WHETHER we like them or not, or even believe in them, the aliens are already here in large numbers. They are even on Rannoch Moor, one of Europe's last great wildernesses.

However, the invaders are not small green men from outer space but introduced plants and animals - biological incursions caused by humans.

And they are wreaking havoc in some environments across the world, inflicting billions of pounds a year in damage on the world economy, and causing serious breakdown of regional biodiversity.

According to the Swiss-based World Conservation Union (IUCN), harm done by such undesirable exports, which can include viruses such as foot-and-mouth, has soared with the expansion in world trade.

''Pests, weeds and pathogens . . . reduce crop and stock yields and degrade marine and freshwater ecosystems,'' Jeffrey McNeely, IUCN chief scientist, told a news conference to launch a report ahead of International Biodiversity Day on May 22.

The foot-and-mouth virus, which attacks hoofed animals and has led to the slaughter of more than two million sheep and cows in Britain, was apparently brought in from Asia, Mr McNeely noted.

''The economic bill runs into tens of billions of dollars a year,'' said the US scientist.

The case of Rannoch Moor and the carnivorous pitcherplant, sarracenia purpurea, is one which is likely to be discussed soon in Glasgow at a unique seminar, the brainchild of Professor Jim Dickson, who holds the chair of archaeobotany and plant systematics at Glasgow University.

He is author of the contemporary and definitive flora of Glasgow and is famous for his botanical detective work on the Austrian stone age glacier mummy, Oetzi. The conference, with speakers from many scientific disciplines, will mark the fiftieth anniversary of the Glasgow Natural History Society.

No-one knows how or when sarracenia, a native of North America, arrived on Rannoch Moor - where 31 plants were recorded seven years ago - or what effect, if any, it will have on the native plantlife.

But the moor, with its vast stretches of blanket bog, peat haggs riddled with ancient Scots pine stumps, and its distinctive vegetation of mosses and lichens, appears to have escaped introductions until this point.

Professor Dickson believes that if keeping Rannoch Moor in its pristine state is regarded as a desirable objective by conservation managers and if action has not already been taken, it should be now to rid an internationally important vegetation of the pitcherplant before extermination becomes impracticable.

He is, in broad terms, relaxed about many introductions, most of which do not stay the course. He points to the Canadian pondweed which became rampant in the nineteenth century in the UK, but which appears to currently have a more modest presence.

''There is more and more evidence that points to local plants and animals learning to make their living from it. It has come under control, which is what I think will happen with other plants such as giant hogweed and Japanese knotweed,'' he said.

The seminar, at Glasgow University on June 15 and 16, will examine whether or not it makes scientific or economic sense to try to eliminate aliens.

In some cases that may already be impossible and in others impossibly expensive.

Among the speakers will be Dr Paul Walton, of RSPB Scotland, an expert on the ruddy duck, which is a successful - sadly so in the views of many - American import to the UK. It is present in the south of Scotland and has spread outwards to Europe where it has successfully hybridised with the very rare white-headed duck, one of the most-threatened species, which is facing extinction as a result.

Control trials paid for by the Department of the Environment in England are already under way, allegedly costing around #1000 a bird. It has also caused a dilemma for the RSPB which wants to see both species survive.

This means reducing the UK ruddy duck population of 4000 to a point where it no longer spreads to continental Europe.

The RSPB says it is ''a regrettable and very serious situation caused by short-sighted action by humans in the past''.

Dr Walton adds: ''It is highly important that people should know about these problems but it is equally important that species should not be demonised for unsound reasons.''

In contrast to non-native fish which have largely been introduced intentionally by humans, non-native invertebrates - the province of Professor Peter Maitland and Dr Colin Adams, celebrated fish biologists - mostly arrive by stealth.

In Scottish waters, these include a crustacean which arrived on board Canadian timber and is now spreading and displacing a native species and several species of flatworm, which came here either on timber from Scandanavia or through the aquarium trade.

Then there is the flatworm, phagocata woodworthi, now present in Loch Ness having been apparently brought there on the undisinfected equipment of American monster hunters. Truth is stranger than fiction.

One of our most ubiquitous and least-liked aliens will also come under the microscope - the sitka spruce.

The seminar will be given an overview of research by the Forestry Commission which suggests that the sitka plantations have the potential to contribute significantly to the conservation of a range of natural flora and fauna in Scotland.

Professor Dickson says: ''Because our flora and fauna were impoverished by two ice ages plenty of niches remain for alien species to flourish. I am against wars of attrition to eliminate plants or other species. Most are here to stay and we have to learn to live with them.''