The introduced red fox spelled bad news for Australia's western brush wallaby - but thanks to a successful control program, wallaby populations are making a comeback.

Credit: Clipart

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Ecosystems around the world are at risk, with invasive introduced species placing among the top three threats to life on our planet.

So says a new report from the Global Invasive Species Program (GISP), which has looked at 57 countries and found that, on average, each has 50 introduced species making a negative impact on biodiversity. The number of invasive non-indigenous species per country was found to range from nine in Equatorial Guinea to 222 in New Zealand.

Among the 542 species documented as invading aliens were over 300 plants, 101 marine organisms, 44 freshwater fish, 43 mammals, 23 birds and 15 amphibian species - all capable of causing damage to the native habitats they infiltrate.

Lead author Melodie McGeoch, from South Africa's Centre for Invasion Biology, said that the increasing number and spread of invasive aliens can be attributed to a substantial rise in international trade over the past 25 years.

Unfortunately, "current evidence suggests that we are losing the war," said co-author Stuart Butchart, from BirdLife International, of the threat. He noted that uncontrolled invasive aliens are having serious impacts on native species, with a growing number of already threatened indigenous plants and animals becoming even more threatened due to the increasing spread of invaders.

A New Zealand-native bird, the Yellowhead, for example, has suffered considerably in recent years due to a surge in invading rat numbers, with two populations now extinct and three more rapidly declining - forcing the species to move from 'Vulnerable' to 'Endangered' on the IUCN Red List of Threatened Species.

Similarly, the chytrid fungus, which was entirely unknown until 1998, is thought to be the cause of the decline and extinction of many amphibian populations around the globe.

While most countries have made international commitments to tackle this problem, only half have introduced relevant legislation and even fewer are taking adequate action on the ground, GISP said.

Where control programs are in place the results can be quite successful. In south-western Australia, for example, the control of the red fox over the last decade has allowed the once-ailing population of the native western brush wallaby to recover to such an extent that it has now been downlisted to a species of 'Least Concern' on the IUCN Red List.

"It's likely to be more cost effective to prevent the spread of invasive species in the first place than to tackle the biodiversity crisis once they have become established," said Bill Jackson, IUCN's Deputy Director General and Chairman of GISP.

"With sufficient funds and political will, invasive species can be controlled or eradicated. This will allow native species to be saved from extinction, but countries need to dramatically improve the way they deal with the problem."