Accueil BiodiversitéModifications de la biodiversitéPathways of introduction

Pathways of introduction

Its openings to the Atlantic and the Red Sea and the intensive maritime traffic in provenance from all the world’s seas make the Mediterranean the greatest receptacle for exotic species in the world.

It is generally accepted that the original population of the Mediterranean dates from the Pliocene. Many species have not evolved or have not evolved in the same way in the Atlantic ocean and in the Mediterranean, others have adapted little by little to Mediterranean environmental conditions and have been able to give rise to new species known as endemic.

For five million years, the Atlantic Ocean and the Mediterranean have been exchanging water and living organisms via the Strait of Gibraltar. This natural passage has made possible a permanent supply of species that are different from the original population.

The development of human activities and trade, in particular maritime traffic, opened up new perspectives for the colonisation of the Mediterranean by exotic species : the opening of the Suez Canal, the transportation of organisms on the hulls of ships and in ballast water, the boom in aquaculture and aquarium keeping.

In the Mediterranean, on average five new species are detected each month, and the phenomenon is on the increase (one species a month in 1995) because of climate warming, among other factors. Nine hundred and twenty five exotic species have now been recorded under the Plan Bleu.

Exotic species, invasive species, introduced species, even alien species … A variety of terms designate a species found outside its known range. The IUCN (International for the Conservation of Nature) has defined the terminology :

-  exotic species (non-native, exogenous, foreign) : species, sub-species or lower taxon present outside its natural range (past or present) and possessing a potential for dispersal (i.e., outside the range occupied naturally or capable of being occupied naturally without direct introduction, or through human intervention). This definition includes the parts, gametes or propagules of the species capable or surviving and ultimately of reproducing ;

-  invasive exotic species : exotic species settling in natural or semi-natural ecosystems or habitats, and which constitute a factor of change and threaten the indigenous biological diversity.

The Strait of Gibraltar is the main pathway of communication between the Mediterranean and other seas. With a maximum depth of 300 m and width of 14 km at the narrowest point, it constitutes a bottleneck for the waters of the Atlantic that enter at the surface and for the waters of the Mediterranean flowing out at depth. It is by this route that throughout time species of Atlantic origin have penetrated and colonised the Mediterranean.

Depending on the glacial or interglacial periods, the migrants came from the north or from the south.

Over the past decades, the arrival of exotic species from the tropical Atlantic Ocean has probably increased because of climate warming. These new arrivals settle along the coasts of Spain or the Maghreb and remain restricted to the western basin where the conditions of temperature and salinity are closer to those of their place of origin than in the eastern basin.

Built by Ferdinand de Lesseps, from 1859 to 1869, the cutting of the Suez Canal, between the towns of Suez and Port Said, opened up a shorter navigable route between the East and the West.

With a length of 161 km, and a width of 80 to 150 m, it brought into contact the fauna and flora of the Red Sea and those of the Mediterranean. Yet until 1960, the freshwater of the Nile which flowed into the Mediterranean formed a sort of natural barrier to the penetration of organisms arriving from the Red Sea. But since the construction of the Aswan Dam - Saad al-Ali in Arabic - in 1960, which retains the water in Lake Nasser, this barrier has lost its effectiveness, and the door is wide open to species coming from the Red Sea.

In homage to the builder of the Canal, these species are known as Lessepsian species. They first settle in the eastern basin where they find environmental conditions similar to those of the sea from which they originated, and many remain confined to the coasts of Turkey, Lebanon, Israel or Egypt.

Their ecological, or even economic, impact there is very heavy : a few species have replaced the native species and are now part of the underwater landscape. After conquering the eastern basin, some Lessepsian species, such as Siganus luridus and Fistularia commersoni, have recently crossed the Siculo-Tunisian Strait and have been found in the western basin.

Cargo ships sail back and forth across all the world’s oceans. Maritime traffic enables organisms fixed on the hulls of ships or contained in their ballast water to reach new lands, and if the new environment is favourable, to settle there.

The animals and plants fixed on the hulls of ships constitute what is known as ‘fouling’ ; they may be accompanied by mobile species. Generally, these species, which have resisted the different climatic conditions encountered during their voyages, possess a very high potential for adaptation. Their first implantation often occurs within harbours : detachment of a few individuals by friction during manoeuvering of the ship or during cleaning of the hull, spawning of fixed individuals or shipwreck.

In order to control their stability and pitch, ships use tanks called ballasts. During the loading or unloading of the cargo or to compensate for the consumption of fuel, the ballasts are filled with seawater or emptied. Numerous organisms may be carried in the ballast water : plankton, various eggs and larvae, crustaceans, molluscs, fishes, at different life stages. During deballasting operations, they are discharged and some of them find conditions favourable for their implantation in a new environment.

Aquaculture and, to a lesser extent in the Mediterranean, fishing are potential vectors of exotic species, either deliberately or accidentally.

The deliberate importation of fishes, crustaceans or molluscs originating in another part of the world for the purposes of aquaculture is a source for the introduction of non-native species : the escape of animals from cages or shellfish farms, the reproduction and dispersal of eggs and larvae.

The other source for the introduction of non-native species is the accidental release of species associated with those imported for aquaculture or human consumption : release, direct or indirect, into the water of eggs and larvae of organisms related to reared animals (species fixed on shells, internal or external parasites) or of materials (species fixed on transportation tanks, fish pots, floats, etc.).

The popularity of aquarium keeping and the development of public aquariums are relatively recent and are on the increase. Exotic species, often more colourful and attractive, are highly prized by the public. The deliberate or accidental release of animals, plants or associated organisms occurs directly or indirectly via the wastewater outlet piping systems.

À consulter