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Tuesday, 22 May, 2001, 03:57 GMT 04:57 UK
Alien species 'cause havoc'
Feral pig Agriculture Western Australia
Alien species can drive out local ones
By BBC Science's Andrew Craig

Conservationists have warned that exotic species of animals and plants are causing havoc around the globe after escaping, often with human help, from their native habitats.

To mark Tuesday's World Biodiversity Day, the World Conservation Union has issued a list of the 100 worst invasive alien species.

Looking down the list, one finds the attractive-sounding water hyacinth and the rosy wolfsnail; the brown tree snake and the feral pig perhaps less so. But, whatever one thinks of their looks, all have proved destructive pests when taken out of their natural environments and introduced into new habitats.

The South American water hyacinth does indeed have lovely purple flowers; but on five continents it has spread from ornamental ponds to choke waterways, stop boat traffic, fishing and swimming, and prevent sunlight and oxygen from reaching plants in deeper water.

Intention or accident

Some pests were originally spread deliberately by humans; the small Indian mongoose was taken from Asia to the West Indies to control rats, but it has wiped out several native birds, reptiles and amphibians, as well as carrying rabies.

Crab BBC
Millions of crabs were killed by imported ants
Others spread accidentally, hitchhiking in ships' holds or in packing cases.

Crazy ants, so called because of their erratic movements, killed three million land crabs in 18 months on Christmas Island in the Indian Ocean.

The World Conservation Union is calling on such bodies as the World Trade Organisation to recognise the threat posed by globalisation of trade - and even by development aid, as agricultural materials can contain the seeds of exotic weeds.

It also wants sea and airports to watch out for invading species, and says authorities must be ready to act quickly when an infestation is detected.

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