Killer slugs and other aliens — Europe's biodiversity is disappearing at an alarming rate

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Article Published 03 Apr 2009 Last modified 11 May 2021
7 min read
Is gardening one of your interests? If so and you live in central or northern Europe, the 'killer slug' is probably one of your personal enemies. The slug, which attacks your herbs and vegetables relentlessly, seems immune to control measures.

Biodiversity [...] represents the planet's natural wealth and as such provides the basis for our lives and prosperity.

The killer slug, known scientifically as Arion lusitanicus, is also called the 'Spanish slug' because it is native to the Iberian peninsula. The slug is hermaphrodite and can spread very quickly. More aggressive than the native black slug it eats weaker slugs.

The killer slug started to spread around Europe about 30 years ago, travelling as eggs in the soil of potted plants. This route is still a major source of infestation today.

The killer slug is just one example of a much wider threat to Europe's biodiversity as alien or non-native species establish and spread across the continent as a result of human activities.

Most arrive as stowaways and are transported unwittingly around the globe. The UN Convention on Biological Diversity identifies the threat of invasive alien species as one of the major threats to biodiversity worldwide.

Alien species have been arriving in new places as long as people have been travelling and trading. Increased trade, exploration and colonisation from the 1600s started the invasion proper with notable species such as brown rats arriving for the first time on ships from Asia.

About 10 000 alien species have been registered in Europe. Some, such as the potato and the tomato, were introduced on purpose and remain economically important to this day.

Others, called 'invasive alien species' create serious problems as pest species to gardening, agriculture, forestry as vectors of diseases or by damaging constructions such as buildings and dams.

Invasive alien species also change the ecosystems they live in and impact on the other species in those ecosystems. For example, a recent study of Knotweed, introduced to Europe in the 19th century from eastern Asia as an ornamental plant, has shown that the rapidly spreading invasive plant is causing serious damage to natural plant and insect species in the United Kingdom and France.


Invasive alien species often exact a high financial cost from their new homes. Alien weeds reduce European agricultural yields and Dutch elm disease — caused by an introduced fungus — has devastated elm trees in the forests of central Europe. The American grey squirrel, introduced to the United Kingdom, not only out-competes the native red squirrel — an impact hard to value in monetary terms — but damages coniferous trees and reduces their value as timber.

The cost in terms of damage and control of invasive alien species in the United States has been estimated at EUR 80 billion each year. Initial estimates put the cost in Europe at more than EUR 10 billion per year. This is without considering the cost of major human pathogens (such as HIV or influenza) or exceptional outbreaks of animal diseases.

Management actions to reduce (or exterminate) established invasive alien species are difficult, cumbersome and costly. The European Commission supports nature management projects in the Member States through the EU LIFE Regulation. The LIFE funds are increasingly being used for projects on invasive alien species and the budget is now approaching EUR 14 million per 3-year period.

Invasive alien species and Europe — increasing impacts

Alien species can be found in all European ecosystems. Globalisation, particularly increased trade and tourism, have resulted in an upsurge in the number and type of alien species arriving in Europe.

Marine and coastal areas are being drastically affected as a result of increased shipping and the building of canals between isolated seas — the Suez canal is still a major source of new species entering the Mediterranean Sea. Released ballast water from ships is such a big source of new organisms that the 'International Convention for the Control and Management of Ships Ballast Water & Sediments' has been established to 'prevent, minimize and ultimately eliminate the transfer of harmful aquatic organisms and pathogens' in this manner.

Control measures

The most efficient defence against invasive alien species is prevention — basically a border patrol blocking new species. A second step is early detection and control.

A striking example is the giant hogweed, Heracleum mantegazzianum, introduced to Europe as an ornamental plant in the 19th century. The plant is now subject to considerable local control efforts as the species has established in grasslands, along railways, roadsides and along river banks. Forming dense stands the hogweed crowds native plants out. It is also poisonous and direct skin contact can result in strong dermatitis. Today, the giant hogweed is most likely beyond eradication in Europe while early actions (up to the 1950s) probably would have had better prospects.

In line with this, the European Commission in the recent communication on biodiversity underlined the need for an 'early warning' mechanism for invasive alien species. In response, the EEA with its network of member and collaborating countries, is planning to establish a European-wide information system that will identify, detect, assess and respond to new and expanding invasions.

