One scientist has likened the biological era we are living through as the ``Homogecene''. The rich biodiversity of the Earth is being homogenised by the movement and replacement of animals and plants from one area into another. As they become established, they quickly decimate the native flora and fauna. In simple Darwinian terms, it is the survival of the fittest.
Professor Morris Gosling, a mammalian ecologist at the Zoological Society of London, explained what is at stake at last week's British Association science festival in Cardiff: ``The natural world is the end product of a complex and wonderful process of evolution and that has produced a rich biodiversity. What's happening is that we are replacing that biodiversity with a smaller number of invading species.'' The haphazard introduction of non-native species is now running a close second to the destruction of wildlife habitats as the primary cause of species extinction, he added.
Sometimes the introductions are accidental, sometimes deliberate. Occasionally, a species is brought in to destroy a pest but becomes a pest itself. Some aliens eat the native residents, others bring nasty diseases with them. A few - notably the ruddy duck - are just too sexually vigorous. But what they all have in common is that they do not fit in to stable ecosystems, evolved for hundreds of thousands of years without them.
``The rate of invasion is directly correlated with the movement of people around the planet,'' Professor Gosling said. As people move from one place to another, then so do other living things that hitch a ride. Between 1985 and 1996, American customs officials intercepted about 5,600 different species of insect pests on freight destined to be imported into the US. Ships also carry large numbers of aliens from one part of the world to another. ``Ships take on water as ballast, go to another part of the world and just expel it,'' Professor Gosling said.
Ballast water contains eggs and microscopic organisms. One of the earliest examples of this ship-borne invasion stems back to the 1860s when ships coming from American waters expelled their ballast water into the Mediterranean and unwittingly introduced a deadly fungus which has decimated the native crayfish population.
To make matters worse, the American signal crayfish (which was immune to the fungus) was introduced to replace its European cousin. In the 1970s, with government blessing, fish farmers in Britain were encouraged to cultivate the American crayfish, but it soon became a rampaging menace. ``It not only kills the native crayfish. It also destroys the freshwater environment because it eats anything else it comes into contact with. It also burrows into river banks and destroys them in the process,'' said David Holditch of Nottingham University.
One of the best-known examples of a species that is totally inappropriate for anywhere but its native homeland is the American mink. Nobody knows the true numbers of introduced mink in Britain but the experts believe its presence poses a genuine threat to virtually every small mammal and bird it comes across, most notably the British water vole.
David MacDonald, a specialist on the American mink at Oxford University, said: ``They are such an amazingly vigorous and competitive species. They are a triumph of adaptability, being one of the most successful mammalian carnivores.'' Unfortunately, this does not help the British water vole, which has adapted to running away from predators that could neither swim nor fit into its narrow burrow. American mink can do both.
American mink first escaped into Britain from fur farms in the 1920s and since then have established a formidable beachhead for a complete invasion of the countryside. Their destructive influence has, however, become far worse in recent years due to the continued loss of wild habitats.
``The impact of the mink on native species may have been exacerbated by generally disadvantageous things. The habitat bordering our rivers has been eroded to a tiny little ribbon in which everything is confined: everything is dangling on this little tightrope. The mink simply travels along it and bangs off the native prey,'' Dr MacDonald said.
Aggressive carnivores are not the only problem facing the beleaguered scientists trying to fend off the invasion force of non-native species. Hedgehogs are perhaps the most docile animals imaginable yet they are helping to wipe out important colonies of ground-nesting seabirds living on the remote islands of the Outer Hebrides. In the 1970s, just seven were introduced by well-meaning residents as garden pets. Nearly 30 years later, they have expanded to an estimated population of 10,000.
Digger Jackson of the Royal Society for the Protection of Birds said that seabird populations have declined by up to 70 per cent since the hedgehogs arrived, the spiny creatures eating the eggs. The hedgehog population, meanwhile, is aided by fewer predators (and even fewer cars) than on the mainland. They have also formed an unholy alliance with another invader, the rabbit, which conveniently digs the holes in which the hedgehogs live.
Trying to tackle the growth of an alien species frequently means introducing another species to attack them. Most of these work, according to Sean Murphy of CABI bioscience, a consultancy specialising in the biological control of pests. ``The record is that there have been more than 5,000 introductions of agents against insect pests worldwide and 1,000 introductions against weed pests worldwide, and that record has been extremely good now there is an international code for the introductions of biological control agents.''
This has not always been the case, however. One of the worst examples is the introduction of a predatory snail called Euglandina into the Polynesian islands, which were threatened with being overrun by another introduced species, the giant African snail. Unfortunately, Euglandina ignored the African snail and made straight for the native Partula snails. The 117 different species of Partula are now under threat of being wiped out. ``The snail story is a bad example of biological control,'' Dr Murphy said.
Each example of alien invasion requires its own unique remedy. Morris Gosling believes, for instance, that it is possible to eradicate American mink, providing someone is prepared to pay the estimated cost of pounds 30 million. Professor Gosling, who spearheaded the successful eradication of the coypu from East Anglia in the 1980s, said the political will to act is critical to the success of dealing with the growing problem of aliens from abroad. Above all, it is important to provide incentives to those engaged in the eradication programmes. As he says: ``There is the central paradox in pest control: pest control organisations don't stay in business by eradicating the pests.''