There are other issues to consider. An invasive species is commonly thought of as a non-native organism whose population is increasing and spreading, and which causes, or may in the future cause, negative environmental, economic or social impacts. But what do we mean by ‘non-native’? The usual understanding is that it’s an organism introduced into a new country by people – on purpose or by accident – rather than getting there ‘naturally’ by walking, flying, swimming or wafting on the wind. In Britain, this often means anything brought here after rising sea levels cut us off from the European continent sometime between 7,000 and 9,500 years ago at the end of the last Ice Age. But what then do we call extinct fauna and flora that we have since reintroduced? The western capercaillie and European beaver, both around long before we became an island, were wiped out less than 300 years ago by hunting. According to the above interpretation, we couldn’t treat them as ‘non-native’, yet both occur in twenty-first-century Britain thanks to human intervention. (The capercaillie was reintroduced to Scotland in the late 1830s, and successful releases of beaver have occurred at several locations across the UK over the last decade or so.)

At least with our current population of capercaillies and beavers we are sure how and why they’re here. But, with many other species, it can be tricky to know when they first arrived, and whether people were involved. The sycamore, first recorded growing in the wild in Britain in 1632, is often regarded as introduced. Some say it was brought over in the fifteenth or sixteenth century, others suggest an earlier, possibly Roman, introduction, but either way we’re looking at a non-native. Or are we? The problem is that sycamores, indigenous to central Europe, are fast-growing, fast-spreading trees well suited to Britain’s temperate climate. While people have planted most of our sycamore stands, we can’t exclude the possibility that sycamore seeds may have also taken root naturally from time to time having been blown across from the continent. If true, our sycamore population might comprise both natives and non-natives.

A question mark also hangs over the white-clawed crayfish. Considered our sole indigenous freshwater crayfish, and the focus of intensive conservation activity, some experts now suspect the crustacean was introduced for food in the thirteenth century. Even with things that we’re 100 per cent sure are foreign, pinning how and when they arrived is a challenge.

Advances in DNA sequencing and analysis techniques are now shedding light on these mysteries. For instance, the presence on the Orkney Islands, off the north coast of Scotland, of a vole found nowhere else in Britain is something of a conundrum. The Orkney vole, as it’s known, is an endemic subspecies of the common vole, a variety found on the European continent. So, did the Orkney vole scamper there naturally on a temporary land bridge from Europe before the last Ice Age, 20,000 years ago, and somehow manage to weather the chill while its British mainland relatives died out? Or, as seems more probable and has long been suspected, is the vole a far more recent introduction by people? Genetic studies seem to support the latter hypothesis, with Orkney voles shown to be more related to those in southwest France and Spain than to geographically closer populations. The voles are now thought to have arrived with humans on Orkney during the Neolithic period some 5,000 years ago in ships directly from the continent. The rodents may have stowed away in consignments of livestock fodder, or were intentionally brought as food items, pets or even for religious reasons.

Another difficulty with the notion of an invasive species as currently defined are those organisms, unarguably native, which behave in a manner that can only be described as, well, ‘invasive’. Take hedgehogs. On the British mainland they couldn’t be more popular; and faced by a catalogue of threats including habitat loss and agricultural pesticides, not to mention the risk of being flattened by motor vehicles, they’re the focus of a nationwide preservation society. Yet on islands, Mrs Tiggy-Winkle goes berserk.

In 1974, someone on South Uist in Scotland’s Outer Hebrides released half a dozen hedgehogs into their garden to keep down the slugs and snails. Within a couple of decades, the introduced mammal’s population had swollen to 5,000 individuals, and had spread across causeways to the nearby islands of Benbecula and North Uist. The Hebridean hedgehogs eschewed garden pests in favour of the chicks and eggs of dunlins, ringed plovers, redshanks, lapwings and other shoreline birds, and are implicated in a 50 per cent decline in their numbers. Since 2001, nearly £3 million has been spent on removing the invaders. At first, they were culled with lethal injection, but tactics changed when animal rights groups kicked up a stink, so right now the hedgehogs are trapped alive and released on the mainland. In New Zealand, where introduced European hedgehogs have also run riot, the authorities have been less squeamish, eradicating them from the 86-hectare Quail Island.

