In February, I travelled to Torridon in north-west Scotland with a group of friends to plant trees. It is a beautiful place – a windswept hillscape of rusted bracken and silvered rock. The ground on the flats is waterlogged in some places, heath in others, all rather stony. But mainly we worked on a steep slope, ankle-deep in thin, brown brash: chopped wet sticks that slid under our feet. These were the remains of rhododendrons.

Only a couple of years ago, this hillside was dense with them. These beautiful plants are a familiar backdrop to life in upland landscapes – heavy with magenta and lilac flowers in the summer and bearing sprays of thick, glossy bottle-green leaves year round. But they aren’t native – they were introduced to the UK in the Georgian era, imported as pot plants and displayed as ornamentals.

The plants had their own agenda. Over the coming years and decades, one variety – the Rhododendron ponticum – took on a life of its own, spreading voraciously through the UK’s woodlands and wild places. It has an almost frighteningly efficient method of dispersal: each flowerhead can produce 5,000 windborne seeds, a single bush a million or more each season. Once established, the rhododendron might also spread laterally, sending out branches horizontally; where they touch the ground, they will put down roots. So the thicket extends, branches interlaced, interlocking, a tight tangle of living matter up to five metres tall.

“It was like a cat’s cradle in places,” says Louise Gray, of the morass that once faced her on this patch of ground. Gray is a partner in the Ben Damph Estate at Torridon, where she has been spearheading efforts to clear 250 acres of ground from rhododendrons as part of a Scottish government-funded programme of removal of invasive species from conservation areas. First planted on a neighbouring sporting estate as cover for game, the plant had spread along the coast, blocking a road and “completely covering” the fertile ground.

Not only do they spread like a wildflower wildfire, but their success actively inhibits other species. Their impenetrable foliage blocks out the light, for one thing, and their leaves are unpalatable for livestock – even poisonous. Even more seriously, they are said to impact on the symbiotic communities of soil fungi in the roots underground (known as mycorrhizas) – devastating the ground’s fertility. “So, what looks like a patch of green is actually devoid of life, except rhododendrons,” says Gray. “This should be full of ancient forest and the delicate lichen and moss and insects and ultimately mammals. But there is nothing there. All I have seen living under the rhoddies are spiders.”

Rhodendrons are so widespread that they have attracted the ire of government bodies across the UK; Gray’s estate was awarded a grant towards the costs of their removal. But it is not a simple task. At Ben Damph, they’ve experimented with almost every method. On accessible land, they used a mulcher – a machine with a flailing arm that munches up the plant and spits it out as bark chippings. Elsewhere they used chainsaws, and teams built pyres of the offcuts and burnt them as they went. In other places, they drilled holes in the woody stems and injected them with glyphosphate, then returned later to clear the dead plants. The remaining stumps had to be sprayed with herbicide to stop them resprouting. It is a violent process, and where it has taken place the ground is littered with the chopped-up remnants. Hence the new trees.

Over a busy weekend in snow, hail and watery winter sunshine, we formed straggling assembly lines – one of us moved ahead with the tree guards, another dug cat holes and shook in fertiliser, another followed to hammer in supports, and finally the planter – the most sought after role – came with a bin bag of tiny seedlings. Over the weekend we planted around 1,000. In the following weeks, Gray and her helpers planted 2,000. On Sunday night we stood back to admire our handiwork. A forest in the making: of birch, oak, rowan, hazel and aspen. All good native species.

* * *

The language of ecology often crosses over with the language of empire. Plants indigenous to a place are “natives”; those that have recently arrived are “aliens” or sometimes “colonisers” or “invaders”. Indeed, the colonial era was a peak time for the transport of species around the world. Imperial powers transplanted crops to new continents – grew rubber in tropical areas where no rubber grew before, brought wheat, tea and countless other foodstuffs to new lands. They shipped livestock around the world, clearcutting vast forests for pasture in which to graze them. Sometimes they took plants and animals with them just because they liked them. They imported pretty potplants home to their gardens – the Rhododendron ponticum among them, which arrived in 1763, in the stock of exotic plant dealer Conrad Loddiges – and exported plants and animals that made them feel at home in the colonies.