The most wanted list

Alien species come in all shapes and sizes. Some are deliberately introduced and economically important, others have little impact but quite a few have been a disaster. As a result, a first step in developing control and management measures, is to identify the most offensive species so that efforts are directed towards these.

In order to get better understanding of the invasive alien species and their impact on European biodiversity the EEA, supported by a number of experts, has established a list of the worst invasive alien species threatening biodiversity in Europe.

The list currently contains 163 species or species groups. Species are added to the list if they are very widespread and/or if they create significant problems for biodiversity and ecosystems in their new habitats.

Species on the list, of which vascular plants are the most common with 39 entries, have a significant impact on native biodiversity at the genetic, species or ecosystem levels. Many also affect human health and the economy. Since 1950, on average more than one of the listed species establishes itself each year and there is no clear sign that the situation is improving ( Figure 1).

The species on the list originate from many parts of the world, most notably Asia and North America ( Figure 2). However, many others have their origin in one part of Europe but have been transported elsewhere on the continent.

Graph Figure 1: Establishment in the pan-European region of the worst invasive alien species threatening biodiversity (all ecosystems)


Graph Figure 2: Area of origin of the terrestrial and freshwater species listed as worst invasive species threatening biodiversity in Europe

Looking ahead

Actions necessary to counter invasive alien species include measures for management and restoration which are usually both difficult and costly.

For example, control measures against the killer slug have been cumbersome and often have only a local and temporary effect. However, they are still important.

Within the EU, attempts are already being made to counter invasive alien species through management and restoration measures, financed by the LIFE Regulation.

Between 1992–2002, EUR 40 million was allocated to projects dealing with invasive species and the investment is increasing. The EU also finances studies of these species within the 'programme for research and technological development'.

The problem of invasive alien species is not going away. Globalisation and climate change (species moving because of changes to the natural habitat) means that more and more of us will come into contact with these species. Increasing public and political awareness is thus needed to put resources to controlling the main pathways of introduction, monitoring of risk areas for early detection and being prepared for immediate action to eradicate undesirable species.


Biodiversity — the wider context

Biodiversity refers to the variety of life on earth. It represents the planet's natural wealth and as such provides the basis for our lives and prosperity. It supports many basic services that we depend on such as the water we drink and the air we breathe. It helps to pollinate crops, put food on the table, regulate weather patterns and clean up our waste.

Without biodiversity we would not be able to survive. As such it can be seen as an insurance policy provided to us by the planet. Its value can be compared with financial markets, where a diverse portfolio of species stocks, as with business stocks, can provide a buffer against disturbances.

Currently, biodiversity is vanishing at an alarming rate mainly because of how we misuse nature to sustain production, consumption and trade in the globalized economy we live in. Habitat loss and fragmentation caused by clearing forests and natural areas for housing, roads and agriculture, the draining of wetlands and damning of rivers for agriculture, and clearing the seas of fish, is the primary cause of biodiversity loss.

Invasive alien species are considered by many conservationists to be the second greatest threat to biodiversity worldwide. Whether introduced deliberately or accidentally, such species can cause havoc to people, ecosystems and existing native plant and animal species. The problem of invasive species is expected to worsen in the coming century through climate change, increasing trade and tourism.

The other main threats to biodiversity come from pollution, climate change and over-exploitation of resources.

As the world's population is forecast to grow from 6.7 billion people today to nine billion in 2050, it is expected that the impacts on biodiversity from the current main threats will grow and losses increase.



DAISIE, 2008. Delivering Alien Invasive Species Inventories for Europe.
EEA, 2007. Europe's environment — The fourth assessment. Copenhagen.
European Commission, 2006. Communication from the Commission. Halting the loss of Biodiversity by 2010 — and beyond. Sustaining ecosystem services for human well-being. COM/2006/0216 final.
IMO, 2004. International Maritime Organisation. Conventions.
Kettunen, Genovesi, Gollash, Pagad, Starfinger, ten Brink & Shine, work in progress.
Scalera, R., 2008. How much is Europe spending for invasive alien species? Report to EEA.
Weidema, I., 2000. Introduced Species in the Nordic Countries. Nord Environment 2000:13.


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