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
"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
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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