In 1890, an “acclimatisation society” in New York released into Central Park examples of every avian species mentioned in the works of Shakespeare.

The ousel cock, so black of hue,
With orange-tawny bill,
The throstle with his note so true,
The wren with little quill.
The finch, the sparrow, and the lark…

Most disappeared into the woods, and were never seen again, but the 60 starlings (“I’ll have a starling shall be taught to speak”) flourished. The American starling now numbers approximately 200 million.

In Australia, wild rabbits were introduced by a British grazier, Thomas Austin, in 1859, to “provide a touch of home, in addition to a spot of hunting”. Over the next 60 years, they spread over four million square kilometres of Australia, their numbers booming to an estimated ten billion, and prompting the erection of the famous “rabbit-proof fence”, 1833km in length. Later, in the 1950s, many were killed by a planned introduction of the deadly disease myxomatosis. It’s still illegal in several states to own a pet rabbit without a license.

These are textbook cases, if you study ecology or invasion biology. When invoked, they bring a chill into the room – the law of unintended consequences, the tragedies that should have taught us a lesson about messing with the natural order. Fear of non-native species, particularly those that make runaway successes of their new situations, is now ubiquitous. The International Union for the Conservation of Nature regularly publishes lists of the world’s “worst invasive species”, while the Environment Agency has released one for the UK. The “killer shrimp” (Dikerogammarus villosus), the American signal crayfish, water primrose, giant hogweed, Japanese knotweed and Himalayan balsam all feature among Britain’s least wanted.

Great efforts are made to “control” these populations now: culling, uprooting or otherwise exterminating these alien invaders. In this way, many conservation bodies devote huge resources to the killing of wildlife. It’s an uncomfortable reality, one that poses deep ethical questions for those devoting their lives to saving the environment.

The grey squirrel, for example, was brought to the country as a charming garden feature for country houses in the 1870s. Now they have spread across the UK, particularly in England and Wales, and number around 2.7 million. They are widely agreed among conservationists to be the greatest threat to the survival of Britain’s 290,000 red squirrels, which are more timid and more susceptible to disease. Fear for the future of the red squirrel, a beloved native animal, has galvanised the public, and government funding backs the attempt to head off their decline.

As a result, efforts are underway across the country to trap and kill grey squirrels in targeted areas, with the hope of slowing their advance into red squirrel heartlands. Methods include trapping and “drey poking” (flushing the animals from their nests), then shooting them with a rifle or air gun. “It’s certainly not something we take pleasure in having to do,” says Gwen Maggs, a conservation officer for the Saving Scotland’s Red Squirrels project. “However, at this time, controlling grey squirrels through humane dispatch is the only method we have available if we want to protect red squirrels.” Controlling their numbers in other ways – administering contraceptives, or increasing numbers of their natural predator the pine martin – “are a long way off,” she says. “We simply can’t afford to wait around to find out if they will work.”

Maggs and her colleagues are well informed and well trained, and she is keen to underline that the culls are not based on a disgust for their target. “We believe it’s important not to demonise grey squirrels for just doing what comes naturally to them,” she says. However, the issue of population control – of prioritising one species over another, with deadly consequences – is morally complex.

One issue is that different groups have different, sometimes clashing priorities. Some rail against Himalayan balsam, an attractive pink wildflower first introduced at Kew Gardens in the 19th century, which grows on riverbanks and wastegrounds and is regarded as a “pest” that smothers local grasses and flowers. The Environment Agency advises landowners to pull them up or spray them with glyphosphate. Yet those who worry about the declining populations of bees love them for their high-sugar nectar, which continues months after most British flowers have dried up. How to balance these competing values?

And what makes a species “invasive” as opposed to simply non-native? “It’s in the eye of the beholder,” says Mark Williamson, professor emeritus of York University, a pioneer of the study of biological invasions. “Possibly the two commonest meanings are ‘spreading widely and abundantly’ and ‘a damn nuisance’.” Officially, the term is defined as referring to a foreign species that threatens native species, he adds, but in practice it means a foreign species that an expert or pressure group believes threatens native species. These beliefs can change. In the early 20th century, great efforts were being made to exterminate a squirrel blighting the Highlands of Scotland – then described as a “devastating menace.” But back then, it was the red squirrel that was seen as the problem, having been freshly reintroduced to the area from English and European stock.

