A unified scoring system applied to five major taxonomic groups: Making diverse impacts of alien species comparable

Background/Question/Methods

Invasive alien species can cause a myriad of changes in the introduced environment, and some of them cause considerable damage. Understanding such impacts is crucial in order to direct management actions. This study therefore aims at unravelling some of the most daunting questions concerning impacts of alien species: Do impacts differ between taxa, and how? How are environmental and socio-economic impacts related? Is the use of “impact elsewhere” as impact predictor useful?
To answer these questions we collected information on impact available in the literature of 309 species belonging to five major taxonomic groups: terrestrial arthropods, mammals, birds, fish, and plants. To make these very diverse impact measures comparable we used a semi-quantitative scoring system with two main classes, environment and socio-economy, with six categories each describing every impact possible for the taxa mentioned. In each category, scores are ranging from zero (no impact known or detectable) to five (the highest impact possible at a site).

Results/Conclusions

We found that overall, alien mammals in Europe reach the highest magnitude of impact while fish the lowest. However for arthropods, the lowest environmental impact was found, and plants had relatively low socio-economic impact. Many introduced mammals are herbivores, and transmit diseases to wildlife. Furthermore, they are damaging agricultural, forestry and animal production, and infrastructure. The main mechanism for birds is genetic pollution through hybridization. Many introduced fish are important predators, whereas arthropods mainly have economic impacts (agriculture and infrastructure). Plants, however, are important competitors for native species.
Magnitude in the two impact classes is overall highly correlated, with socio-economic impacts increasing faster than environmental; but patterns differ between taxa.
For mammals and birds, environmental impact elsewhere (in introduced areas outside Europe) is similar to environmental impact in Europe, showing that impact elsewhere can generally be useful as impact predictor. However, there is some variation among orders, with passeriform birds having slightly higher documented impacts outside Europe and rodents and anseriform birds within Europe. Also socio-economic impacts between elsewhere and Europe are correlated, but here carnivores seem to have higher documented impacts outside Europe.
All these patterns should be considered when trying to predict impact magnitude.