70 % of the species constitute a risk

On the basis of the assessments the species are classified into one of the following five risk categories: no known risk NK, low impact LO, potentially high impact PH, high impact HI or severe impact SE. Ecological risk quantifies the influence alien species have on biodiversity.

Of the 1473 assessed alien species for mainland Norway and its maritime waters, 127 species (9 %), are assessed as severe impact, 106 (7 %) are assessed as high impact, 103 (7 %) as potentially high impact and 703 (48 %) as low impact. 434 species pose no known risk NK to biodiversity. These have no known ecological effects based on current knowledge, and it is also unknown whether they are spreading. It is nevertheless important not to dismiss such species from posing a potential risk to biodiversity in Norway in the long term.

Among the 12 regionally alien species which are assessed 4 have severe impact, 2 high impact and 6 species have low impact.

Regionally alien species

Invasion potential and ecological effect determine the risk category

The relationship between invasion potential and ecological effect determines the risk category. Species with severe impact have moderate or large invasion potential at the same time as the ecological effect is medium or large. Species that constitute a low impact risk have small, limited or moderate invasion potential, combined with no known, small or medium ecological effect.

Species with potentially high impact include species that either have the lowest degree of invasion potential combined with the greatest degree of ecological effect or the greatest degree of invasion potential combined with the lowest degree of ecological effect. This category therefore includes two groups of species which have a completely different type of risk.

Alien species with potentially high impact

Geographic variation in risk

Norway is a long, slender country with a large variation in the bioclimatic conditions and other environmental conditions. Alien species can therefore have different ecological risks in different parts of Norway. Species with a relatively broad distribution will therefore experience different environmental conditions.

In 2018, the ecological risk was limited to certain climate zones for 77 species, of which 37 are door knockers. Good examples are European hare Lepus europaeus and swallowwurt Vincetoxicum rossicum. Both can be found in south-eastern Norway and are assessed as severe impact.

133 species, of which 41 are door knockers, have an ecological risk which is limited to specific nature types. The ecological risk for eg. dwarf mountain pine Pinus mugo is particularly connected to southern sand dune areas and to coastal heath. Dwarf mountain pine is assessed as severe impact.

The risk category is based on the greatest impact the species can have in Norwegian nature and is not influenced by geographical variation.