Invasive alien species, ranging from disease and plants to rats and goats, are one of the top three threats to life on this planet, according to a new publication coordinated by the Global Invasive Species Programme (GISP).
Most countries have made international commitments to tackle this threat, but only half have introduced relevant legislation and even fewer are taking adequate action on the ground.
The paper entitled 'Global indicators of biological invasion: species numbers, biodiversity impact and policy responses', published in the journal Diversity and Distributions, looked at 57 countries and found that, on average, there are 50 non-indigenous species per country which have a negative impact on biodiversity.
The number of invasive alien species ranged from nine in Equatorial Guinea to 222 in New Zealand. A total of 542 species were documented as invasive aliens, including 316 plants, 101 marine organisms, 44 freshwater fish, 43 mammal, 23 bird and 15 amphibian species.
According to Professor Melodie McGeoch, lead author on the publication and member of the Centre for Invasion Biology (Stellenbosch University), these numbers are a significant underestimate. "We showed that regions with low development status and little investment in research have lower than expected numbers of invasive aliens."
An increase in the number and spread of alien species, which adversely affect the habitats they invade, is nonetheless attributed to a substantial rise in international trade over the past 25 years.
"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", says Dr Stuart Butchart, BirdLife International's global research and indicators coordinator.
"This shows that although we are winning some battles in the fight against invasive species, current evidence suggests that we are losing the war." If left uncontrolled, invasive alien species can have a serious impact on native species. Yellowhead Mohoua ochrocephala, a bird endemic to New Zealand, has suffered considerably in recent years due to a surge in the number of rats.
Two populations of the yellowhead are now extinct and three more are significantly falling in number, leading the species to move up from Vulnerable to Endangered on the IUCN Red List of Threatened Species.
Similarly, the pathogenic 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. The disease caused by the fungus can be spread by humans and a host of other species, ranging from exotic fish to African clawed frogs Xenopus laevis.
But the impact of invasive alien species can be successfully controlled. Black-vented shearwater Puffinus opisthomelas, a seabird native to Natividad Island off the Pacific coast of Mexico, was under threat from cats, goats and sheep. But since they've been eradicated, the status of the bird has been reduced from Vulnerable to Near Threatened on the IUCN Red List.
Similarly, the control of the red fox Vulpes in south-western Australia in the last decade allowed the population of the endemic western brush wallaby Macropusirma to recover sufficiently for it to be downlisted on the IUCN Red List to Least Concern.
"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", says Dr Bill Jackson, IUCN's deputy director general and chairman of GISP.