Horse chestnut. Sycamore. Sweet chestnut. Apple trees. Larch. Spruce. Snowdrop. Wild poppy. Cornflower. Corn marigold. Snake’s head fritillary. Burdock. White campion. Shepherd’s purse. Hemlock. Mayweed. Pheasant. Little owl. Rainbow trout. Rabbit. Hare. Fallow deer. Edible dormouse.
Each of the animals and plants on this list of species that Britons are all familiar with was introduced to Britain by human agency, either accidentally or on purpose, which, technically, makes them all aliens. And yet, despite such aliens being (according to some reports) one of the world’s worst environmental problems, I doubt you’ve ever felt particularly threatened by any of them.
The lesson I take from this commonplace observation is that although a dislike or fear of alien species is widespread, it is not automatic, and certainly not innate – it has to be learned. In fact we can go further than that: the knowledge of nativeness, or alienness, has to be learned – there is no operational definition of alienness. By which I mean that if I took you to a wood consisting of, say, oak, beech and sweet chestnut, there is nothing you could measure that would tell you that one of those trees is an alien and the other two are native. And I don’t just mean that the measurement would be difficult or expensive, I really do mean literally nothing. Given all the time in the world, and as many scientific resources as you like, up to and including the Large Hadron Collider, there is no way to distinguish a native species from an alien one. In fact for a long time many of the species mentioned at the outset – for example, sweet chestnut and snowdrop – were assumed to be native. Not only that, not being native is no barrier to being the object of active conservation efforts. The brown hare, for example, is a UK Biodiversity Action Plan priority species (one of the species ‘identified as being the most threatened and requiring conservation action under the UK Biodiversity Action Plan’). Nor is the brown hare unique – the white-clawed crayfish (introduced from Europe in early medieval times) is also a UK BAP priority species, felt to be in need of protection from the more recently arrived American signal crayfish.
The logic behind decisions about which species need protection is clearly far from straightforward. Still, the overwhelming majority of professional biologists and conservationists have strong – and rigid – opinions about aliens and natives. Natives (animals and plants that evolved where they live now, or spread there without human assistance) belong, and thus merit our concern and, if necessary, our care. Aliens, introduced by humans, even if in the relatively remote past, do not belong, and thus deserve only indifference (at best) or active persecution. As an ecologist, I began my career feeling the same way. As an undergraduate, I was more or less obliged to regard the snowdrop (a classic ‘nice’ alien) as a UK native, because the then standard book on British flora, written by my taxonomy tutor, Professor Tom Tutin, said it was. Later, as a university ecologist, began to look a bit more objectively at the published research on alien species, and I soon realised that much of it was, if not positively dishonest, at least being economical with the truth.
Many professional biologists and conservationists seemed happy to promote erroneous ideas about the relative value of native and alien species. Alien species that appear to have some kind of negative impact (often described as ‘invasive’) are obvious targets for research funding, and bad news is always more newsworthy than good news, so ecologists often choose to study the most damaging alien species we can find, even if they are atypical of alien species as a whole. And, not content with studying the relatively few species that are genuinely detrimental, we also start out by choosing to study the times and places where they appear to be having the largest effects, if only because dramatic effects are easier to notice and measure than small ones.
The result of all this is both predictable and well documented (in a 2013 paper in Trends in Ecology & Evolution). If we begin by studying an alien species where it appears to be having the most severe effects, it’s inevitable that further work on the same species will reveal smaller impacts. Eventually, researchers get around to looking at places where it’s having no effect at all, or even positive impacts. But this is all too late, because it’s only the most alarming findings that make the headlines, or are worth reporting at all. Two well-studied European invaders in America, purple loosestrife and tamarisk, or salt cedar, both illustrate this pattern. And don’t forget that the overwhelming majority of alien species that seemed to be just quietly minding their own business were never worth investigating in the first place.
Along the way, we (both ecologists and the public) adjust (consciously or otherwise) what we mean by ‘negative impact’, first by assuming our chosen alien is causing economic or environmental harm, and then looking for the evidence to support that view. So we often define the ‘costs’ of an alien species in a circular, question-begging way. For example, it’s not easy to see how aliens ‘harm’ ecosystems, given that ecosystems do not feel pain, or have interests, hopes or ambitions that could possibly be harmed. But if we define any change to the state of an ecosystem as harmful, then aliens are often harmful by definition. Further, the cost of an alien routinely includes – and indeed sometimes consists entirely of – the price tag we put on its eradication. But the eradication of almost any species, native or alien, would be expensive, and that tells us nothing about the real cost of the economic or environmental harm it may be causing, if any. Anything on the credit side of the balance sheet is routinely ignored.
