The alien invaders threatening Scotland's wildlife

There are fears muntjac deer could appear in Scotland

Scotland's wildlife is increasingly at risk from non-native species, according to the country's nature agency.

NatureScot says there are more than 180 species - 122 plants and about 60 animals - which pose threats such as killing wildlife or damaging habitats.

But how have these species arrived here - and which are seen as the next big threat?

Where have they come from?

Pink salmon, which are native to Pacific Ocean waters, have been found in Scottish rivers

Some reached Scotland on the hulls or in the ballast of ships, while others have escaped from gardens, private animal collections or zoos.

Other species have spread from elsewhere in Great Britain and continental Europe.

Pink salmon, a fish native to Pacific Ocean waters, have been appearing in Scottish rivers in recent years. It spread to parts of northern Europe after being released into rivers in Russia in the 1960s.

"Unprecedented numbers" were found in Scottish rivers in 2017 and were filmed spawning in the River Ness in the Highlands. Anglers reported seeing the fish again last year.

Abandoned exotic pets also pose a potential threat.

Last week, several tropical fish were found in the River Ness at Inverness. The warm water fish, which did not survive long in the cold river, may have had diseases harmful to native species.

The worst offenders

Crayfish pose a threat to native freshwater species

NatureScot said the worst invasive species included carpet sea-squirt, a marine animal originating from Japan that "smothers" seabed habitats. It has been found in the Clyde and parts of Argyll.

Brownish with a leathery texture, carpet sea-squirt grows quickly and covers over areas of seabed, including fish spawning and feeding grounds.

North America signal crayfish have been found at more than 20 locations in Scotland, including lochs in Dumfries and Galloway and a quarry pond in the Highlands.

The crayfish have been described as an "aggressive" predator that kills native freshwater animals and burrows into riverbanks, causing erosion. They feed on fish eggs, as well as young salmon, trout and plants.

The threat they pose to wildlife is considered so serious that it is illegal to put even accidentally-caught crayfish back into water. The advice is to kill them instead.

Plant strife

Giant hogweed can be found on riverbanks

Some of the plants posing risks to native species were brought to Scotland by private collectors in the 19th and 20th Centuries.

They have spread into the wild and in some places have taken over. In these areas native species have become scarce or died out.

Dense growths of Japanese knotweeds, giant hogweed and Himalayan balsam have been found on riverbanks.

They support fewer species of insects than native plants, and when they die back in winter they leave river banks bare and prone to flood erosion.

Efforts to tackle their spread include work in North Harris to control giant rhubarb and efforts led by Forestry and Land Scotland to remove rhododendron. In Argyll, NatureScot has funded a community project to eradicate rhododendron in Glen Creran.

What are the next threats?

Killer shrimp could be one of the next invasive species to appear in Scotland

NatureScot - formerly known as Scottish Natural Heritage - is asking people to look out for species it expects could soon arrive in the country.

They include 3cm (1in) long freshwater killer shrimp, which were first recorded in the UK in 2010 but have not yet been seen in Scotland. The shrimp can outcompete rivals for food.

It preys on an array of species, including other shrimp, damsel flies and aquatic insects like water boatmen.

Killer shrimp have been linked to local extinctions in other countries, and research has suggested that it could be altering the behaviour of other species through fear alone. Scientists found the creatures it preyed on expended more energy trying to evade the shrimp than on normal behaviours.

There are also fears about the introduction of muntjac, a deer that originates from China and is described as "one of the most destructive non-native animals in Britain".

Dubbed the "Asbo Bambi" because it is considered such a nuisance, the deer can damage woodlands, crops and gardens. They have been present in England since the late 19th Century.

In Scotland, it is illegal to the keep the deer without a licence and it is an offence to release them into the wild.

What's the scale of the problem?

Exotic fish have been found dumped in the River Ness

NatureScot said there had been a "dramatic increase" in the number of non-native species arriving and becoming established since the start of the 19th Century.

The State of Nature Scotland 2019 report suggested there was no sign of this trend slowing down.

The UK's Department for Environment Food and Rural Affairs (Defra) believes about 25 invasive species have become established in Great Britain in the last 20 years.

It estimates that between 36 and 48 new species could become established in the next two decades.

NatureScot said non-native species were one of the biggest threats to Scotland's wildlife, and cost industries like agriculture and forestry hundreds of millions of pounds each year.

The agency aims to prevent their introduction and spread in the first place.

"Everyone has a responsibility to help stop the spread of invasive non-native species," said a spokeswoman.

"Pets should never be released into to the wild - it's cruel as well as dangerous for wildlife."

How do you tackle a voracious predator?

Mink have almost been eradicated from parts of the Western Isles

NatureScot says the American mink is a "voracious predator" which preys on ground-nesting birds and water voles.

Those found in Scotland and other parts of Great Britain are descended from mink that escaped or were deliberately released from fur trade farms in the 1950s and 60s.

There has been an effort to control their numbers and even eradicate them from some areas.

Trapping by the Hebridean Mink Project in the Western Isles has reduced their numbers to "very low" in Lewis and Harris, where no young mink have been caught since 2015. The animals have also been "largely removed" from Uist.

Mink are also a target species for the Scottish Invasive Species Initiative.

The project has been working with 10 fisheries trusts and the University of Aberdeen to control mink along with other invasive species at sites in a large area of northern Scotland.