Invasive Aliens. Dan Eatherley

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