We are being invaded by aliens. At least that's what you might believe if you're a regular newspaper reader. We're talking alien species, of course. This year the papers have been full of the influx of killer harlequin ladybirds, originating from Asia and unknown in the UK before 2003. Slightly larger and more extravagantly patterned than most native ladybirds, they have spread across England, competing with - and eating - British ladybirds, butterfly larvae and other bugs.

Some ecologists are predicting that the exotic harlequin will soon cause the extinction of several native insects, like our seven spot and tiny two spot ladybirds.

Last week there were also headlines about parakeets multiplying across the country so quickly that they were threatening native birds and crops. There have been dozens of similar stories over the past year or so (see panels below), a symptom of a rapid and permanent change occurring in the British countryside.

A 2005 audit recorded 2,271 non-native species resident in England alone, roughly a third of them animals, and two thirds plants. The number has been rising in the past few decades as a result of globalisation and global warming.

We are inclined to be over-hysterical

But are our food, health and way of life under threat? Maybe not as much as we might think. Environmental experts and psychologists agree that we are particularly susceptible to becoming over-hysterical about the threat posed by non-native species. Our in-bred instincts to repel all boarders make it very hard to assess such threats objectively.

The arrival of ring-necked parakeets in the UK - probably descended from escaped pets - is one example. There are a reported 30,000 parakeets now living in Britain, and obvious concerns about the implications of this for native wildlife. An expert from the British Trust for Ornithology has cautioned that the livid green birds may be stealing nest sites from other birds. However, parakeets have been with us since the 1970s and as yet there has been no conclusive evidence that they are damaging our wildlife.

Sebastian Selge, an environmental psychologist at the Macaulay Institute, is conducting research into why we abandon objectivity and become alarmed when we hear about non-native species, calling them “aliens”, “invaders” and “immigrants”. What drives our worry, he says, is our sense that what is happening is completely out of our control.

“My research with focus groups has also found that people find it much harder to accept dispersal of non-native species that has been caused by Man, say mink escaping fur farms, than ‘natural dispersal', say a bird arriving of its own accord, such as the little egret, a type of white heron which first started breeding in southern England ten years ago.”

Dr Bundy Mackintosh, a senior psychology lecturer at the University of East Anglia and an expert on fear, goes farther, and says that a primitive form of xenophobia is at work.

“We didn't evolve to be accepting. We evolved to protect our own and fight off aliens; this is not unique to human beings - most species do it. We instinctively evaluate anything that's different or novel, and if there's any doubt in our minds whether it's dangerous, we'll fight it, because in survival terms it's safer to make the error of not letting something in, than letting something in.”

Environmentalists also say that lack of understanding can make us unnecessarily overwhelmed by the idea of destructive invasions. There have been 1,500 introductions and invasions of species in the past 100 years and, according to the Centre for Ecology and Hydrology, an environmental research council, there are more and more arrivals every year.

But, according to the Environment Agency, the government body with the task of protecting and improving the countryside, just 10 per cent of all the foreign species that get introduced into the British countryside establish themselves and, of these, just 10 per cent become “invasive” and cause problems for man or other wildlife. Most work their way into natural ecosystems without causing much damage.

Causing damage to the economy

So we should reserve our concerns for those that pick an argument. First, those that threaten the UK's biodiversity, such as the harlequin ladybird, and others such as the American signal crayfish, which compete for food with native crayfish, and are killing them by passing on a fungal disease.

Secondly, there are many that cause damage to our economy as well as our environment; species such as mitten crabs (see panel below) damage our river banks, and outbreaks of new crop-damaging insects can necessitate expensive control measures. About 188 invasive species in England have a negative economic impact, largely on farming. Most can be controlled, but it costs the economy several billion pounds a year.

And then there are one or two invaders that can cause human health problems. The Health Protection Agency is predicting outbreaks of malaria in the coming decade as the UK becomes a comfortable habitat for the malaria mosquito. Giant hogweed, a Victorian import which can grow up to 5m (16ft) high, releases toxin-containing sap when brushed against. This sensitises the skin and causes it to develop serious blisters when exposed to sunlight.

The oak processionary moth, a native of continental Europe which established itself in West London two years ago, is causing concern because inhalation of dislodged hairs from its caterpillars can cause a potentially life-threatening allergic reaction.

Rather than getting into a lather as a result of our primitive instincts, it's more constructive to do something to prevent further threats to Britain's farms, gardens and countryside. Environmental bodies have plenty of suggestions of ways to stop future invasions, and minimise the effects of existing ones (see box). These measures are likely to become more relevant as climate change continues to increase the likelihood of new species becoming established.

“You could say that the aliens will inherit the earth,” says Trevor Renals, from the Environment Agency, the government-run body for protecting the environment. The trick will be

to learn to love some, and to do all that we can to control the others.

