Alien species are spread by moving individuals into an area where the species previously has not been naturally found. The risks with spreading alien species to natural biotopes where they have not been previously found are great. Many wild animals and plants have been extirpated by alien species and entire ecosystems can be affected. During the last two hundred years, the introduction of alien species in different areas has increased enormously because of the increased mobility of humans. The documentation of alien species is defective. Alien species are found in all or Sweden and all Swedish environments.

More than 1300 alien species have been introduced to the Nordic countries. These alien species are described in the project "Nordic Network on Introduced Species".

In Swedish lakes and rivers, (in pdf-format, 30 kB) about 50 alien species have been documented. In the Swedish marine environment approximately 100 alien species have been documented Species in Coastal and Marine Water (in pdf-format, 22 kB). In the eastern part of the Baltic Sea, many alien species have established and caused great ecological problems Baltic Sea Alien Species Database .

In the Swedish terrestrial environment, 700 alien species have been documented. In the Swedish Environmental Protection Agency's report Introduction of Alien Species in the Terrestrial Environment (in pdf-format, 113 kB) approximately 300 introductions are described. The number of alien species can be much larger than what has previously been documented, as knowledge about invertebrates is very deficient.

For more information about alien species in Sweden read Spread of Alien Species .

Alien populations and genes are spread by moving individuals of a species to areas where the same species is found, but where the genetic composition of the natural population is different from that which the individuals has been taken. If the stocked or released individuals reproduce with their wild relatives, the alien genes are spread into the wild population. The spread of organisms with the alien genetic background into natural populations is a genetic manipulation which can have consequences for biological diversity at the gene, as well as the species and ecosystem level. Alien genes are spread in large scale in for example, fisheries management and forestry. The effects of spreading alien genes is very poorly investigated and documented.

A special type of alien population/genes is composed of genetically modified organisms, GMO, that is organisms whose genetic code is altered with the help of molecular techniques. The spread of such organisms in nature poses a potential threat to the biological diversity of natural flora and fauna. If a certain GMO is able to reproduce in nature and spread its modified genetic code to naturally existing species or to closely related species, this can have have very great consequences, in the worst case for the entire ecosystem. There are specieal regulations for risk analysis for GMOs.