Popular Science: How to score an invasion?

Massive transport and an increasing rate of human activities around the world enable more and more species to profit from the situation and to get to regions far from their native range of occurrence, establish populations there and spread into natural habitats, inhabited by native species. Such biological invasions can have negative environmental and socioeconomic effects (impacts) on the occupied area (a decrease in species diversity and in numbers of populations of native species, alteration of ecosystem properties, spreading of diseases, damage of infrastructure), and therefore the alien species are in the center of attention of nature-conserving organizations, or crop and live-stock producers. Many of the alien species are not that harmful, however, there are so many invasive species with a high impact that a prioritization is needed in order to effectively invest time and money in the meaningful and truly necessary management limiting their spreading.

The worst alien species according to the list created in this study is the Canada goose (Branta canadensis). Photo: Charles J Sharp [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons

One of the most popular lists of alien species is the “100 of the world’s worst invasive alien species” compiled by the IUCN Invasive Species Specialist Group and a similar list exists for Europe. However, these and similar lists are usually not fully objective, because they focus rather on detecting representative examples of invasive species with various types of impact and on a balanced inclusion of individual groups of plants and animals from different types of habitats.

In the present study, the researchers used the Generic Impact Scoring System (GISS), a tool that compares alien species from different taxonomic groups based on the already-published information about their impact on native species and ecosystems. The impact is classified into 12 categories, each containing a scale from 0 to 5 according to increasing intensity and severity; and each level having an unambiguous detailed description. From publications, previous lists and other sources, the scientists evaluated 486 species that were introduced to Europe from other continents and represent a wide range of taxonomic groups. In the end, they included in the final list the 149 absolutely worst species with the highest scores; i.e. 54 plants, 49 invertebrates, 40 vertebrates and 6 fungi. The top-ranking organisms are: one bird species (specifically the Canada goose; Branta canadensis), four mammals (for example the brown rat; Rattus norvegicus or the muskrat; Ondatra zibethicus), one crayfish (red swamp crayfish; Procambarus clarkii), one mite (Varroa mite; Varroa destructor) and four plants (for example the silver wattle; Acacia dealbata); in general, most species are terrestrial, followed by fresh-water and sea organisms. (The complete list can be found directly in the publication).

Silver wattle (Acacia dealbata) is the worse alien plant species.
Photo: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=296875

Of course, the list is not exhaustive and will need to be updated on a regular basis; at the moment the authors count with ten-year intervals. GISS can be considered to be an objective and transparent tool that can help to prioritize the management of certain alien species and to assess their eventual inclusion in the list of priority invasive species in the European Union created by the European Commission two years ago. However, for local needs of individual member states, it is necessary to elaborate similar lists focused on site-specific situations.

Nentwig W, Bacher S, Kumschick S, Pyšek P, Vilà M (2018): More than “1000 worst” alien species in Europe. Biol Invasions, 20: 1611-1621.

Author of the text (c) Darina Koubínová
Popular Science editor