What are our reference points for taking a stance on whether something belongs in the ecosystem…?
Why do we give introduced species such a hard time? Or at least those we dislike, or consider a threat to other species, whether or not this is justified. Perhaps, deep down, we still believe humans should have ‘dominion over nature’. Conventional nature conservation can be interpreted as this: from scattering wildflower seeds in a garden, to changing the ecosystem of a forest, from a one-day Himalayan balsam clearance, to a 20-year habitat rehabilitation project. We strive to both conserve nature and to mould it into our ideals. No more so than when it comes to deciding what should thrive and what should be suppressed or even eliminated. Perversely, nature constantly tries to thwart us. Consequently we wage war on many successful species whilst desperately trying to conserve the strugglers. We are particularly prejudiced against species that have been moved, deliberately or accidentally, from one place to another by human agency.
We are influenced by a key frame of reference: nature and wildlife as we ‘discovered’ it during the 19th and 20th Centuries, coupled with the ecological processes, systems and services we then identified. This is but a snapshot in time and place. The 500-million-year-old natural world has experienced constant change. There is a sense in which anything belongs anywhere it can live, and all species and habitats are merely occupying temporary places. We seem to think everything should stay as it was between about 1850 and 1950.
This leads to a muddled and inconsistent approach to those species which have travelled from where they were first found. We rail against many such migrants. We use pejorative language , calling them invasive and aggressive aliens. We depict them as threatening our beautiful and virtuous native species. In a triumph of values over science, we exempt those we like from this generality, favouring introduced cornfield annuals in our wildflower mixes, advising people to plant Buddleia, and organising snowdrop (Galanthus nivalis) walks. Introduced species such as the brown hare (Lepus europaeus) and white-clawed crayfish (Austropotamobius pallipes) even have their own Biodiversity Action Plans .
Language is important: as ‘exotic’ and ‘alien’ have the same meaning I have long argued for using the word exotic when referring to introduced species. Exotic has positive connotations, alien entirely negative ones, which one is used affects perceptions. In an episode of Springwatch this year, ring-necked parakeets (Psittacula krameri), usually referred to as aliens, were being filmed (in my neighbourhood as it happens). In this case the presenter, who obviously liked the parakeets, enthusiastically described them as exotic. In our multi-ethnic and unsettled world engaging people in nature conservation is hampered by references to ‘invaders’ and ‘aliens’. And xenophobia does play a part: after the 9/11 attacks President George W. Bush moved staff responsible for invasive species from the Animal and Plant Health Inspection Service into his newly created Department of Homeland Security, whose mission is “to secure the nation from the many threats we face”.
Another example of how the choice of words conveys different impressions relates to wild boar (Sus scrofa). Their impacts are often referred to as ‘damaging’ or ‘threatening’ to woodland ecosystems, as in the Forest of Dean. The boar are certainly influencing and affecting the ecosystem, but probably more positively than negatively. They were, after all, a natural part of such ecosystems for far longer than they have been absent. The same factors relate to feral big cats and the expanding population of pine martens (Martes martes), although this is more the case amongst the general public than professional ecologists.
Perhaps also we do not consider ourselves as part of nature – we think it ‘natural’ if migrating creatures carry organisms and propagules on their journeys, but apparently not so if people do the same. Unless it was long enough ago of course, then we don’t seem to mind. The Romans brought us many species, including : millet, lentil, fig, olive, medlar, pear, almond, peach, garlic, onion, shallot, leek, cabbage, pea, cucumber, lettuce, turnip, radish, asparagus, rosemary, thyme, bay, basil, walnut, sweet chestnut, and eating apples . Our allotments and orchards would be pretty bare if we treated these in the same way as Japanese knotweed (Fallopia japonica). The Romans also brought rabbits (Oryctolagus cuniculus) which in some places are now considered vital to the conservation of acid grassland.
Curiously, we worry when native species which benefit from our activities, such as farmland birds or suburban blue and great tits (Cyanistes caeruleus and Parus major) decline in number. We even use them as barometers of conservation success, often without acknowledging that their numbers would be naturally much lower without our cultural landscapes of gardens and farms. Surely there is very little difference between moving species into favourable habitats and incidentally providing such habitats for them in an otherwise hostile world? Habitats, natural, semi-natural or cultural are not though the subject of this article, but in passing I would recommend you look at the history of the Serengeti and other supposed ‘pristine’ habitats around the world. Many places are not what they seem.
There are some important caveats to bear in mind when reading that which follows. Island wildlife and ecology is a particularly important one. The introduction of previously absent fecund, agile and carnivorous mammals, such as rats (Rattus sp.) and cats (Felis catus), to islands has indeed resulted in the extinction of many other species, especially flightless birds. Neither is it being argued that we should not protect and try to improve the status of endangered and vulnerable species. What is argued is that we should not be scapegoating introduced species and their undoubted impacts, when the true causes of decline and possible extinction often lie with unrelated human activities or the impacts of native species. Very often the new species are merely taking advantage of the situation, as well as sometimes contributing to ecological restoration.