Natives don’t just act up on islands. Foxes and carrion crows, both indigenous to the UK, can obliterate nesting bird colonies. Red and roe deer can be every bit as destructive to woodlands and agricultural crops as their introduced counterparts: muntjac, fallow and sika deer. Common ragwort, bracken, brambles and nettles – natives all – often spread out of control, suppressing other plants, and are sometimes regarded as weeds. Even beech trees, indigenous to England, upset Scottish conservationists when their seedlings throw shade north of the border. A further complication is that a non-native might misbehave in one location but elsewhere – typically back home – cause no trouble at all, indeed may even be endangered. So, it’s seldom fair to tar the entire species with the invasive brush. The hedgehog was one example but there are plenty more, such as the rhododendron, which spreads aggressively here but not in New Zealand, or the Japanese knotweed, whose penetrating roots are blamed for weakening buildings across Europe but which in Asia is surprisingly scarce.

Professor Helen Roy, a leading British expert on biological invasions based at the Centre for Ecology and Hydrology in Oxfordshire, suggests that perhaps the only time when an understanding of whether a troublesome organism is native or not truly matters is when an incursion is still recent, or hasn’t yet occurred, since it offers a possibility of intercepting it. Most of the time, though, the focus might be better spent on understanding, managing and, where practical, reducing any negative impacts of the troublemaker in question.

Despite these complexities, the term ‘invasive species’, as a label for fast-spreading, harmful non-native organisms, seems embedded in the common psyche. So how many do we have now in Britain? This is difficult to answer but thinking about it in terms of the invasion process can help. Scientists recognise several key hurdles any aspiring invader must clear. First, the organism needs an ‘invasion pathway’; in other words it must get itself transported to a new region by humans. We may intentionally facilitate this, as with organisms brought for agriculture, hunting, horticulture, aquaculture, as biological control agents and for countless other reasons. But plenty of things use us to move around without our consent; think of plankton suspended in the ballast water of ocean-going vessels, the legions of wood-boring beetles holed up in internationally traded furniture, the seeds and spores peppering the mud of a tourist’s hiking boots, the soil-borne invertebrates hitching a ride in plant pots. Crucially, the globetrotter must survive transit. This is no mean feat; the journey might take weeks during which the stowaway may be subjected to extremes of temperature, lack of food or moisture, and other privations. And that’s even before the prospective invader has to contend with strict quarantine measures imposed by vigilant customs officials.

Before moving on, an important clarification is perhaps required. Should a new species reach our shores from its region of origin with no direct human intervention – be it by flying, rafting on an ocean current, blowing in the wind, catching a ride on a (non-human) animal, or in some other fashion – then this organism is classed as a ‘natural colonist’, and not counted in the statistics. A classic example would be the Eurasian collared dove, which started spreading west from Asia in the nineteenth century and was first recorded breeding in Norfolk in 1955. The distinction between natural colonist and human-mediated invader is not always as clear-cut. For instance, how to treat all those weird and wonderful new species set to colonise Britain – both on land and off our coasts – as the climate begins to warm? We may not directly be involved in their spread, but since human activities are driving global climate change, we are not innocent of this process. We might be happy to welcome recent natural colonists such as the tree bumblebee and small red-eyed damselfly, but are liable to baulk at accepting a malarial mosquito.

Importantly, though, an element of the natural about part of a new organism’s journey movement won’t earn it a pass as a natural colonist, if humans are known to have played a key role somewhere along the line. Take the Asian hornet and harlequin ladybird. Both insects are thought to have made incursions from the continent as a result of individuals blowing across the English Channel, but both only reached Europe in the first place as a result of human activities: the shipping of pottery from China, in the case of the hornet, and introduction as an agent of biological control, in the case of the ladybird.

On arrival, the introduced organism then has to escape and reproduce. Those that maintain a viable population, without further human intervention, are regarded as ‘established’ or ‘naturalised’, the rest dismissed as ‘casuals’. (Incidentally, the term ‘feral’, according to Sir Christopher Lever, a British author of several well-known books on introduced animals, should be reserved for creatures that have ‘lapsed into the wild from a domesticated condition’, not simply escaped from captivity. Noting that populations of the American mink established in the British countryside are often referred to as ‘feral’, Lever insists that to regard the non-native mammal as ‘domesticated’ is ‘preposterous and wrong’.)

Only around 10 per cent of organisms brought to a new country persist unaided in the wild. Disagreements over what constitutes non-native flora and fauna, along with the patchiness of data, have led to varying estimates of the number in Britain. The most recent figures, for 2017, suggest that 3,163 species were present in England, Scotland and Wales, of which 1,980 – mostly plants – had established and were reproducing in the wild. In Ireland, there are at least 1,266 non-native species, of which two-thirds are plants.

Finally, for something to be regarded as truly invasive, it needs to spread and expand its population enough to cause measurable negative impacts. On average, a minority of established non-natives register as a problem, although the likelihood of invasiveness varies: just 4 per cent of introduced insects in Britain are classed as invasive, compared with 32 per cent of non-native fish and 85 per cent of exotic plants.