Drawing attention to any of this is not the way to make friends and influence people. A decade ago I was one of the authors of a short paper in the journal Nature that made the (I still think) modest proposal that we should stop worrying about where species come from and simply treat them on their merits. A swift rejoinder was entitled ‘Non-natives: 141 scientists object’ – in other words, ‘We must be right because lots of people agree with us.’ More recently, a whole new class of offence has been created specifically for those who think like me: ‘invasive species denialism’. Unbelievably, calling for a less hysterical attitude to alien species is often lumped together with denying the evidence for climate change and evolution, downplaying the risks of tobacco smoking and exaggerating the risks of immunisation, and originates, apparently, from ‘a vested interest in opposition to the scientific consensus’, according to a paper published in Trends in Ecology & Evolution in 2017. And if you’re curious about the nature of that vested interest, the same paper claims that ‘deniers typically consistently reject scientific evidence on a range of different topics and there is a strong correlation with support of free-market ideologies such as laissez-faire regulation’. As someone with a lifelong and passionate belief in the primacy of scientific evidence, I find these accusations fairly ludicrous.
But before we go any further, do those who accuse me of ‘invasive species denialism’ have a point? Am I guilty of denying the negative impacts of alien species in every case? No, I’m not. I would be the first to admit that some alien species have had some very undesirable effects indeed. This is especially likely on remote islands, where specialised floras and faunas have evolved in the absence of the usual set of predators, herbivores and competitors. Guam would undoubtedly be better off without the brown tree snake, Australia without the cane toad, several Pacific islands without the predatory snail Euglandina, and a long list of oceanic islands without introduced rats, cats and goats. But extrapolating from these examples to the whole world is simply wrong; studies that claim to show big effects of aliens on native biodiversity are always heavily biased by data from remote islands.
To return to the question underlying all this discord: why do we think we disapprove of alien species when so many of them are familiar – even valued – parts of the landscape? I genuinely don’t know, but if we are alarmed by some kind of environmental problem, it’s a lot easier to blame any aliens we find hanging around the crime scene than it is to look for the real, underlying (and often intractable) cause of the problem. History teaches us that we love a convenient scapegoat. For example, it’s increasingly apparent that American grey squirrels might never have become established in the wild in the UK if we had not driven their only serious predator, the pine marten, to the verge of extinction. But it’s a lot easier to blame the squirrel than to admit our past mistakes.
And, of course, fear and alarm sell newspapers, and drive traffic to Facebook and other online sources of (mis)information. On these platforms, alien species are just one convenient source of fear and alarm, along with everything that might give you cancer, which, if you read enough online, eventually turns out to include every foodstuff you’ve ever heard of.
Only the brave, or foolhardy, swim against this powerful tide. For instance, as everyone knows, Japanese knotweed ‘[chews] through buildings, destroying walls and ripping up transport links’, and ‘tears through brickwork and concrete’, according to one disturbing report from the Daily Mail in 2013. Paranoid insurance companies and banks, and companies that profit by controlling Japanese knotweed, are only too willing to promote such beliefs, leading to its presence on residential properties regularly being used as a reason to refuse mortgage applications – a response out of all proportion to the threat. In reality, reports of knotweed actually damaging buildings are rare, and even if we look at a genuine, worst-case-scenario Japanese knotweed disaster area, such as when the plant is growing among crumbling, derelict buildings, knotweed is only rarely associated with damage. And when it is, it’s clearly an ‘accessory after the fact’, exploiting existing cracks or other damage. Trees, either by pushing walls over or simply by falling, are far more destructive. All this is documented in a 2018 paper in the journal PeerJ. It’s also worth noting that Japanese knotweed in the UK is effectively sterile, and its only means of long-distance dispersal is human carelessness.
To complete our picture of Britain’s favourite pantomime invasive alien villain, the latest research, published in Biodiversity and Conservation in 2018, reveals that Japanese knotweed is a terrific late-season source of nectar for both bees and hoverflies, but that’s not much of a headline, is it?
Something else that makes it hard to think rationally about alien species is the issue of timescale. The most far-sighted among us struggle to look much beyond the next election, and our usual attention span is closer to that of a goldfish. And although the mills of natural selection eventually grind exceeding small, they do grind slowly, at least on a human timescale. Take, for example, giant hogweed, a plant so huge and apparently unstoppable that it even inspired its own hit pop song (although you do need to be of a certain age to recall ‘The Return of the Giant Hogweed’ by Genesis).
