What ecological risks do exotic plants and animals bring along?

The distribution area of indigenous species is shrinking considerably as a result of competition from exotic plants and animals or a changing habitat. Some exotic plants and animals are even becoming a nuisance. These are referred to as invasive species. The common slipper limpet (Crepidula fornicata), which was imported with oysters from America at the end of the 19th century, became a real pest for local oyster breeding. However, there are no known cases of species disappearing in the North Sea as a result of the introduction of an exotic species, although this is the case in fresh water environments and estuaries.

The reproduction of our species may also be threatened by exotic plants and animals. The American and indigenous lobster can mate, but their offspring are sterile.

Mass stranding of the American jack-knife clam as they often occur on Belgian beaches.

It may be thought that the introduced species increase biodiversity, but the newcomers are usually fast growing species that are able to withstand disturbance and pollution. They feel very much at home in environments that are created by man or that are heavily influenced by man, such as harbour and coastal areas. There, numerous artificial hard substrata are constructed or we find areas that have been greatly impoverished and uniformized as a result of huge fishing pressure. Such areas are therefore highly suitable for relatively undemanding immigrants which displace the indigenous species. Consequently, there is a worldwide risk that marine flora and fauna will become similar and the differences blurred. So, even if introduced species may result in greater diversity locally, on a worldwide scale they may lead to the impoverishment of biodiversity.

There is also a real danger that exotic plants and animals bring with them all sorts of associated organisms and diseases, to which the indigenous species are not immune. For instance, oysters from Japan brought a single-cell parasite that is harmful to indigenous oysters. The introduction of micro-organisms, such as species of phytoplankton, can cause toxic plankton blooms, making oysters or mussels unfit for consumption.

What measures can be taken to limit the risks?

In Belgium, the deliberate introduction of exotic plants and animals is forbidden by law (Article 11 MMM act). Throughout the world, treatment techniques are being sought for ballast water which will prevent species being imported through this channel. Because aquaculture is becoming more and more important as a source of introduction, measures are also needed there.

 Text of MMM act (PDF, 27pp, 317KB, French and Dutch only)