The 2005 Millennium Ecosystem Assessment report, Ecosystems and Human Well-being: Biodiversity Synthesis, identified “invasive alien species” as one of “the most important direct drivers of biodiversity loss and ecosystem service changes”. This clearly implies that change is always bad, whereas of course it is both inevitable and equally likely to be beneficial.
A Sense of perspective
One of the main references for this article is Ken Thompson’s book ‘Where do Camels Belong?’ The answer is intriguing and complex. The camel family originated in what is now North America. Recently they have been associated mainly with the Middle East, but apparently the only place where truly wild camels now live is Australia. Populations elsewhere are domesticated or feral. Their closest relatives, such as alpacas, llamas and vicunas are now in South America. Thompson poses the questions where should we consider a species native, and where introduced? Should it be where we know it first evolved and lived, where it has lived longest, where it was first encountered, or where it thrives best? The answers for camels are, in order, North America, North America, the Middle East, Australia .
The same questions could be asked about horses (Equus ferus caballus). They also evolved and lived in what is now North America for millions of years, but became extinct there about 8,000 years ago before, in an early, if unconscious, example of rewilding, being re-introduced by the Spanish 500 years ago .
As long as we limit our time frame to a few human generations, where we found things is where we think that they should be. It may not even be the right question. A better one I suggest is: “Where will a particular species make the greatest contribution to nature conservation and increasing biodiversity?” Article 8( h) of the 2000 Convention on Biological Diversity Multilateral Treaty states that: “Each Contracting Party shall, as far as possible and as appropriate: Prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species”. (There’s that word threaten again.) This is a curious and negative way of looking at how nature actually works, and perpetuates the myth that everything is fixed in time and space. It is not, nature is dynamic, restless, unpredictable, constantly changing. Extinction is a natural process; generalists become specialists, specialists become vulnerable, the vulnerable become extinct. This is a wholly natural process. Any species, introduced or not, will have impacts on the ecosystem of which it is a part. Predators, for example, are commonly thought to ‘threaten’ their prey species, although often the prey species evolves defences which favour its continuing viability.
Sometimes it happens the other way around as with the cane toad (Rhinella marina) and its predators in Australia. The toad was introduced from South America in 1935 as a biological control agent, to eat beetles in Queensland’s sugar cane fields. From an initial population of 60,000 the toad quickly became an uncontrolled agent, its numbers and range exploded, and potential natural predators were killed when they ingested the poison gland at the back of the toads’ heads. Fifty years later ecological Armageddon was being forecast, and by 2009 they had reached Western Australia. And now? Some predators avoid the toads altogether and some, such as crocodiles and crows, have ‘learned’ to eat the toad but not the poison gland. Some snakes are evolving resistance to the poison, and the blacksnake (Pseudechis porphyriacis) within a few generations has evolved a smaller head which means it can only tackle smaller, less toxic toads .
The toads have an 85-year history in Australia, about three human generations. Some people, therefore, lived their whole lives knowing or assuming only that they were a problem. But 85 years is but a blink in ecological and evolutionary time. Rick Shine of the University of Sydney has said “Rarely has an invasive species been so widely reviled, but our predictions were dramatically in error. No native species have gone extinct” .
Destroyers or disruptors?
So just how justified are the fears expressed about non-natives? Their impacts can be profound, at least over short timescales, but over long timescales those effects may be attenuated, as with the cane toads. Sycamores (Acer pseudoplatanus) seem to have been, if grudgingly, accepted as part of our flora in Britain, and are no longer the villains they once were. Perhaps non-natives should be thought of as disruptors rather than destroyers. After all, according to Stace and Crawley (2015) Britain has more introduced plants (2068 species, or 59%) than native plant species. The incomers include iconic wildflowers such as snowdrops, as mentioned above, and fritillaries (Fritillaria meleagris).
They have this to say about fritillaries: “Fritillaries, the first record in the wild is 1736, and even that is an outlier; no one admits to seeing it again until 1776. That’s a very late date for a real British native, especially such a colourful, unmistakable and attractive one. In other words, if you believe Fritillaria is native, you have to assume that generations of seventeenth-and eighteenth-century botanists conspired not to mention it, a plot rivalled only by NASA faking the American moon landings”.
No one though thinks that fritillaries are a threat to anything. So how about those well-known villains, Rhododendron (Rhododendron ponticum), Himalayan balsam (Impatiens glandulifera), Japanese knotweed and giant hogweed (Heracleum mantegazzianum)? All were enthusiastically introduced by Victorian gardeners when the average size of a garden (other than cottage gardens) was considerably larger than is the case today. Japanese knotweed was once described as “undoubtedly one of the finest herbaceous plants in cultivation”  and Himalayan balsam was recommended because of its “splendid invasiveness” . They are not prevalent away from people and generally disturbed land but remain frequent in urban and residential areas and alongside paths and waterways. This means we notice them and because of this “all four are victims of the kind of hostile treatment that no native species would receive” .
