Citation: Sandvik, H., S. L. Olsen, J. P. Töpper, and O. Hilmo (2022) Pathways of introduction of alien species in Norway: analyses of an exhaustive dataset to prioritise management efforts. Journal of Applied Ecology, 59, 2959–2970.

Key words: Ecological impact, escape, introduction pathway, invasive species, spread, stowaway.

1. Alien species constitute one of the major threats to global biodiversity. Stopping alien species at an early stage, preferably before establishment, is crucial for the effectiveness of management actions. To enable early detection and prevent future introductions, knowledge of pathways of introduction and their absolute and relative importance is crucial.
2. Based on an exhaustive impact assessment of alien species in Norway (all multicellular neobiota), the relations of taxonomy, lifestyle and ecological impact of alien species to their pathways of introduction are investigated. This taxonomically and ecologically unbiased dataset contains 2267 unique pathways of 1180 alien species.
3. Ecological and taxonomic patterns indicate that terrestrial organisms were predominantly introduced by means of escape (mainly perennial plants escaped from gardens), parasites as contaminants (mainly fungi and insects parasitising plants), freshwater organisms by release (mainly vertebrates), and marine organisms as stowaways (mainly invertebrates and algae). Unaided introductions were most common among insects and marine organisms.
4. Alien species with high ecological impact were mainly introduced along the same pathways as other alien species. In relative terms, high-impact species were overrepresented among released species, even though this pathway was subordinate in absolute terms. The number of pathways and the overall introduction pressure were important predictors of ecological impact, especially of the species’ invasion potential, and area of occupancy.
5. Introduction rates of novel alien species have seen recent increases in all taxa and along almost all pathways. This acceleration was especially pronounced for insects and fungi introduced as contaminants and for marine organisms introduced as stowaways. In absolute terms, introduction rates were highest for plant escapes, reaching more than five novel species per year.
6. Synthesis and applications. Introductions of new alien species cannot be prevented by closing one or two introduction pathways, since none can be singled out as the main pathway of high-impact alien species. Yet each pathways closed makes a difference, as this reduces the overall introduction pressure. The highest priorities for management are the pathways that are easiest to address, such as release, and those with the highest volumes, such as plant trade.

Full text: © 2022 the authors. If you accept (i) the conditions specified in the Creative Commons "Attribution" 4.0 licence, and (ii) that printouts have to be made on recycled paper, you may download the article here (pdf, 4.8 MB).

Supplementary material: The article has three Appendices,

  • Appendix S1 (Generic Ecological Impact Assessment of Alien Species; pdf, 0.3 MB)
  • Appendix S2 (definitions of pathway categories and subcategories; pdf, 0.3 MB)
  • Appendix S3 (supplementary methods and results; pdf, 0.2 MB)

The data used in the analyses are available from the Dryad Digital Repository at doi:10.5061/dryad.4b8gthtg7.

Related publications: The information on pathways used in this paper has been collected in connection with the 2018 Norwegian impact assessments of alien species. The method used and the results obtained during these assessments have been described elsewhere.


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