Throughout history, species invading the territories of others have fuelled the motor of extinction. And as human economic activity spreads to every corner of the globe, the process is speeding up.

After direct habitat destruction invasive species now pose the second greatest threat to global biodiversity. The international response to control and eradicate these aliens and restore the balance in disrupted ecosystems.

The tropical ocean islands were the first ecosystems to become victim to alien invaders. Rats arrived aboard ships with the first human visitors to this bountiful paradise - and they've been competing successfully for the food of the islands endemic species ever since. The drastic measure of laying poisonous bait is being applied in order to eradicate rats once and for all and restore the Seychelle's delicate natural balance of species.

The hedgehog is the first documented case of a species threatening an internationally important bird population. In 1974, seven hedgehogs were introduced to the Scottish island of South Uist to control garden slugs. Thirty years on, with no natural predator, their numbers have exceeded 6,000. With an appetite for almost anything, the population of hedgehogs has been expanding on food normally eaten by local bird colonies, causing the numbers of Dunlin to crash dramatically. The RSPB estimates that within 20 years the Dunlin could disappear entirely from the island. Urgent conservation measures are being considered, including a hedgehog-barricade to halt their progress to other Hebridean islands. But the only long-term answer is a final solution of total eradication.

As well as the huge ecological impact of invasive species there's also the financial impact. In the United States, alien weeds cost ranchers 144 million dollars a year. Theoretically one alien species can be eradicated by a carefully selected second alien species, a process commonly known as 'bio- control'.

In America's North West, rangelands are being cleared of cattle-toxic leafy spurge by the European flea beetle which feeds exclusively on the weed. And it seems to be working. Now ranchers are locking away their shotguns and rifles and taking up sweep nets in a bid to bio- control this poisonous weed.

Today, bio-control species are tested exhaustively for ecological 'side-effects'. But this wasn't always the case. Without proper testing, bio-control species could destroy the ecosystems they were meant to restore.  The Eurasian weevil, introduced to stamp out musk thistle on Nebraska's prairies, is now attacking native American thistles and out- competing native insects such as the picture-winged fly.

It's clear that increased global trade inevitably raises the risk of transporting organisms to new environments where they may become invasive.  The World Trade Organisation's agreements to control the spread of 'pests' offer only limited protection. A country must provide proof that a species is harmful if they wish to introduce regulations to protect their territory. With 32 million species in the world, finding out which are going to be a problem is a tall order and, once they've arrived, eradicating them is difficult and costly.