* * *

The narrative of foreigners usurping or out-competing struggling locals is a common refrain at the moment, although its usually heard in the context of human movement, not animal. “The point is, they use the same kind of language, and the same kind of thinking is involved: the assumption that anything foreign is suspicious, if not downright bad,” says veteran science writer Fred Pearce.

In his book The New Wild: Why Invasive Species Will Be Nature’s Salvation (Icon Books), he argues that although humans have speeded it up, the movement of species around the world is a natural process. Opposition to “alien” species is often misplaced and the dangers overplayed – especially when compared to equally aggressive native species like nettles or brambles – due to what he terms “green xenophobia”.

Some refer to “climax ecosystems”: the idea that the different species in a habitat come, over time, to live in a steady-state, symbiotic relationship with one another, each in a different biological niche they have co-evolved over millennia to fulfil. “If you take that view, then almost anything that comes in from outside will be regarded as intrinsically bad or untrustworthy,” says Pearce. But, he adds, nature is far more dynamic and adaptable than we give it credit for. If new species arrive and do well, then that’s a biological success story.

“We’ve made a huge mess of this planet,” says Pearce. “We need nature to make a comeback in many ways, for many reasons. Often alien species are actually very good at recolonising areas that we’ve messed up – they can be part of the solution rather than part of the problem.”

In some locations, non-native species play an important role – in Puerto Rico, for example, forests of invasive trees like the African tulip have come to harbour rare species like the native coquí frog – some say more successfully than natural forests (although this is debated in ecological circles). In Mauritius, a conservation group has even taken to translocating non-native tortoises to the island to replace a similar species that has been extinct there since the 19th century. They hope that the Aldabra Giant Tortoise, from the Seychelles, will fill a role in grazing and seed dispersal similar to its lost forebear.

In the UK, beavers have been reintroduced from continental stock after going extinct in the 16th century, and there is a proposal to “bring back” lynx after a gap of 1,300 years. Both moves have wide support in the environmental community, partly because they were – at some point in the past, however distant – native. “Personally, I think the idea of there being a ‘bad’ or a ‘good’ way to intervene in nature is built on pretty flimsy foundations,” says Pearce. “By all means, let’s help the red squirrel recover – to take one example. But let’s recognise that we’re doing that for us, for our aesthetic, rather than for nature. Ultimately, I don’t think nature cares one way or the other.”

Still, it can feel uncomfortable to call into question the motivation of conservationists, many of whom devote their lives to helping the environment, and think deeply of the ethical implications of their work. Pearce argues that a non-native species finding a toehold in a new location increases local biodiversity – and therefore its capacity to adapt to changes, such as those related to climate. Others point out that if incomers succeed at the expense of local species that exist nowhere else, there is a net loss of biodiversity. The crux of the issue for many seems to rest upon the question: did we do this?

* * *

This is how Gray, my tree-planting friend, sees it. “Humans cause so much damage when they interfere,” she says. “The rhododendron problem was started by mankind. It is our duty to reverse it.” She acknowledges their beauty, but adds: “In Wester Ross they have taken over some of the most beautiful and precious remnants of wilderness we have. They have destroyed whole swathes of land that should be Atlantic rainforest.”

As I picked my way over the hillside, doling out fertiliser, I saw signs of new life. Pines were sending up seedlings, bristling with needles, and tufts of grass. But there were also smooth, strong shoots in a vivid shade of green: the rhododendrons making their revival. They have the capacity to grow back from a cut branch, or disembodied root, or a stump that has weathered the chemical onslaught.

It is astonishing and unsettling to witness this resurrection. “They are triffids,” says Gray, with a shiver. To rid Torridon of rhododendrons, they will have to kill them over and over again for years, decades even. “It will never be over,” she says. “I’ll be pulling up rhoddies til I die.”