Invasive species

Some alien species cause significant damage

Alien species are without doubt an important and integrated part of Swedish agriculture and forestry. Alien species thrive in our gardens. However, alien species can also be a cause of concern and in worst cases create serious problems. Certain species simply thrive too well in their new environment and can proliferate enormously and thereby affect indigenous biodiversity or cause socio-economic damage. These species are usually called invasive alien species.

Alien species must fulfil several criteria to be considered invasive. First and foremost they must have been transported from their place of origin to a new area. Other than natural dissemination there are many other dissemination vectors for alien species (ballast water, passive stowaways on boats or aeroplanes, unintentional passage through living plant matter etc.). On arrival the species must be able to establish a viable population. In most cases the conditions are such that their presence is only temporary. A few species succeed in both establishing themselves and spreading into the surrounding environment. Of these there are a few that can be classed as invasive and these cause problems in terms of spreading and population expansion.

How does a species become invasive?

In order to move to another area a species requires a vector (through human intentional or unintentional intervention). On arrival the species must be able to adapt to its new environment so that it can reproduce. To become permanently established the species must be able to grow in population size, and to become invasive must be able to spread to new areas. See the figure on the right.

Proliferation of invasive species due to climate change

In general the problem with alien species is greatest in tropical and warm temperate areas. Sweden with its northern location is relatively free from invasive alien species. As the temperature rises, the risk increases significantly. Many species that are considered problematic in Central Europe are currently only present in small and harmless numbers in Sweden. However, as the climate becomes more favourable the risk of spreading increases and from being an exciting, rather exotic species it can cause real problems. Furthermore, there is a risk that species that have yet to become established at these latitudes are given the opportunity to proliferate.