Invasive alien species pose greater risks than previously thought for biodiversity, human health and economies, according to two new reports from the European Environment Agency (EEA).
An alien or non-native species is an organism which humans have introduced –intentionally or accidentally –outside its previous range. It is deemed ‘invasive’ if it has negative effects on its surroundings, for example by outcompeting or predating on native species that have evolved without specific adaptations to cope with them. In such cases populations of native species can be devastated. Evidence shows that in a growing number of cases invasive alien species even cause harm to human health and society.
There are more than 10,000 alien species present in Europe, and the rate of new introductions has accelerated and is still increasing. At least 15 % of these alien species are known to have a negative ecological or economic impact. However, non-native species — for example, some food crops — can also have huge benefits.
The first report, The impacts of invasive alien species in Europe, details the effects and spread of some species. The second report, Invasive alien species indicators in Europe discusses the methodological approach in bringing this data together.
The most common reason species are introduced elsewhere is for horticulture, while others may be brought into new areas for other reasons including farming, hunting, and fishing, or as pets, the report notes. Transport is not always intentional — for example, zebra mussels have stowed away in the ballast water of ships to proliferate in European lakes.
Increasing trade and tourism in recent decades may have led to increasing numbers of alien species. Climate change may also play a role in the spread of these species, the report says, making some areas more favourable to plants and animals originally from elsewhere.
Invasive alien species are one of the main threats to biodiversity. Of the 395 European native species listed as critically endangered by the IUCN Red List of Threatened Species™, 110 are in danger because of invasive alien species. The EU Biodiversity Strategy 2020 has policy targets which aim to address the problem.
For humans, one of the most dangerous effects of invasive alien species is as a carrier of disease. The Asian tiger mosquito has been linked to more than 20 diseases, including yellow fever and chikungunya fever. It has come to Europe mainly through the intercontinental trade in used tyres, and is now prevalent in several southern European countries, especially Italy. Climate change projections show that the mosquito will likely extend its range further north in coming years.
Climate change is also enabling the spread northward of the common ragweed. The plant is originally from North America, the seeds first coming to Europe in mixes of grain intended as bird feed. It is a powerful trigger of hayfever and other allergies.
Changing landscapes are another result of invasive alien species. For example, the red palm weevil is destroying large numbers of palms in the Mediterranean region, transforming the green spaces in cities.
There are also effects on ecosystems which indirectly affect humans. In some cases ecosystems altered by invasive alien species may be less able to provide important ‘ecosystem services’ which support human activity. For example, the pollination carried out by honeybees may be affected by invasive alien species — the yellow-legged hornet, native to Asia, has been found to devastate beehives in France.
Invasive alien species cost Europe around € 12 billion per year, according to one estimate. Species such as the Spanish slug, now found in most European countries, can devastate crops. Other species such as the pervasive zebra mussel can also cause high costs by fouling water filtration plants and water cooling reservoirs of power plants.
Invasive alien species harm native species through predation, as is the case of feral cats killing smaller creatures. Cats have been introduced to approximately 180 000 islands worldwide, and have a significant impact — in Britain alone, cats are estimated to kill 25 – 29 million birds every year.
The American mink was originally brought to Europe for fur farming. Many animals have since escaped or been intentionally ‘liberated’, so the species is now common in the wild in many areas of Europe. It is now outcompeting its European cousin in many areas, and has had devastating effects on local wildlife, particularly ground-nesting birds.
Amphibians around the world are in decline, in part due to the invasive chytrid fungus. Other alien species can spread diseases, as is the case with the red swamp crayfish, which carries the ‘crayfish plague’. The disease often proves deadly to European crayfish, as they have not evolved to cope with the disease.
Hybridisation can also be a problem. For example, Japanese knotweed is a particularly virulent hybrid of two alien species originating from different parts of Asia, which first came into contact as alien species in central Europe. The hybrid has been found to spread faster than its parents, outcompeting other plants and altering ecosystems with effects on other species.
(Source: European Environment Agency news release, 21.02.2013)