Non-native species are organisms that have been purposely or accidently introduced to an area outside its original geographic range. Often, these non-native species become invasive where they thrive in their new habitat and can aggressively start to take over the environment by outcompeting some of the native species. This alteration can not only cause harm to the environment, but to the economy as well.

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There are thought to be more than 12,000 non-native species in Europe and several projects have been launched to tackle invasive species and protect ecological systems, but there are few resources and a limited amount of money available.

To help prioritize spending, a group of European scientists put together a list of the 10 most damaging invasive species in Europe, listing sika deer, Canada geese and zebra mussels to name a few.

But according to a new study, many of these invasive species claims are not backed up with hard evidence and this might be skewing priorities when it comes to dealing with them.

'Some invasive species are possibly getting a harder time than they deserve,' says Claire McLaughlan, a NERC-funded PhD student at the University of Cambridge, who led the study.

'It's an emotive subject but it needs to be looked at in a balanced way. For many of the species in the list of Europe's top ten worst invaders, we could find little evidence of their reported effects in the literature.'

'If this is the literature behind the worst species, then what is the evidence like for others?'

They found that the damaging effects of invaders are often assumed, rather than based on hard evidence. Some invasions, particularly those in environments, which are already severely damaged, could even enhance ecosystem services.

'It's context-dependent,' explains McLaughlan. 'For example, with the zebra mussel, if they were to invade a stream full of rare, native species, they would obviously be very damaging.'

'But if they were to become established in a large man-made reservoir with very few species and an algae problem, they could help to process the algae and improve some ecosystem services.'

'We're not for a moment suggesting that you should introduce invasive species anywhere; prevention is always better than cure.'

'But with the species that are already there, there's only limited money available and we need new ways to prioritize which species we tackle. That has to be based on the evidence of their effects.'

The study is published in Acta Oecologia.

See more at Planet Earth Online.

Canada geese image via Shutterstock.