The world is engaging in a new defensive battle against invasive species. But these invaders aren't from another planet, they're indigenous to our world.

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.

Now scientists are rising to a new challenge: to control and eradicate these alien invaders and restore the balance in disrupted ecosystems.

Earth Report travelled to the Seychelles to find out how 200 years of devastation by rats is coming to an end.

Rat catching in the Seychelles:

The tropical ocean islands were the first ecosystems to become victim to alien invaders. Rats hitch-hiked aboard ships with the first human visitors to this bountiful paradise - and they've been gorging on the food of the islands endemic species ever since. They're a menace that's driving local fauna to extinction.

Now scientists are using the drastic measure of laying poisonous bait in order to eradicate rats once and for all and restore the Seychelle's delicate natural balance of species.

The prickly problem of the island-hopping hedgehog:

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, hedgehogs have been dining out 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. 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. Could this mean the end for the island-hopping hedgehog?

Purging the leafy spurge:

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.

However, help is at hand. Scientists have discovered that 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.

Bio-control 'out of control'?:

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.

Global trade in aliens:

It's clear that increased global trade inevitably raises the risk of transporting organisms to new environments where they may become invasive. And there's not much we can do about it.

WTO 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.

Even more worryingly, scientists fear that world trading will allow species a global passport to new environments. Will we end up with an homogenous world of mono-cultures of single species?