Invading species in the Baltic Sea

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Invading species

Over 100 new species established themselves in the Baltic Sea during the 20th century. This is a significant addition to the biodiversity of this species-poor, brackish sea, which only supports approximately 900 species altogether. Since the Baltic is young in evolutionary terms (<3,000 years old), its colonisation is probably not complete, and newcomers are easily filling available space and ecosystem functions[1].

Examples of invading species

For example, the roundhead goby (Neogobius melanostomus) from the Black and Caspian Seas is now playing the previously-vacant role of small, coastal predator on bivalves. At present, there are no documented examples of any negative ecological impact of these new species on the Baltic ecosystem.

A number of new gammarid amphipods frequenting the southern Baltic coast are well mixed with resident species and all show irregular ups and downs in abundance. However, small-scale problems for humans have been recorded, such as local problems with pipelines clogged with the zebra mussel Dreisena, or fishing nets covered by the copepod Cercopagis.

A number of former invaders are now playing an important role in the benthic system. The bivalve Mya arenaria, which arrived from North America during the medieval period, is now one of the key species and sediment bioturbators, and it is also a source of food for fish and birds. The sessile barnacle, Balanus improvisus, a 19th-century invader, is now one of the few species that builds stable biogenic structures (it is a bioconstructor and habitat builder) in this system[1].

Changes in native species

So far, the only documented extinction from the Baltic Sea is the sturgeon, Acipenser sturio, a species that is now also believed to be a medieval invader from North America.

Shifts and changes have occurred before. In the early 20th century, the dominant top predators in the Baltic were marine mammals (gray, ringed and common seal and harbour porpoise). The seal populations declined by about 95% during the last century as a result, initially, of hunting (1900–1940) and later of toxic pollutants (1965–1975)[1].


Climate change-related increases in temperature will provide the opportunity for more new species to settle in the Baltic Sea. However, all of the new species will have to cope with the effect of decreasing salinity, which forms the key environmental factor structuring this ecosystem, and it will probably continue to remain a key factor in the future[1].

Related articles

Baltic Sea
Effect of Climate Change in the Baltic Sea Area
Non-native species invasions