World's 100 most destructive species named

Rare plants and creatures face extinction as vigorous invaders devour everything in their path, conservationists warn

Scientists have identified the world's 100 deadliest invasive species, which are responsible for the extinction of hundreds of other species and causing havoc to ecosystems and the birds and mammals that inhabit them. Fish, ants, snails and even seaweed are named for their aggressive growth, biological pollution and catastrophic impact on habitats around the world.

Scientists have identified the world's 100 deadliest invasive species, which are responsible for the extinction of hundreds of other species and causing havoc to ecosystems and the birds and mammals that inhabit them. Fish, ants, snails and even seaweed are named for their aggressive growth, biological pollution and catastrophic impact on habitats around the world.

The species are published in a new and updated booklet compiled by the World Conservation Union (IUCN), one of the world's leading conservation agencies. It will be distributed this week at the IUCN's world conservation congress in Bangkok, where delegates will be warned of the urgent need to curb invasive species or face a world where only "hardened survivors" such as pigeons exist, at the expense of parrots and other exotic creatures.

Among the leading culprits in the booklet, entitled One Hundred of the World's Worst Species, is the Nile perch, which has contributed to the extinction of more than 200 endemic fish species in Africa by eating not only their food but also the fish themselves. Another is caulerpa, a seaweed in the Mediterranean that smothers the beds of native sea grass which act as nurseries for many species. The IUCN considers caulerpa to be a threat to the whole of the Mediterranean.

Many invasive species initially appear benign: the water hyacinth's large purple and violet flowers make it a popular ornamental plant for ponds but it is fast-growing and can quickly block waterways, limiting boat traffic, swimming and fishing.

Britain also has its share of invasives in the booklet, including Japanese knotweed, the grey squirrel and Dutch elm disease. Even the humble domestic cat takes its place in the list, for its role in killing both familiar garden birds in the developed world and exotic bird species further afield.

An invasive species is an animal or plant that has been taken from its native habitat and introduced to another area. Though the international trade in species is responsible for many invasives, some instances can be accidental. Caulerpa is believed to have been emptied into the Mediterranean as waste from the Monaco aquarium. On other occasions it is deliberate: rabbits and stoats were introduced to New Zealand in the 19th century with catastrophic results. Birds in New Zealand had evolved for millions of years without predators and many had lost the need to fly: several species, including the kakapo, a flightless parrot, and the yellowed- eyed penguin, the world's rarest penguin, have been brought to the point of extinction by stoats and feral cats.

Invasives are invariably aggressive and hardened to their new habitat. "This booklet really spells out what the dangers and impacts are," said Dr Maj de Poorter, co-ordinator of the IUCN's invasive species specialist group. "Invasives are taken out of the ecosystem that evolved with them and put in a new ecosystem which may have none of the tough predators or pests they evolved to fight. The ecosystem into which they are introduced has never had a need to deal with them. In New Zealand, nothing had evolved to cope with stoats."

But invasive plants and animals have wider implications for humans and the planet, according to Jeffrey McNeely, the IUCN's chief scientist. "Climate change is altering our ecosystems at a great pace." he said. " Invasive species adapt to new and quick-changing environments and so they are likely to respond most quickly to climate change. Our biodiversity is going to become homogenised and the richness of our planet will be lost.

"Do we want pigeons or parrots? People like diversity, they like lots of flowers in their garden. In the end, it's a matter of how we want to live."

The 10 most destructive

1 Nile perch: introduced to Lake Victoria, Africa, in 1954 to counteract the drastic drop in native fish stocks caused by over-fishing. Instead, it has contributed to the extinction of more than 200 endemic fish species through predation and competition for food.

2 Water hyacinth: this South American native is one of the most insidious aquatic weeds in the world. Its flowers make it a popular ornamental plant for ponds but it is now found in more than 50 countries on five continents, infesting waterways and making fishing impossible.

3 Caulerpa: introduced to the Mediterranean around 1984, it is thought, as waste from the Monaco aquarium. Hardier than most tropical seaweeds, it has adapted well to cold waters and smothers habitats.

4 Crazy ant: so named because of their frenetic movements, crazy ants have invaded native ecosystems and caused environmental damage from Hawaii to the Seychelles and Zanzibar. On Christmas Island in the Indian Ocean they have formed supercolonies in all habitats of the rainforest.

5 Small Indian mongoose: this voracious and opportunistic predator is native to areas as widespread as Iran and the Malay Peninsula. It was introduced to Mauritius, Fiji, the West Indies and Hawaii to control rats but has caused the local extinction of endemic birds, reptiles and amphibians.

6 Feral pig: introduced to many parts of the world, it damages crops, stock and property and transmits many diseases such as leptospirosis and foot and mouth. Diet includes juvenile land tortoises, sea turtles and sea birds.

7 Dutch elm disease: deadly fungus, spread by the elm bark beetle, which can kill an elm in three weeks by clogging its water-conducting vessels. Spread in the UK in 1968 from imported Canadian timber to deadly effect, killing more than a third of southern England's 23 million elms.

8 Grey squirrel: the American grey has devastated Britain's native population of red squirrels, which are barely half the size. First appeared in the English countryside between 1876 and 1929, possibly after having been accidentally released from London Zoo.

9 Japanese knotweed: introduced from Asia to Europe in the mid-19th century as an ornamental and fodder plant. Grows rapidly, prevents native seeds from germinating and thrives on being uprooted - can even regrow after being rooted out and washed downstream.

10 Giant African snail: introduced to the islands of the Pacific and Indian oceans as a food source for humans but has a voracious appetite and has been recorded as attacking more than 500 different kinds of plants. Has spread to parts of South America. Tropical in origin, but copes comfortably with snow at other latitudes.