Citation: Sandvik, H., B.-E. Sæther, T. Holmern,
J. Tufto, and S. Engen (2011) Quantifying risks and effects:
The Norwegian classification system for alien species. – Talk given at the XIth
Conference on the Ecology and Management of Alien Plant Invasions in Szombathely.
Most national classification schemes for alien species are based on
qualitative sets of criteria. Qualitative criteria have the benefit of requiring few data and
comparatively little time. Quantitative sets of criteria, on the other hand, can be argued to
outweigh these benefits by a number of important advantages: a classification of alien species
based on quantitative criteria is less vulnerable to subjectivity and arbitrariness; it is
transparent and testable; it is easily adjustable to new knowledge about the biology of the
species or to changing environmental conditions; it is comparable across different taxa;
it allows a standardised treatment of uncertainty or data paucity. All of this increases the
legitimacy of the resulting classification, both in terms of scientific legitimacy of the
results, and of political legitimacy of the management action derived from these results.
We present a novel classification system of alien species, which is based on quantitative
criteria, and which will be the basis of the revision of the earlier Norwegian Black List.
The main feature of the proposed classification system is that it is two-dimensional:
alien species are assessed along two independent axes, a spread axis and an ecological
effect axis. By addressing these two properties of alien species separately, the resulting
classification is more informative.
The placement of alien species along each of the two axes is determined on the basis of a
number of different criteria. Spread/establishment risk is determined based on the estimated
spread velocity; on the populations’ expected lifetime as derived from a population viability
analysis; and on the proportion of different habitat types that are projected to be occupied
by the species. Ecological effect is inferred from interactions with native species; from
state changes brought about in natural habitat types; and from the probability of transmission
of genetic material and/or diseases or parasites to native species. Effects on threatened
native species or native key species, and state changes in threatened or naturally rare
habitat types, receive greater weight.
Along each axis, the species receives the highest category in which at least one criterion
is fulfilled. This takes the precautionary principle into account, and allows at the same
time the classification of species with incomplete data. Uncertainty is incorporated by means
of confidence intervals.
Related publications: The set of criteria has now been published in
Biodiversity and Conservation.