Why Scotland should learn to love hated Grey squirrels and Japanese knotweed

Many conservationists regard invasive alien species as one of the biggest threats to native wildlife. The government's wildlife agency, Scottish Natural Heritage (SNH), puts the cost of non-native species to the Scottish economy at £244 million a year.

Grey squirrels from North America chase out native red squirrels, rhododendrons from Spain smother natural forests, and Japanese knotweed bursts through floorboards. American mink kill seabirds, voles and salmon, while North American signal crayfish eat indigenous fish, amphibians and insects.

But we should not be demonising them, says Pearce, but giving them a warm welcome.

Pearce is an award-winning science author with 15 books to his name. He has been environmental and development consultant to the magazine, New Scientist, since 1992 and has reported from 85 countries.

His new book, 'The New Wild: why invader species could be nature's salvation', is a full-frontal attack on the conservation orthodoxy about alien species. "In the Anthropocene (the modern human geological era), fortress conservation is a doomed enterprise," he says.

"Humans are an inescapable part of the landscape. There are no pristine ecosystems and no blueprints for what they might be," he adds. "The old wild is dead. But the new wild is flourishing."

In Scotland Pearce points out that native red squirrels live best in non-native conifer plantations, and that native red deer have become "the curse of the highlands". The old certainties about native and alien species no longer hold, he argues.

"Scottish conservationists would love to go back to mythical wilderness, but it's not going to happen. We have changed things too much, including the climate. Efforts at rewilding end up as glorified gardens, constantly tended to keep them 'natural'," he told the Sunday Herald.

"What we need is a new idea of nature, in which alien species mingle with the natives, in which the old labels are abandoned. Alien species are nature's go-getters and colonists. In Scotland, as everywhere, we will need them to keep nature in rude health in the 21st century."

One of Scotland's biggest conservation groups, the Royal Society for the Protection of Birds (RSPB), however maintained the position that alien species are a worry. "Successful non-native invasive species really are admirable in terms of their extraordinary tenacity and vigour, but this does not make their introduction a good thing for nature," said RSPB Scotland's head of species and habitats, Dr Paul Walton.

"RSPB Scotland never demonises non-native species. But we do recognise their impact as a massive driver of global biodiversity loss - with human actions firmly to blame."

There was a need to be much smarter about how organisms were moved around and develop sensitive solutions, Walton suggested. "This is really about protecting and enhancing the incredible living diversity of our planet."

The Scottish Wildlife Trust accepted that alien species were "rarely a black and white issue" and that many were benign. But it pointed out that some could do serious harm, warning that unless grey squirrels were stopped they would make red squirrels extinct within a generation.

"These are complex ecological and ethical questions which, in the view of the Scottish Wildlife Trust, should be assessed on a case by case basis," said the trust's chief executive, Jonathan Hughes.

"We shouldn't demonise alien species, but we must be vigilant and take action on those few species which pose a severe threat to our rich natural heritage."

SNH's non-native species adviser, Stan Whitaker, thought that most conservationists took a pragmatic approach. "Only a small minority of non-native species become invasive, but those that do can have a serious impact on our native wildlife, economy and health," he said.

"The threat is greater than ever because of the unprecedented rate at which new species are being introduced, largely due to the increase in the global trade in plants and animals. Our focus should be on preventing non-native species becoming established because this is the most cost-effective, least environmentally damaging option."

Pearce's analysis was supported, however, by Professor Jim Dickson, a botanist and honorary senior research fellow at the University of Glasgow. There had been "hysteria" about Japanese knotweed, he said.

"It's absolutely inevitable because of globalisation that plants, insects and animals will move," he added. "Millions and millions of pounds are being spent on Japanese knotweed, rhododendrons and others, but I think we ought to learn to live with them and have a minimalist approach."

Six invasive alien species

Over 1,100 plants, animals and other wildlife from abroad have established themselves in Scotland, according to the government's conservation agency, Scottish Natural Heritage (SNH).

But only 185 of them are thought to damage the environment, and action is being taken to control less than a dozen species regarded as particularly harmful.

Grey squirrels

Introduced from North America in the late 19th and early 20th century, grey squirrels are bigger and stronger than native red squirrels. As a result reds have been pushed out of most of England and Wales, with less than 120,000 left living in Scotland.

Grey squirrels have also brought diseases with them like squirrelpox that have proved fatal for reds. This is "possibly the single greatest risk to the future of red squirrels in Scotland", says SNH.

Since 2006 a joint project involving landowners, government and voluntary agencies, Saving Scotland's Red Squirrels, has been working to control grey squirrels by trapping and killing more than 7,000 of them.

Rhododendrons

Introduced from southwest Spain in 1763, rhododendrons started in gardens but have spread widely across Scotland, particularly on the west coast. The rate at which the shrubs have taken over has accelerated in the last 50 years.

Though they look beautiful when flowering, rhododendrons form dense thickets that smother and damage native woodlands, killing plants and preventing saplings from growing. They are also thought to produce chemicals that may harm other plants.

According to SNH, the spreading shrubs are damaging 68 natural features on protected areas across Scotland, half of them woodlands. There are major programmes to cut back and uproot them.

Japanese knotweed

Japanese knotweed was introduced from eastern Asia in the 19th century. It grows to about two metres tall, with large triangular leaves and spreads fast underground via its roots.

It's become a major nuisance for property developers in some areas, as its bamboo-like stems can sprout between floorboards. It grows in urban areas, on riverbanks and on waste ground.

Japanese knotweed is difficult to control because surviving small fragments can start regrowing, which is why there are strict rules governing its disposal. It is killed by spraying or injecting herbicides.

American mink

American mink escaped from fur farms and became established in the wild in the 1950s. They have now spread across most of the country, apart from the very north of Scotland.

Mink are opportunistic predators that eat a wide range of animals, birds and fish. They have been blamed for the drastic decline of water voles, for damaging salmon populations in some rivers and for decimating seabird colonies in Argyll.

There have been major programmes to eradicate mink from the Western Isles, though they are still present on other Hebridean islands.

North American signal crayfish

Imported to Scotland in the 1980s, North American signal crayfish originated from west of the Rocky Mountains. They have since spread to many Scottish rivers, including the Tweed, the Clyde and the Tay.

Signal crayfish grow to a length of 16 centimetres or more, and live for 16 years. They eat aquatic plants and insects, and can also predate on fish and amphibians.

According to SNH, signal crayfish damage native flora and fauna by eating and displacing them, as well as maybe spreading disease. Their burrows can also destabilise riverbanks, and increase erosion.

New Zealand pygmyweed

Introduced into England in 1927, New Zealand pygmyweed has been recorded at 50 sites in Scotland and is probably present at many more. It grows underwater in ponds, lakes, reservoirs, canals and ditches, and produces small white flowers.

Pygmyweed grows vigorously and can form dense masses of vegetation that deprive other species of light and oxygen. Its dominance, says SNH, can squeeze out native plants, and the wildlife that depend on them.