Natural spreading
It is natural for a species to spread into new areas. Natural spreading is happening without any human aid, and usually it is a gradual process within the landscapes and geographic limits that determine the natural distribution of a species. Naturally spreading species are termed indigenous species.
Spreading barriers
The spreading of indigenous species is normally limited by natural spreading barriers; biological barriers such as forests or open areas, or non-biological barriers such as mountains, ocean or unfavourable climate. Indigenous species are also distributed over separate areas due to such barriers, thereby increasing the global biodiversity. Distinctive characters of natural landscapes, and also occurrence and variation, are related to the existence of spreading barriers.
Spreading of alien species
Human activities may, directly or indirectly, facilitate the spreading of a species across a natural spreading barrier. Most commonly this happens through:
a) Species are deliberately introduced into nature
b) Species escape from captivity and breeding, or grow astray from cultivation or industrial
activities
c) Species are introduced as stowaway during transport of animals, plants, goods or humans
d) Species spread from wild populations in neighbouring countries, where their origin is due
to a), b) or c)
e) Norwegian species spread into new areas in Norway as a consequence of human activities
f) Species are spread with the aid of humans, in cases where knowledge on the spreading
method is lacking
In most cases, introduced organisms will not be able to adapt and eventually they become extinct. A few species, however, will succeed in adapting to the new environment.
Species equilibrium in nature
Indigenous species are adapted to their habitats, and have a particular location and role in a landscape type. They are also adapted to each other and live in an surrounding ecological equilibrium.
The effect of an alien species that spread into the area is an unknown factor in this equilibrium. If the equilibrium is changed, the introduced species is considered as harmful in many cases. If the introduced species has little or no effect on the equilibrium, it is considered harmless.
Harmless alien species
Alien species may find their place in an ecosystem without altering the ecological equilibrium between indigenous species in any significant way. These species are of little threat to the biodiversity, and there are many such species in Norway. Several species have been here for a long time, and are considered as a natural part of the Norwegian biodiversity.
Harmful alien species
Alien species may have properties that change the living conditions for one or more indigenous species, and in such cases they have an adverse effect on biodiversity. As a result, the introduction of a harmful alien species is a disturbance to the local species composition, and it may lead to changes in the ecological properties of the landscape. Such changes may work as a domino effect: several ecological functions are influenced in turn, and several species are affected. At worst, local species become extinct or industrial interests are adversely affected.
Breeding species are also alien species
Some Norwegian indigenous species have been selected by humans for breeding, and kept in captivity either as food or as pets. The resulting population of a breeding species is considered an alien species, since it represents a genetic variant that is different from wild individuals.
Breed individuals which escape and spread into nature may pose a threat to wild populations of the species. This is usually a problem only if the population is very small, or in cases where many individuals of the breeding population are released into nature.