The ten most wanted beasties

The Environment Agency's list of foreign species in need of containment

1. Japanese knotweed

Fallopia japonica

Native to East Asia, but invasive in most European countries and North America, the plant was popular in Victorian gardens, but soon escaped from cultivation. It crowds out other plants and its prolific roots and shoots can damage walls and pavements.

2. American signal crayfish

Pacifastacus leniusculus

Over-sized, over-sexed and over here, the signal crayfish was farmed in England and Wales for food in the 1970s and 1980s but many escaped into our waterways. It carries a fungal disease to which native crayfish have no resistance.

3. American mink

Mustela vison

Native to North America but invading worldwide, mainly as the result of accidental escapes from fur farms or releases by animal rights activists. Depletes populations of ground nesting birds and small mammals.

4. Giant hogweed Heracleum mantegazzianum

This form of giant cow parsley, which can grow up to 5m (16ft) high, originated in the Caucasus mountains, but was introduced as an ornamental plant in the 19th century. It chokes out other plant species and its sap can cause severe burns to human skin.

5. Floating pennywort

Hydrocotyle ranunculoides

Native to North America, it was brought to Britain in the 1980s as a plant for tropical aquariums and garden ponds, and had spread into the wild by 1991. Its dense interwoven vegetation can grow 20cm (nearly 8in) a day, starving the water below of light, nutrients and oxygen, and blocking waterways.

6. Himalayan balsam

Impatiens glandulifera

Introduced in 1839, this purple-flowered plant thrives on riverbanks where it shades out other vegetation.

7. Australian swamp stonecrop

Crassula helmsii

First introduced from Tasmania in 1911, it has since been sold as an “oxygenating plant” for garden ponds but has been aggressively spreading in the wild for the past 40 years.

8. Chinese mitten crab

Eriocheir sinensis

First spotted in the Thames in 1935, having probably arrived in the ballast water of large ships. Now entering estuaries and rivers up and down the country where they are predating on native crayfishes and fish eggs.

9. Parrots feather

Myriophyllum aquaticum

Causing problems to waterways similar to Australian swamp stonecrop and floating pennywort, parrots feather was another import for garden ponds from South America.

10. Topmouth gudgeon

Pseudorasbora parva

This Japanese fish found its way into our waterways in the 1960s amid deliveries of carp for fish farms. It eats the eggs of native fish species and spreads a deadly parasite to salmon.

Five that haven't made the headlines... yet

1. Horse chestnut leaf miner

Cameraria ohridella

First found in Wimbledon, South London, in 2002, this tiny moth originating in Greece has larvae that cause significant damage to horse chestnut trees. It has spread across most of southeast England.

2. Zebra mussel

Dreissena polymorpha

In 2002, a survey of the Thames found 25 of these shellfish, originating from the freshwater lakes of Russia. A new survey has found 850 per square metre in many areas. They form reefs that which can block outlet pipes.

3. Eel swim bladder nematode

Anguillicola crassus

A 3cm parasitic worm, originating from Asia, that can kill eels and prevent them breeding.

4. Mediterranean fruit fly

Ceratitis capitata

This small fly, which is constantly imported with fruit, causes serious damage to fruit crops. May become established in the UK as temperatures rise.

5. Sika deer

Cervus nippon

Introduced in many areas of the world as an ornamental species, populations of this pretty deer in the UK have expanded by 5 per cent a year since 1972. They have become a serious forest pest, causing significant damage to trees.

What can we do

Report sightings Future control measures are made much easier if ecologists have exact information about the spread of species. There is a harlequin ladybird survey at www.harlequin-survey.org

Better controls Call for better controls on imported plant products. The freer the trade, the more species moved. Introducing non-native species to Britain (for example, through agriculture or garden centres) contributes to the UK economy, which means that environmental factors can sometimes be outweighed in policy decisions, according to the Parliamentary Office for Science and Technology.

Go native Consider using garden centres less and planting native plants in your garden. But get plants and seeds from stockists and growers; digging them up from the wild causes more problems.

Take alerts seriously The Colorado beetle, which can destroy potato crops, has failed to establish itself in the UK despite 160 outbreaks because government publicity has encouraged the public to report sightings, and control measures have been introduced rapidly.

Prevent escapees Most non-native garden plants are quite safe to grow. But never dump garden waste into the countryside, where your seeds and plant fragments might flourish.

Make yourself a challenge to compost all debris within the bounds of your own garden. Never release pet animals, insects or fish into the wild.

Complain If a nursery or garden centre you visit is stocking a plant you know to be invasive - for example montbretia, an attractive orange iris-like flower, or floating pennywort - then make an official complaint.

Check When you buy a plant, check that there isn't anything hitching a ride. New Zealand flatworms, which prey on native earthworms, are an increasing problem.

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