North America for fur farming. Escapees have established the British contingent.
Distribution: Recorded sightings in many parts of Britian, including the Hebrides.
Estimated population: At least 110,000 and growing.
Effect: Fierce predators, mink deplete fish, mammals and wildfowl. Native competitors, such as the otter and bittern, are losing out.
Name: Edible dormouse - "The fat dormouse" (Glis glis).
Origin: Imported from Hungary to Tring Park, Hertfordshire, in 1902, by Walter Rothschild (later Lord Rothschild).
Distribution: Spread fairly slowly throughout the Chilterns.
Estimated population: 10,000.
Effect: Damages forestry plantations in the Chilterns; mainly larch, also pine, spruce and beech. Can cause domestic problems such as noise and chewing. Protected under the Wildlife and Countryside Act 1981; licence needed to trap them.
Name: Grey squirrel (Sciurus carolinensis).
Origin: Imported from North America in 1828. Earliest documented sighting at Macclesfield, Cheshire, in 1876. Refused entry in 1938.
Distribution: Widespread throughout England and Wales, except around the Scottish border and the Wash, which lacks tree cover.
Estimated population: 2,520,000 and increasing.
Effect: Displaced the red squirrel with disease and competition for resources. A major forest pest which regularly debarks trees.
Name: Muntjac deer (Cervoidea muntiacus)
Origins: Brought to Scotland, and from there to Woburn in England, in the 19th century. Thought to have originated in India.
Distribution: Woodland in England and Wales.
Estimated population: between 50,000 and 200,000.
Effect: Can cause much damage to young woodland by eating saplings. Has been known to get into gardens and destroy them.
Name: Canada goose (Branta cana- densis).
Origin: Introduced by Charles II in the 17th century.
Distribution: Lakes and ponds all around the country.
Estimated population: Currently 60,000; but expected to double by the end of the decade.
Effect: Prolific consumers of agricultural and garden crops. Each goose produces about 2lb of excrement per day.
Name: American bullfrog (Rana catesbeiana).
Origin: North American swamplands.
Distribution: Widespread sales as pets have led to escapee colonies establishing across Britain.
Estimated population: Several thousand.
Effect: Highly poisonous.
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