This is all a long-winded way of saying that a tiny proportion of introduced species will ever earn the title ‘invasive’. According to Helen Roy’s team, which keeps a running score, in 2017 at least 275 (about 9 per cent) of the non-natives established in England, Scotland and Wales cause negative impacts. While around 5 per cent of the 1,266 introduced species recorded in Ireland are classed as invasive. These numbers will almost certainly climb.

So, let’s turn to those impacts which can be classed as environmental, economic or social. Again, there is much debate, but the number one ‘environmental’ charge against invasives is that they harm natives, through predation, competition or, perhaps, by spreading disease. Invasive species are increasingly listed alongside habitat destruction, pollution and overhunting as key threats to wildlife. The 2005 Millennium Ecosystem Assessment says they’re a major driver of biodiversity loss. A recent review of 247 kinds of plants and animals around the world that had vanished since 1500 found invasives to be the second most common cause of extinction (hunting, fishing or harvesting came first). For the amphibians, mammals and reptiles on this list of the disappeared, invasives were the number one culprit. Among the most frequent offenders were rats, cats and goats, along with diseases, such as avian malaria and chytridiomycosis, a fungal condition that is wiping out amphibians around the world.

Many of the most often cited cases of extinctions caused, or at least hastened, by introduced species come from islands. Famous examples include a near-flightless wren wiped out by the lighthouse-keeper’s cat on New Zealand’s Stephen Island; the dozen sorts of birds thought to have been extirpated by the brown tree snakes on Guam in the Pacific; or the eight varieties of endemic rodent dispatched on the Galapagos Islands by ship rats. A well-known non-island case comes from Lake Victoria in East Africa where the Nile perch was released by colonial Brits for sport-fishing in the late 1950s. This fast-growing predator has since been blamed for the loss of two-thirds of the lake’s 300 types of endemic cichlid fish, although the introduction may merely have delivered the coup de grâce to dwindling populations already threatened by decades of over-harvesting and pollution.

Back here in Britain, concrete evidence for extinction is scarce, but we can’t ignore two examples where introductions have threatened other species and could lead to their demise. While grey squirrels don’t directly interfere with native red squirrels, they outcompete them for food, especially in deciduous woodland, and also pass on a lethal virus. The red’s population crashed in the wake of the grey’s arrival, so it’s hard to avoid the conclusion that grey squirrels are a big part of the problem. Disease is also a reason that signal crayfish, brought from North America in the 1970s for aquaculture, are displacing Britain’s white-clawed crayfish (which, as mentioned, may or may not be a true native). In this case, the signals pass on a fungal-like pathogen to the white-claws, which die within weeks of being infected. To be fair, ‘crayfish plague’ was already expunging the white-clawed population before signals came on the scene. Indeed, it was the signal’s resistance to the plague that had recommended the crustacean to fish farmers in the first place.

There’s no doubting, however, that even where an introduced species doesn’t kill off a native, it can contribute to significant population declines. And if the impacts are only felt locally, it’s still a concern. For instance, pirri-pirri burr, an Antipodean plant invader which reached Britain at the beginning of the twentieth century, probably as a hitch-hiker in sheep fleeces, will never trigger a national emergency, but in certain places – notably, Minsmere in Suffolk, and Lindisfarne island off the Northumberland coast – it threatens local wildlife.

Another problem, as some see it, wrought by invasive species is hybridisation. Even if non-natives seldom exterminate our home-grown wildlife, the tendency of many to interbreed with them is beyond the pale. Perhaps the best outcome is when the offspring prove sterile, although this represents, for the native, a waste of valuable breeding effort. More serious are cases where viable progeny arise and in turn back-cross with the indigenous species; this sort of thing can erode the gene pool, reducing the population’s genetic variation and leaving it vulnerable to extinction. Hybridisation between native red deer and smaller introduced sika deer – an Asian variety – in parts of Scotland is a well-known example. Over time as the genes mix, red deer are starting to get smaller and sikas larger. As the two deer approach each other in size, this facilitates further hybridisation and risks accelerating negative impacts. Occasionally, hybridisation results in a more vigorous strain, as seems to be happening with the bluebell. Half the entire global population of this much-loved wildflower is found in this country, but a fertile hybrid has also established here, the result of a cross between the native bluebell and a Spanish variety introduced by horticulturists in the late nineteenth century. Many bluebells in Britain’s gardens and urban areas turn out to be this hybrid, although even experts struggle to tell the difference and, for now at least, the hybrid bluebell does not seem to be invading woodlands. Indeed, recent research suggests that the Spanish bluebell is less fertile, and sets fewer seeds, than its British counterpart.

