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Our Oceans, Seas and Coasts

Descriptor 2: Non-indigenous Species

“Non-indigenous species introduced by human activities are at levels that do not adversely alter the ecosystems”

What are non-indigenous species?

The Convention on Biological Diversity (CBD) defines non-indigenous species as species whose introduction or spread threaten biodiversity. Non-indigenous species are species introduced outside their natural past or present range, which might survive and subsequently reproduce. These species are introduced in situations where exchange of people or goods takes place between countries and continents, by shipping for example.

Non-indigenous species in the marine environment

North Sea coasts

Oyster bank © Annemieke Kouwenberg, 2010

Dreissena polymorpha © Lars Peters

Dreissena polymorpha © Lars Peters

D vexillum © Woods Hole Science Centre

D vexillum © Woods Hole Science Centre

Along the whole coast of Europe many species of seaweed, seagrass, molluscs, fish and birds were introduced last century. Some notable examples are:
  • Along the Atlantic and North Sea coasts, the American jack knife clam (Ensis directus) and the Pacific oyster (Crassostrea gigas) are two molluscs that form an ordinary image for people strolling over beaches and mudflats. They are originally from the Pacific coast of the United States.
  • The Soft shell clam (Mya arenaria), which is now very common in Europe, is a non-indigenous species which was introduced in the 16th or 17th century from the United States. It is said to have been introduced via ballast water of freighters.
  • The ‘Whangamata sea squirt’ (Didemnum vexillum) is a species that has been spreading in harbours along the European coast since the introduction at the end of last century. Possibly originating from Japan, it was probably introduced through shellfish imports as an accompanying species. It can rapidly expand in places with hard substrate, like harbours, and forms a threat to, amongst others, shellfish.

In many cases non-indigenous species do not harm the regional ecology and economics. However, in certain cases, non-indigenous species can become “invasive” species and have enormous and long-lasting impacts on the region. The Marine Directive states therefore that non-indigenous species should be at levels that do not adversely alter the ecosystem.

Why should we pay attention to non-indigenous species?

Non-indigenous species can harm a local ecosystem. They can expand rapidly if they can outcompete indigenous species and when natural enemies are absent. By competition for space, food or other factors, non-indigenous species can sometimes replace indigenous species in the area. Non-indigenous species can also prey upon indigenous species or introduce diseases to a local system, to which indigenous species are not resistant. Non-indigenous species are referred to as ‘invasive species’ if they expand dramatically. Highly invasive species are very often species, which reproduce fast, quickly adapt to a broad range of situations (water quality, food availability), have a diverse gene pool and/or are associated to human activities.

The number of introductions of non-indigenous speciesWhy does it matter if non-indigenous species replace local species? First of all, such invasive species can dominate a whole area, reducing natural biodiversity and making ecosystems less resilient to changes. In addition, if these non-indigenous species are not preyed upon by local species or if they outcompete and replace a local species that serves as a major food resource, they can have a significant impact on the local food web. Another important impact of the invasion of alien species is the economic damage it causes if the introduced species expels commercially valuable species. In addition, management measures taken to control invasive species are costly: controlling invasive species and repairing the damage they do is estimated to cost European economies at least €12 billion each year.

The number of introductions of non-indigenous species has increased globally and at the European level over the past century, as illustrated by this graph from the European Environment Agency, due to the increase use of maritime transport for food and goods over the world.

What are the sources of introduction of non-indigenous species?

Several human activities form pathways for the introduction of non-indigenous species:

  • Figure 1 Map of maritime routesBallast water of freighters is a possible way of introducing non-indigenous species. If a ship is not fully loaded, water is used to maintain the draft, stability and strength of the ship. This ballast water is usually taken up locally in harbours during unloading or en route on rivers and oceans. With the uptake of ballast water local species (such as, for example, larvae in the plankton) can be taken up, transported and released into a new environment in other parts of the world.
  • Fouling of ships is another source of importation of invasive alien species. Species attach to the ship hull travel thousands of kilometres and are released in completely new waters and ecosystems.
  • ‘Hitchhiking’ of non-indigenous species with goods transported for trade is a possible way of introducing non-indigenous species. For instance, if mussel seed is transported abroad for the purpose of aquaculture, other species like crabs or algae can be transported as accompanying species with the mussels.
  • The introduction of species can occur by foreign tourists that sample particular species for an aquarium or garden at home.
  • In the Mediterranean Sea an overwhelming number of species was introduced after the connection of the sea to the Suez canal (i.e., Lessepsian species).

Species can also be introduced intentionally, for instance in aquaculture, foreign species are introduced on a large scale for economic reasons. For new enterprises, it is attractive to invest in the exploitation of well-known species like the Pacific oyster (Crassostrea gigas) because a lot of knowledge is available on the best practices for their breeding. Moreover, a consumer market already exists. However, these introduced species may be invasive in the local environment.

What can be done?

Recognizing the stranger!

Recognizing the stranger! Early detection of an invasive species is important to be able to stop it from expanding. Detection of species occurs for example during standard monitoring programmes for local species, by people who are commercially involved in aquaculture or by recreational amateurs such as divers. These people recognise species that do not belong in the region from their experience in the marine environment. Non-experienced people can be trained for detection of known invaders to enhance early detection of non-indigenous species.

Policy and legislative measures

In 2004, the International Convention on the Control and Management of Ships’ Ballast Water and Sediments (BWM Convention) was adopted. It aims to prevent the potentially devastating effects of the spread of invasive species carried by ships' ballast water from one region to another. This is be done through the strict control and management of ships' ballast water and sediments. The entry into force of the BWM Convention is a crucial step towards the reduction of the spread of non-indigenous species regionally and worldwide.

Addressing the issue of invasive species is one of the six key objectives of the EU Biodiversity Strategy. The European Commission is currently working on a dedicated legislative instrument on Invasive Alien Species, which is due to be adopted in 2012. In its Communication "Towards an EU Strategy on Invasive Species", the Commission proposes a number of possible options to tackle the issue of invasive species, including the immediate setting up of a Europe-wide early warning and information system to report new and emerging species and voluntary codes of conduct to encourage responsible behaviour by retailers and consumers.

In addition, in 2007, the EU adopted a regulation on the use of non-indigenous and locally absent species in aquaculture aiming to create a framework governing aquacultural practices in order to ensure adequate protection of the aquatic environment from the risks associated with the use of non-native species.

Useful links

What can YOU do?

  • 1. As a diver, learn to detect new marine species.
  • 2. As a fisherman, remember to take your excess bait and wrappings home at the end of the day and clean your boat before moving it to another stretch of coastline.
  • 3. As a recreational boat owner, regularly clean the hull of your boat, in particular, if you travel long distance.
  • 4. As an aquarium owner, dispose of the content of your aquarium responsibly: do not pour the content of your aquarium in a body of water or storm drain. If a home cannot be found for the contents of the aquarium, bury the animals in the ground and dump any water in your backyard, far away from storm drains.
  • 5. As a teacher, there are many different fun ways to teach your students how to recognize invasive (alien!) species. For instance they could make and trade colourful cards depicting “good” native species and “bad” invasive species.

IUCN, Marine Menace: Alien invasive species in the marine environment, 2009