Ranking alien species based on their risks of causing environmental impacts: a global assessment of alien ungulates.

For an efficient allocation of the limited resources to alien species management, the most damaging species should be prioritized. Comparing alien species based on their impacts is not straightforward, as the same species can cause different types and magnitudes of impacts when introduced to different contexts, making it difficult to summarize its overall impact. The Environmental Impact Classification of Alien Taxa (EICAT) systematically summarizes and compares detrimental impacts caused by alien populations to native biota and has been adopted by the International Union for Conservation of Nature (IUCN). For each alien species, all reported impacts to native populations within the introduced range are classified into five levels of severity, from negligible impact to irreversible local extinction. Currently, EICAT only compares alien species based on their highest impact, thereby ignoring variation in impact magnitudes. Here, we used information on the variation in impact magnitudes of alien species to estimate their risks to cause high impacts if introduced to a novel environment. We demonstrate the usefulness of this approach by classifying the global impacts of alien ungulates. We found impact reports for 27 of the 66 alien ungulate species established worldwide, highlighting substantial knowledge gaps in invasion science. We classified a total of 441 impacts to native fauna and flora caused by these 27 species. Twenty-six of the species were found to cause harmful impacts (native population declines or local extinctions). Mouflon (Ovis orientalis, Gmelin, 1774) and dromedary (Camelus dromedarius, Linnaeus, 1758) had a higher risk of causing local extinctions if introduced to a novel environment than sika deer (Cervus nippon, Temminck, 1838) and goats (Capra hircus, Linnaeus, 1758). Including risk of high impacts allows to discriminate among species with the same EICAT classification and improves alien species prioritization for management.