And though some alien species--such as horses, cattle and sheep--are important parts of our culture and commerce, many are pests.

CNN: Fish tale

However, times change and the presence of the alien species presents problems to those who want to maintain the integrity of native woodland.

BBC: NEWS | UK | Northern Ireland | Gardeners urged to repel boarders

Quarantine rules will prevent still more alien species arriving, and park officials will step up efforts to control or get rid of those already installed.

ECONOMIST: Galapagos

Population increase, over-consumption and galloping urbanisation have multiple consequences, including the increasing rarity of certain species of plants and animals, pollution, the over exploitation of land and the sea, as well as the spread of invasive alien species.

UNESCO: MEDIA SERVICES

Organised by conservation groups IUCN and Birdlife, the high-level debate at the European Parliament in Brussels will consider measures that could be taken within the EU policy framework to mitigate the present and future threats from invasive alien species.

BBC: Report tracks threats from Europe's alien invasion

"Because of this ongoing environmental catastrophe, we need to put all of the management options on the table to try to find ways of reducing the rate at which our biodiversity succumbs to the impacts of invasive alien species, " he says.

BBC: Trumpets of outrage in the outback

Since no plausible evidence has yet emerged of risks to human health from eating GM food, the environmentalists' main remaining objection is that GM plants may cross-pollinate others, spreading their alien genes to other species with unpredictable environmental effects.

ECONOMIST: Invasion of the transgenics