THE presence of invasive species is an everyday reality for millions of Scots.

So commonplace are the grey squirrels leaping around our parks or the giant hogweed growing on our greenbelts that many of us don’t notice or think about whether they should be there in the first place.

Yet as we move through a time of biodiversity crisis the existence of these plants and animals within our borders seems ever more incongruous.

If Scotland’s natural heritage is to survive the coming storms of climate change, the moment to act upon invasive species has well and truly arrived.

The Scottish Invasive Species Initiative (SISI) is the biggest non-native species control programme in the UK.

It coordinates efforts to tackle invasive plants and animals in nearly all of mainland Scotland north of Perth.

“There are lots of invasive plants and animals that arrive into Britain every year,” said Callum Sinclair, a SISI project manager.

“A great many of them are detected on arrival and removed. But a very small proportion become invasive, which means they’ve escaped into the wider environment and are starting to compete with native species.

“For example, American mink is a new predator which is very adept at hoovering up our native water voles.”

Mink have been living wild in the UK since escaping (or being released) from fur farms in 1950s and 60s. Their taste for water voles has contributed to the rodents disappearing from 94% of their former habitat.

Mink also prey upon ground nesting birds and their eggs, causing problems for conservationists encouraging the recovery of fragile populations. “They’re fantastic animals,” said Sinclair.

“Fantastic predators and colonisers and very mobile. “When they reproduce they will move approximately 20 kilometres to find a new breeding area. But some move as far as 80 to 100 kilometres.

The National: A raft used to detect and catch American mink A raft used to detect and catch American mink (Image: Scottish Invasive Species Initiative)

“So, the tactics for American mink are not really about eradication because we’re always going to have animals that are looking to move in.

“Instead, what we’ve done is draw a line from Perthshire to the West coast, with everything north of that area coming inside the boundary where we try to depress their numbers as low as possible to ensure that we protect areas like Caithness where they haven't currently reached.”

To do that work requires a network of volunteers who regularly monitor nearly 700 kitchen table sized floating rafts on top of which sits a tunnel lined with clay.

Once mink footprints are detected, one of the 350 volunteers across Scotland lays a live-capture trap and proceeds to check it every day.

Between 2018 and the end of 2022, 560 mink have been caught and humanely dispatched.

Sinclair added: “The manpower required to run that network just wouldn’t be feasible without volunteers.

“It’s the equivalent of 56 people working full time on the project for a year. We’ve just not got the money to pay for that level of labour, so the volunteer effort really is essential.”

But it isn’t just animals causing problems for our environment.

Rhododendron – a richly-blooming species brought over to the UK from Spain or Portugal around 1793 – is considered to be Scotland’s most threatening invasive plant.

“Deer and rhododendron are the two biggest threats to our native woodlands,” said Arina Russell, a policy and advocacy manager for the Woodland Trust in Scotland.

In Scotland’s rainforest zone on the west coast, which boasts some of the world’s rarest lichens, the plant has found a particular foothold.

“It does better in these areas because of the year-round mild temperatures and the high level of rainfall,” said Russell.

“But it essentially chokes the woodland. It takes over and shades the woodland floor, preventing the spread of lichens and bryophytes that make the forest so special.

The National: A worker using rope access to eradicate rhododendron A worker using rope access to eradicate rhododendron (Image: Forestry and Land Scotland)

“It also spreads like wildfire and it’s so widespread now that the longer we wait, the more expensive it is going to be to tackle.”

But getting rid of rhododendron isn’t easy. While methods such as stem injection, cutting and spraying with herbicide can be all effective, the plant is so abundant and fast spreading that it’s necessary to scour large areas to ensure every last one has been despatched.

Russell said: “It has to be done at scale and it has to be long-term.

“You can’t just do it in an individual garden or an individual woodland because if an adjacent landholding has rhododendron it will just makes its way back.

“We’ve had people using rope access on really steep cliff to remove plants there, which is a costly thing. But we have to do it otherwise there’s no point in doing the rest of the woodland.”

Over the last decade, Forestry and Land Scotland (FLS) has treated rhododendron within approximately 11,000 hectares of rainforest zone on the west coast. That’s required an investment of some £13.5 million, with only follow-up treatment needed on around 8000 hectares.

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“We’ve made steady progress in a lot of areas but the two most effective advances in recent years have undoubtedly been that advent of drones and a greater degree of cooperation and improved partnership working,” said Colin Edwards, national environment manager for FLS.

“The aerial view that drones give us are incredibly valuable in helping us to assess the extent of the rhododendron issue at any given site and helping us to prioritise, plan and execute our treatment operations to make them as effective as possible.

“That unparalleled mapping ability has given us a better picture of what we’re dealing with and has also helped to galvanise opinion amongst our neighbours and partners that concerted action needs to be taken.”

Climate change may very well result in further complications to Scotland’s invasive species landscape, which is why ecologists stress the importance of making habitats as healthy as possible right now.

“We know that pests and diseases will be exacerbated by climate change. But that’s why we need to build better resilience in all of our landscapes,” said Russell.

“It’s all about joining up fragments of woodland, connecting habitats and enlarging the space they inhabit. “I really want Scotland’s rainforest to be around for future generations and not be choked by rhododendron.

“We can’t be left thinking ‘Oh, I wish we’d done something’”.

The Scottish Invasive Species Initiative runs several volunteer groups across Scotland and encourages everyone to get involved. Details can be found at