Natives forced to slug it out, but are proving no match for green invader

But the bad news is that their place is being taken by a pushy relative which is arriving en masse in Scotland.

The native black slug (Arion ater) is under attack from the invading force originating in Spain (Arion flagellus) to such an extent that scientists believe the endemic species could eventually be forced out altogether.

The black slug is already under pressure from hybridisation with other alien species and climate change is now helping the visitors, used to warmer climes, make their mark here.

Dr Les Noble, from the University of Aberdeen, warned that the invader, which he calls the "Spanish green slug", or "Spanish stealth slug", is flooding Scotland and wiping out the natives.

Dr Noble, a specialist in molecular zoology and genetics, said Arion flagellus were reported in Spain in the 1800s and later in Ireland, before their discovery in the 1970s in the Wirral peninsula and in Durham. It is also known now as the "Durham slug" and has spread across the country.

"These things provide an increasing problem for us. They hide away really well and the eggs are about half the size of the endemic species and are difficult to find.

"But once they get into an area they go like wildfire, they have a reproductive capacity at least twice that of our native slugs. Wherever they come in the native molluscs really get badly hit."

Dr Noble said he has seen the problem at first hand on his croft in Aberdeenshire, "All the big black slugs have just vanished," he said. "The green slugs are showing all the signs of being an invasive species, but we don't know what is allowing them to push the other ones out.

"It may be they are bringing in some sort of parasite or pathogen that they have evolved with so they can cope with it, but our endemic species can't."

Dr Noble said: "The reason I call the Arion flagellus the stealth slug is not a lot of people in this country are aware of it. It's a major concern for me and I want to do further research.

"Maybe a lot of people would not care, but our endemic black slug is now becoming an endangered species. It is on the verge of being pushed off the edge.

"The black slug is being driven to the mountains and the moorlands and on higher ground."

He said warmer temperatures from climate change are helping the foreign mollusc thrive.

"People will start to see more of these Spanish green slugs because this time of year is ideal for them because it is warm and moist. It will help them increase in size - we are in the middle of an invasion.

"If we start to lose the big black slugs in a garden, does that mean you will get particular weeds start to become more prevalent that they would otherwise keep in check? Will we get the parasites and pathogens transmitted from the green slug to other things like snails and such like?"