Given the right conditions, especially on riverbanks, giant hogweed does indeed start out by just steamrollering other plants into oblivion, and for the first thirty years or so these effects continue to increase. By which point you could be forgiven for assuming that they will last forever – but they don’t. Careful work in the Czech Republic, published in Ecology Letters in 2013, shows that eventually hogweed goes into a gradual decline and the associated natives begin to recover. By fifty years this recovery process is well under way, but we don’t know the eventual outcome because there just aren’t enough really old hogweed populations to study. Experiments reveal that the likely cause of hogweed’s decline is a build-up of soil pathogens.
In every case where data is available for a long enough time, the same pattern emerges: however invincible an alien appears at first, competitors, predators and diseases eventually catch up, as natural selection says they must, and as they always have throughout the history of life. This is as true for animals as it is for plants. For example, their high social and economic value means that brown trout have been introduced to practically all parts of the globe, starting with the European settlement of North America and Australasia in the mid-nineteenth century. The result is always the same: large negative impacts on native fish and other aquatic species, but gradually these effects decline, and after a century or more they can no longer be detected. Cynics among you will suspect that this is because a hundred years is how long it takes species sensitive to predation by trout to go extinct, but the evidence demonstrates that this is not the case, and the true cause appears to be adaptation by species that either eat trout or are eaten by them.
If you still think one hundred years is a long time to wait for the locals to catch up with a successful invader, remember that the biosphere has been around for hundreds of millions of years, and with any luck will be around for another few hundred. From that perspective a century is much less than the blink of an eye.
Does any of this matter? Should we care if most people believe, wrongly, that Japanese knotweed is both far worse than it is and typical of alien species in general? I think we should. On the most simple level, the world has enough real problems without worrying about imaginary ones. Resources devoted to a war against alien species (a war that is almost always unsuccessful and often counterproductive) are resources that are not available for dealing with anything else. In 2015, the UK government stated that invasive aliens cost at least £1.7 billion per year, while in 2017, the US government spent an estimated$3 billion across a range of federal agencies and activities in an effort to prevent, control and eradicate invasive alien species. Most of this expenditure is ineffective; for example, by 2009, Australia had spent more than twenty years and over AUS $500,000 trying to eradicate the Mexican weed Martynia annua from a single national park. It is, of course, still there.
There can also be subtle negative effects of worrying too much about potentially invasive aliens. Within the conservation community, a debate rumbles on about the advisability or otherwise of ‘assisted translocation’: moving species to new regions to allow them to keep pace with a changing climate (some species will manage this on their own, but a lot won’t). The chief argument against assisted translocation is a misplaced fear of the translocated species becoming invasive.
Believing that alien species are all out to get you, or are at best useless, has other undesirable consequences. British gardeners have been happily growing thousands of species of alien plants for centuries, mostly without knowing or caring where they originally came from: for example, roses from Asia, fuchsias from South America and dahlias from Mexico. Fifty or more years ago, there was a widespread perception (among both gardeners and scientists) that those alien plants were unable to nurture and sustain native wildlife, and therefore that gardens were basically wildlife deserts. That perception has now been completely overturned, beginning with ecologist Jennifer Owen’s monumental thirty-year study of her Leicester garden, which found an astonishing selection of diverse wildlife, including not only species previously unrecorded in Britain, but a handful of species new to science.
The transformation in our understanding of garden wildlife was continued by the Sheffield Biodiversity in Urban Gardens project and completed by the recent four-year-long Plants for Bugs project carried out by the Royal Horticultural Society. The lesson from all this work is that yes, the wildlife value of alien plants is inferior to that of natives, but only marginally, and the effect of plant origin is completely overwhelmed by the impact of vegetation quantity. In short, more flowers and more plants (of any sort) equal more wildlife, and how you garden is far more important than what you grow. Jennifer Owen, who made no particular effort to grow native plants, suggested at the end of her study that collectively Britain’s private gardens may be our most important nature reserves, and indeed together the UK’s gardens exceed the area of all of our National Nature Reserves combined.
You will not be surprised to learn that this discovery has gone down badly with those who never liked alien plants as a matter of principle, and especially with those for whom ‘native plants for native wildlife’ was essentially a religious belief, not susceptible to either confirmation or refutation by the normal rules of evidence. This belief shows every sign of being almost perfectly elastic, so that knocking it on the head in one place always leads to its popping up somewhere else, even in places where you were starting to hope people knew better.
