Quantifying the impacts of alien species: towards an IUCN Black List

One of the major transformations of the planet from human activities is the wholesale redistribution of species, through the deliberate or accidental translocation of their populations to areas outside their native range. These “alien” species have in many cases caused substantial deleterious impacts to the recipient environment (e.g. extinctions of native populations and species, the disruption of soil nutrient and water cycling, and natural disturbance regimes). Preventing and mitigating such impacts is a major drain on limited conservation resources. There is thus a considerable incentive to understand which species are currently, or are likely to be, the most damaging (and concomitantly, which have minimal impacts), so that we can direct legislation and prioritize action more effectively to preventative biosecurity, early eradication, or long-term control programs. A fundamental problem here is how to compare the enormous range of impacts attributable to diverse alien taxa, acting on different levels of ecological complexity, and at different spatial and temporal scales.