Invasive Aliens: Rabbits, rhododendrons, and the other animals and plants taking over the British Countryside

The point is that Hampstead Heath is populated by lots of living things from other parts of the world, many of them breeding and difficult to control. And I haven’t yet mentioned some of the more notorious: plants such as Himalayan balsam, Japanese knotweed and giant hogweed; insect pests like oak processionary moth and harlequin ladybird; and virulent pathogens, such as Dutch elm disease, ash dieback and Massaria disease, which attacks plane trees. But, in this respect, Hampstead Heath is nothing special. I could have gone to pretty much any park in London, or indeed anywhere in Britain, and seen the same things, and far more besides. And it’s not just parks: our rivers, lakes and streams; our forests and farmland; our estuaries and coastal waters; our homes and gardens; even our own bodies; all host a wealth of introduced species.

Many Brits pride themselves as stoic defenders of a green and pleasant land, boasting a record of resistance against aggressors dating back centuries, be it weathering the Spanish Armada or defying Hitler’s Blitzkrieg. This patriotic fervour, and its clarion call ‘to control borders’, may in part explain the 2016 Brexit vote. Yet, a cursory examination of the natural world reveals that while many interlopers of the human variety have been kept at bay, our islands have throughout history been colonised by a succession of animals, plants, fungi and other organisms that apparently belong elsewhere. Indeed, it’s often hard to sort out the native from foreign.

Philosophers and scientists have long noted the spread and impacts of introduced animals and plants around the world. In his Naturalis Historia, published around 78 CE, Pliny the Elder wrote of Spanish rabbits, whose ‘fertility is beyond counting’, bringing such famine to the Balearic islands by ravaging the crops that the inhabitants begged the Emperor Augustus for military aid. Charles Darwin likewise observed the rampant spread of a European thistle across several South American islands during his nineteenth-century voyage on the Beagle. And, closer to home, in 1920 the Scottish writer James Ritchie highlighted the challenge to ‘Nature’s order’ posed by ‘many thoughtless introductions’, arguing that ‘the alien stowaways which become established in a country include more economic pests than the native fauna they invade’. Yet, until the 1958 publication of The Ecology of Invasions by Animals and Plants, the march of non-native species was largely invisible to the wider public.

Its author, the pioneering British ecologist Charles Elton, believed that we faced a decisive battle whose outcome would determine the fate of the world. The book was an expansion of his series of BBC radio lectures entitled ‘Balance and Barrier’ and opened with a warning of the existential threat presented not only by nuclear bombs – the Cold War was by then gearing up – but by ‘ecological explosions’. Elton defined these as ‘the enormous increase in numbers of some kind of living organism – it may be an infectious virus like influenza, or a bacterium like bubonic plague, or a fungus like that of the potato disease, a green plant like the prickly pear, or an animal like the grey squirrel’. He identified ‘the movement around the world by man of plants, especially those intentionally brought for crops or garden ornament or forestry’ as among the primary reasons for the spread and establishment of new organisms. ‘Just as trade followed the flag,’ added Elton, ‘so animals have followed the plants.’

In the decades since, international commerce has continued to grow and the ecological explosions have kept on detonating. In 2016 – the year Asian hornets were discovered in Britain for the first time, sparking a military-style response to make Elton proud – the first specimen of the Obama flatworm slithered into Britain. Native to South America, the species reaches seven centimetres in length and devours earthworms and other important soil invertebrates. This one turned up in an Oxfordshire garden centre courtesy of a pot plant from the Netherlands. The flatworm wasn’t christened in honour of the former American President but instead derives its name from the Brazilian Tupi for ‘leaf animal’. That same year, three new infestations of the ‘Asian super ant’ were discovered. Hailing from Turkey and Uzbekistan, this social insect forms supercolonies numbering in the millions and is also nicknamed the ‘electricity ant’ for its inclination to congregate in power sockets and light switches, chewing cables and threatening black-outs. Recent years have also seen harlequin ladybirds from Asia kill off our home-grown varieties; Pacific sea squirts carpet our marinas; and buddleia and Japanese knotweed move down the rail network more efficiently than most trains.

Public awareness of the issue is higher than ever before, with sensational news headlines stoking our fears. Giant hogweed, introduced as a horticultural curiosity from the Caucasus mountains in the 1820s, has been recast as Britain’s ‘most dangerous plant’ with sap that ‘melts’ a child’s skin. ‘Monster goldfish’ are on the prowl. ‘Sex mad Spanish slugs’ are terrorising our gardens. Emotive terminology isn’t just the preserve of tabloids: even serious scientists will talk about ‘demon shrimps’ and ‘killer algae’ with a straight face. Some of the language has a xenophobic flavour: introduced plants and animals are ‘ex-pats’ or ‘immigrants’, which ‘pollute’ our pristine environment and need to be ‘bashed’ and ‘sent home’. Perhaps it’s telling that the Nazis were among the first to take against non-natives, drafting a ‘Reich Landscape Law’ in 1941 banishing exotic plants from pure German landscapes. Some argue that the current fixation with non-indigenous wildlife is bound up with subliminal, and not so subliminal, antipathy to arrivals of the human kind. Concerns about non-natives and immigration to our small, overcrowded island are, they say, all of a piece. Even the term ‘invasive species’ has its drawbacks, perpetuating Elton’s notion that we are somehow under assault, as if rhododendrons, grey squirrels and Asian hornets were working to a strict battle plan. The word ‘alien’, which remains in wide use, particularly among botanists, can have similarly unfortunate connotations. Worries about many non-natives can be whipped up unnecessarily, and sometimes for unsavoury political ends. But we shouldn’t avoid talking about them: new organisms are arriving all the time, the pace of arrivals is rising and, yes, a handful of them do appear to cause problems.

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.