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BONN, Germany -- A wide range of species not native to Europe's Wadden Sea have invaded its ecosystem, threatening the biodiversity of the World Heritage Site, the United Nations Environment Programme (UNEP) said in a report unveiled today.
A diverse range of alien species are increasing at an alarming rate in the sea, which borders the Netherlands, Germany and Denmark, according to the report, delivered at a conference in Bonn, Germany, to mark Wadden Sea Day. Many species have become abundant and several can be regarded as invasive with a significant impact on the recipient ecosystem.
The Wadden Sea includes mud and sand-flats, salt marshes, islands, dunes, estuaries, gullies and open waters that stretch over 500 kilometres along the North Sea coast. It is one of the last remaining natural inter-tidal ecosystems in Europe and supports a huge number of plant and animal species. Between 10 and 12 million birds visit the Wadden Sea during their migratory journeys every year.
The alien species could also become a serious problem to human health, according to the report. For example, the sharp shells of Pacific oysters can cause injuries to the feet of mud-flat walkers and oysters or other aliens may carry agents that cause infections. Oysters covering blue mussel beds reduce fishermen's yield.
Grasses, mussels and jellyfish are among the most damaging invaders. The common cord-grass (Spartina) is the main invasive plant in the Wadden Sea as it facilitates the build-up of sediment, thus transforming the sea's tidal flats into salt marshes. The plant was deliberately introduced into the Wadden Sea to enhance the development of such salt marshes. Efforts to eliminate the plant failed and the spreading of the species increased.
Pacific oysters were introduced from Asia in the 1990s. Since then, they have begun to invade native blue mussel beds and create their own oyster reefs throughout the Wadden Sea, causing a food shortage for birds that feed on blue mussels.
Although there has been an increase in blue mussel populations in the Dutch parts of the Wadden Sea, numbers in the Danish and German areas have dropped. There are major concerns that the Pacific oyster might displace domestic blue mussel beds.
An invading jellyfish species could also be threatening fish populations. The sea walnut is originally native to the coastal waters of the western Atlantic Ocean and was first recorded in the Wadden Sea in 2006. It is thought that the species was introduced via ballast water – water carried by a commercial ship for stability that is then discharged upon arrival at its destination.
The sea walnut consumes zooplankton, crustaceans, other jellyfish and the eggs and larvae of fish. Elsewhere, this species is being blamed for the striking decrease of anchovy in the Black and Caspian Seas. Conservationists are concerned that the same phenomenon might occur in the Wadden Sea if the numbers of sea walnut continue to rise.