The scheme, described in the journal PLOS Biology and co-authored by Lincoln University Professor of Plant Biosecurity, Philip Hulme, proposes a standardised approach for ranking alien species relative to their negative environmental impact. In so doing, globally recognised ‘Black Lists’ of unwanted species can be produced.
 
By emphasising the impact of an alien species on the environment as a whole, rather than purely economic considerations, the approach can be used to rank invasive species in a way that can help governments and conservationists prioritise their efforts and direct appropriate programs for preventing, controlling or eradicating these species.
 
The ‘Black List’ scheme has been devised as a standardised system suitable for worldwide use, and is similar to the International Union for Conservation of Nature (IUCN) Red List, used to categorise species under threat from extinction.
 
In New Zealand, controlling plant or animal species that have been moved outside their native range by humans (either deliberately or accidentally) is essential for the survival of the country’s unique native plants and animals. Invasive weeds can threaten native plants and landscapes, while introduced predators like possums, rats and stoats threaten many native bird species with extinction.
 
This scheme classifies such invading species according to the magnitude of their impact on the environment: from minimal, where little or no effect is seen; to massive, where significant changes in native plants or animals occur, or an ecosystem is fundamentally altered. This means that for the first time comparisons can be made on the relative impacts of invasive plants, insects, birds and mammals.
 
The team have described scenarios where species cause increasing levels of damage to the environment through mechanisms like competing for food, transmitting disease, preying on other species, causing poisoning, toxicity, or altering an ecosystem’s structure or function.  

“In contrast to economic impacts, ranking species in relation to their environmental impacts has been a major challenge.  This approach uses clear guidance to combine the wide range of impacts on populations and ecosystems into a single metric,” says Professor Hulme, a lead researcher at the Bio-Protection Research Centre based at Lincoln University.
 
“This approach should enable consistent ranking across the world and allow the threats from invasive species in New Zealand to be judged in a global context,” he says.
 
This is the first time that a standardised system has been devised for comparing the impacts of non‑native species and, like the IUCN Red List, it is likely to be a useful guide and indicator of current and future trends.
 
The published study can be found at: http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001850