Ecologists also fear that invasive organisms could alter ecosystems in far more profound ways. These could include anything from changing water quality or soil nutrient levels to disrupting food webs, reducing pollination rates and generally messing about with the ‘balance of nature’. Examples at random from around the world include the Mediterranean tamarisk tree, blamed for drying up marshes and salinising the soil in California, or zebra mussels altering nitrogen and phosphorus levels in freshwater habitats. One school of thought suggests that since ecosystems are dynamic and ever-changing, perhaps we shouldn’t be too bothered. Such an attitude is simplistic and defeatist. Much of what humans – the most ‘invasive’ species of all – have done, from cutting down rainforests to spilling oil into the sea, from landfilling toxic waste to pumping out carbon dioxide, has upset ecosystems, and we need to understand and combat those negative effects however subtle. Right now – not for want of research – our understanding of how ecosystems function, how different organisms interact, and what makes these complex systems more resilient, or less, remains limited, with plenty of knowledge gaps left to fill. We are instinctively concerned each time a species is lost from a natural system through our actions (or negligence); we should also perhaps feel a similar disquiet whenever we cause a new one to be added.

If the invaders left people alone and restricted their impacts to the degrading of natural ecosystems, that would be bad enough – not least as we ultimately depend upon these systems for our survival and wellbeing. But some non-natives harm us directly. Notwithstanding the odd pinch on the privates from a crayfish, the obvious threat is their role as agents of disease. The most notorious example in history is offered by the Black Death, inflicted by a strain of bacteria originating in Asia which, from the fourteenth century onwards, has killed tens of millions across Europe, the Middle East and North Africa. While nothing on that scale has recently been visited upon us here in Britain, new parasites and pathogens are on the radar, many transmitted by mosquitoes and other biting insects. At the moment, it’s a bit chilly for these to get a foothold here, but with climate change all bets are off.

Judging by the growing scientific literature devoted to the economic impacts of biological invaders, these species hurt our pockets too. Much of the cost arises from direct impacts such as insect pests reducing yields from agriculture and forestry, fish stocks wiped out by disease or the erosion caused when signal crayfish or Chinese mitten crabs tunnel into river banks. To the ledger we must add the eye-watering sums spent on preventing, monitoring and eradicating invasives. In excess of £5 million is spent every year in Britain removing Japanese knotweed alone. Various indirect impacts, trickier to calculate but just as real, and many times greater than the direct costs, can also be attributed to invasives. This is a complex area, but it boils down to the loss of valuable ecosystem services like nutrient cycling, pollination or flood prevention.

Overall costs incurred by invasive non-native organisms are estimated to amount to 5 per cent of the global economy. Across Europe, invasives inflict some £9-billion worth of damage every year. In the UK alone, the figure has been put at about £1.7 billion annually. Although these are ballpark estimates, resting on plenty of assumptions and subject to much debate, governments the world over are taking notice as never before. Invasive species are fast becoming public enemy number one. In 2016, the European Union banned 37 of the most problematic plants and animals from being kept or traded without a permit. These include signal crayfish, raccoons and American skunk cabbage. On this side of the Channel, the Great Britain Non-Native Species Secretariat was set up a decade ago and tasked with detecting and containing invaders, as well as helping to predict and prevent future incursions. Tackling troublesome non-natives is complex: the measures taken can be extraordinary and sometimes cause more problems than they solve, even hurting the very ecosystems they’re intended to protect.

An emerging school of thought is suggesting that the threat of invasive species has been exaggerated, that we should stop worrying about non-natives and even welcome them for the benefits they can bring. At the other extreme, a growing band of conservationists is going beyond simple calls for the eradication of non-natives to campaign for the deliberate reintroduction of a menagerie of native British plants and animals which have become extinct at the hands of humans. To its critics, the ‘re-wilding’ movement is pure eco-nostalgia.

For me though, most fascinating of all is that non-native organisms, invasive or otherwise, from rabbits to rhododendrons, mink to muntjac, hold up a mirror to our own species. Yes, the pace of invasion is higher than ever before but problematic non-natives aren’t a modern phenomenon: they’ve been with us from the outset, as unavoidable a corollary of the human way of life as cleared forests and piles of garbage. From the earliest settlement of our islands and first experiments with farming, through the Roman and medieval times, and the age of exploration by Europeans, to the current period of globalised free-for-all, the story of invasive species is the story of our own past, present and future.

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