General information about alien species

When alien species are mentioned, you may think that they come from another planet. In biology, however, we use the term alien, or non-native species for organisms which have been transported by man to a region where they did not occur naturally. Non-native species include a wide variety of organisms: viruses, fungi, plants and animals, and they can also differ in the pathways of introduction. Because of this, it is not always esasy to identify non-native species as such. In recent literature, the definitions which are agreed in the context of the Convention on Biological Diversity are used. These definitions are somewhat more extensive, but they are most suitable for the variety of non-native species. Therefore, we use these definitions on this website as well: 

• Alien species:  (non-native species) a species, subspecies or lower taxon, introduced outside its natural past or present distribution; includes any part, gametes, seeds, eggs, or propagules of such species that might survive and subsequently reproduce.
• Native species: (indigenous species) means a species, subspecies, or lower taxon, occurring within its natural range (past or present) and dispersal potential (i.e. within the range it occupies naturally or could occupy without direct or indirect introduction or care by humans.) This is includes regions which the species can independently reach for instance by walking, flying, transport with water or wind or other natural ways of spreading.

The introduction of non-native species is a direct consequence of the mobility of people and the transport of goods. While this trade is ongoing for thousands of years, it has continuously increased in the last century. With it, also the number of non-native species increases, which reach new regions with the aid of human transport. The impacts of these non-native species are presented on the next subpage. First, let’s have a look at the stages through which species pass when they establish themselves in a new environment.

Every year, hundreds of new non-native species are imported in Europe, both intentionally and unintentionally. Not all of these become invasive. In fact, most of them enter unsuitable living conditions and do not survive, or they cannot reproduce because too few individuals are imported and they do not find a potential partner in the new environment. Some species, called casual alien species survive in the new environment for a considerable time, either because of a long individual lifespan or because of occasional reproduction. 

Some alien species do not only survive in the new environment, they gradually adapt and start to reproduce (either sexually or vegetatively). Non-native species which maintain populations without further human assistance are called naturalized species. These categories of alien species are defined as follows: 

• Casual species are alien species which can reproduce either sexually or vegetatively in the new environment, but fail to maintain their populations for a longer time. They are sometimes called Acclimatized species. 
• Naturalized species are alien species which reproduce in their new environment independently from human interventions, but without noticeable impact on economy or ecosystems. 
• Invasive alien species (CBD) An alien species whose introduction and/or spread threaten biological diversity, or: (IUCN) an alien species which becomes established in natural or semi-natural ecosystems or habitat, is an agent of change and threatens native biological diversity

This text was prepared in the framework of project Tuja 2, which was financially supported by the Swiss Contribution to the Enlarged European Union. 

Author: Jana Kus Veenvliet, last updated in January 2018, translated by Paul Veenvliet

Literature: Richardson, D. M., P. Pyšek, M. Rejmánek, M. G., Barbour, F. D. Panetta & C. J. West, 2000.   Naturalization and invasion of alien plants: concepts and definitions. Diversity and Distributions 6: 93-107.