People try to trap them and sell them to restaurants."However, not all the aliens are equally unwelcome, and some do not prosper. 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. 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. 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. On a second visit their haul was down to 20, but consisted of larger specimens, and then last Thursday 40 to 60 were collected. 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.
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. "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. 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. 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. 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. 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. An invasion of aliens is under way, threatening our native species with death and disease. Rogue animals from all corners of the world have colonised Britain, multiplying with ease despite often hostile environments which are quite different from those of their homelands. Tasks will vary from measuring the height of ocean waves to charting the polar caps or sea ice.
It can also plot large-scale desert erosion or crop rotation in individual fields. It can be targeted to provide information on the effects of natural or man-made disasters.. Construction has started at their Portsmouth plant on the ASAR for the European Space Agency. The design will enable almost the entire globe to be scanned in a day, providing radar information at global, national or regional level.