Humanity is also part of the natural environment.
Invasive species threat growing globally, experts warn
Threat to native species from alien invaders is growing and posing one of the greatest threats to wildlife around the world, conservationists say
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* guardian.co.uk, Friday 22 January 2010 11.14 GMT
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Japanese knotweed, a damaging and invasive species in the UK. Photograph: David T. Grewcock/Corbis
Hundreds of invasive species - from rats to diseases - are posing one of the greatest threats to wildlife around the world, conservationists warned today.
A study of 57 countries coordinated by the Global Invasive Species programme (pdf) found 542 types of animals and plants were putting native wildlife at risk in places where they are not naturally found.
On average, around 50 non-native species are having a negative impact on existing plants and animals in each country, ranging from nine in Equatorial Guinea to a massive 222 in New Zealand.
On the list of invasive aliens are 316 plants, 101 marine species, 44 freshwater fish, 43 mammals, 23 birds and 15 types of amphibian.
And the threat to native species from alien invaders is growing, the experts warned.
Invasive plants and animals are those which threaten native wildlife, by eating native species, laying eggs, damaging their habitat, spreading disease or by competing with them for the same "niche" in an ecosystem.
Many invasive species are successful because they have no natural predators in their new environment.
Examples of non-native species causing problems in the UK include grey squirrels, whose spread has led to widespread declines in red squirrels, the rampant plant Japanese knotweed, American signal crayfish and water primroses.
The study said that, globally, the increase and spread of invasive species is the result of a substantial rise in international trade in the past 25 years.
In some places invasive species are driving native wildlife towards extinction, for example in New Zealand where the yellowhead bird is now endangered because of a surge in the number of rats, while the chytrid fungus is spreading around the world causing massive declines in amphibians.
There are examples of success stories, in which threatened wildlife has bounced back after control measures were taken against the invasive species.
On Mexico's Natividad Island, the black-vented shearwater was at risk from the introduction of cats, goats and sheep, but its numbers are now recovering following an eradication programme.
And control of the red fox in south Western Australia has allowed the western brush wallaby to recover sufficiently for it to be downgraded on the "red list" of endangered species compiled by the International Union for Conservation of Nature (IUCN).
But while most countries have made commitments to tackle the threat of invasive species, only half have introduced legislation and even fewer are taking enough action on the ground, according to the IUCN.
Dr Stuart Butchart, from Birdlife International and one of the authors of the study, said: "While some threatened species on the IUCN red list have improved in status as a result of successful control or eradication of invasive alien species, a growing number are more threatened owing to increasing spread and threats from non-indigenous species.
"This shows that although we are winning some battles in the fight against invasive species, current evidence suggests that we are losing the war."