For example, Garden Birds, published by the Royal Society for the Protection of Birds in 2019, chooses to ignore thirty years of evidence and issues a pretty stern injunction to those gardeners (virtually all of them, that is) who might have thought it was okay to grow alien plants: ‘If you want plenty of birds in the garden you need plenty of insects, and for the insects you need native plants.’ That this view stems from a straightforward dislike of aliens of any sort is confirmed by the fact that the book almost airbrushes out of existence altogether the non-native ring-necked parakeet (now a familiar bird in London, and spreading elsewhere in the UK), perhaps on the principle that if we ignore it, it will just go away?
But we know gardeners like to grow alien plants, so if they are told that filling their gardens with native plants is what it takes to attract wildlife, many will probably conclude that it’s just not worth the effort. The most likely result of ordering gardeners who want to do their best for wildlife to grow native plants is to discourage them from engaging with wildlife gardening at all – surely the exact opposite of what anyone intended. Of course, many sterile, double-flowered cultivars have little to offer wildlife, but that’s as true for native plants as it is for aliens.
In truth, pretending that the native–alien dichotomy has much practical value is lazy, out of date and pessimistic. Lazy because conservation is all about values, but there’s surprisingly little debate about what those values are. ‘Aliens are nasty, natives are nice’ has been one of the very few things everyone thought they could agree on without having to think too hard, so a lot of practical conservation has involved digging up, poisoning, burning or shooting alien plants and animals. This military-industrial approach has rarely been successful, and has often simply created suitable conditions for the establishment of yet more aliens.
Losing sleep over aliens is out of date because trying to control or eradicate alien species frequently stems from a yearning to return to a pristine, alien-free state that was often a romantic illusion anyway. Today’s wildlife managers, like it or not, have to recognise that the natural systems of the past have gone forever thanks to climate change, pollution and urbanisation, among other things. It can’t be much of a surprise, when you stop and think about it, that in the face of such pressures, many natives are no longer the best-adapted species in any particular spot, and conservation of a functioning countryside in the face of climate change may even require introducing aliens, particularly trees from warmer climates to mitigate the failure of our native trees to respond quickly to rising temperatures and changing weather patterns. Much of the modern natural world now consists of a mixture of long-term residents and new arrivals, and ecosystems are emerging that never existed before, sometimes living in climates that never existed before either. Restoring such ecosystems to some ‘rightful’ historical state is no longer possible, even in theory, so we should stop trying, embrace such ‘novel ecosystems’ and determine to get the best out of them, for humanity and for the planet.
Finally, disparaging aliens is profoundly pessimistic. No one would dispute the fact that we have a global biodiversity crisis, but biodiversity means different things to different people, and as long as it means just ‘native biodiversity’, the only way is down; the very best that we can hope for is to prevent biodiversity in any specific habitat or region from declining. But once we allow aliens to contribute, then local biodiversity can – and does – increase. Of course every time a forest is converted to oil palm or soya beans, local diversity plummets. Nevertheless, there’s plenty of evidence to show that regional biodiversity (that is the number of species per country, state or island) is increasing, as the number of introduced species establishing themselves exceeds the number of native species that die out. For example, introduced aliens have added nearly 2,000 established non-native species to the British flora (some of them in the list that opened this article), and there’s no evidence that any native species has suffered as a direct result. Nor is Britain at all unusual; the total number of species on the planet may be declining, but all the evidence points to increases in the number of terrestrial species in most of the world’s regions over recent decades and centuries.
It’s even possible for introduced species to add to global biodiversity. In the British flora, over a hundred hybrids, either between a native and an alien plant, or between two aliens, are now widespread. History teaches us that such hybridisation is often a stepping stone to speciation, and half a dozen hybrids have taken that step and become new naturalised species (our commonest salt-marsh grass, for example, started out as a native-alien hybrid). Once you’ve put aside the alien-bashers’ frothing at the mouth about ‘genetic pollution’, this is actually a profoundly hopeful development, and suggests that the trajectory of biodiversity in the Anthropocene might not be downhill all the way after all. But it’s a crumb of comfort that, if you insisted on remaining behind the sandbags in the ‘natives good, aliens bad’ camp, would be denied you.
Photograph © Walter Schels, Hare, 2000, Courtesy of Galerie Peter Sillem