The impacts of non-natives are much more complicated than either ‘good’ or ‘bad’. In a study by Rob Marrs of Liverpool University it was discovered that bramble (Rubus agg.), bracken (Pteridium aquilinus) and ivy (Hedera helix) did four times more damage (whatever that means) to woodland biodiversity than sycamore, Himalayan balsam and rhododendron . There is a parallel between Himalayan balsam here and purple loosestrife (Lythrum salicaria) in the USA where it is widespread and very conspicuous. There are fears that it is ‘choking’ wetlands and threatening native species. In 1999 however, research showed that plots with loosestrife were no less diverse than those without, there is no correlation between its presence and species richness, and native species are no less likely to occur where it grows .
Nature’s resilience and turbulence
A common scenario seems to be that when a new species appears somewhere that favours it, it rapidly increases its range and abundance. Often it seems to dominate, as for instance with Himalayan balsam and harlequin ladybirds (Harmonis oxyridis). This phase may last for several decades and prompts great concern and (often unsuccessful) eradication and control programmes. Then, perhaps almost imperceptibly, natural processes begin to first limit its success, and then to reduce its negative impacts, before it settles down as a relatively well-behaved member of the ecosystems of which it is a part. An example is Canadian waterweed (Elodea canadensis) which was once of great concern, but it has decreased in abundance, and in fact was never as aggressive as some of the species mentioned above .
Extinction is, at least for plants, still a rare event in Britain. According to Plantlife only 20 species of plant have become extinct here, plus another seven “extinct in the wild” . This, bear in mind, in relation to a total plant species count of more than 3,300 according to Stace and Crawley . “Overall there are probably about 2,000 non-native plant and animal species in the wild in Britain. To date not one native species has gone extinct because of them” . (My italics)
Related to this is the very idea of ‘species’. Hybridisation is common enough to demonstrate that nature is much more flexible than we are about this. We are happy to take advantage of it when dealing with domestic livestock, but don’t like it when nature does the same. The sorry story of ruddy ducks in Britain is an example of this. Darwin of course had much to say about species, including: “I look at the term species as one arbitrarily given to a set of individuals closely resembling each other”.
Our anomalous attitudes lead to some real absurdities. For example Stace and Crawley say this about American pondweed (Potamogeton epihydrus): “(it) has existed in unheated canals in Lancashire and Yorkshire since 1907. This species, however, is outside the scope of our book, because in 1943 it was discovered as a native in the Outer Hebrides, as one of the tiny group constituting our North American element”. So a plant which is almost certainly non-native in northern England is being classed as a native because it naturally occurs on a remote Scottish island.
This comes of relating distribution to political, rather than natural, boundaries. If a native species spreads from northern Mexico into the USA it becomes non-native there. We tend to think this way on a county and vice-county basis in this country. Perhaps appropriately so, because the concept of native and alien has been attributed to the originator of vice-counties, Hewett Cottrell Watson (1804 – 1881) . What will happen with the wildlife and flora of Slovakia and the Czech Republic, which was once all in the one country of Czechoslovakia?
Unlike the favoured brown hare and white-clawed crayfish, botanists removed 30 plant species from the third edition of the British Red Data Book in 1999 just because they were no longer considered native. So much for one of the oft quoted reasons for conserving wildlife, its intrinsic value. These 30 species were stripped of this merely because some people thought that they were in the ‘wrong place’ .
This contrasts with the pool frog (Pelophylax lessonae) saga. A small population of this species in East Anglia was considered non-native. Genetic research led to the conclusion that this was an error, and it was indeed a native species. This happened at about the same time as the population died out, resulting in an expensive species recovery programme, using Scandinavian pool frogs to re-establish the species. Again, nativeness trumped all other considerations.
We hear much about the harm that non-native species may cause, and very little about their positive aspects. They can, for instance, compensate for lost native species in an ecosystem. The test of novel ecosystems, with a mix of natives and non-natives, is surely how effectively they deliver ecosystem services, not how ‘natural’ we think they are. Where non-natives are added to an existing mix they are more likely to be contributing to a local increase in biodiversity, not a loss.
Another plus is the evolution of new species and the adaptation of existing ones. We now know that evolution can happen far more quickly than once thought. It has only taken a few generations for those snakes in Australia to evolve the smaller heads which enable them to safely prey on cane toads. The toads themselves have evolved longer legs to help them on their travels.2
Hybridisation also leads to an increase in biodiversity when it results in viable new species. Stace and Crawley recognise a category of plant called ‘neonative’. This is a plant that has originated as a result of hybridisation or evolution involving both native and non-native species. One of their examples of this is Oxford ragwort (Senecio squalidus) . Non-native sika deer (Cervus nippon) in Scotland can breed with native red deer (Cervus elaphus) . Is it not likely that a new species of deer will eventually arise from these two parent species? Insects feeding on native plants often take to new species on the block. This is the case with a species of fly in America which used to feed only on hawthorns, but some now also feed on introduced apples. After 150 years of this there are now two different populations which hardly interact with each other. One species becoming two seems inevitable .
Then there may be a straightforward conservation benefit. The much-maligned Himalayan balsam is a valuable late source of pollen. A different type of example is the Chinese water deer (Hydropotes inermis). The buck has tusks, suggesting that they are a primitive form of deer as they developed before antlers evolved. This is a vulnerable species with only about 10,000 left in the wild. About 1,000 of them are in Britain, having been brought here in the 1890s . Would we rather such species become extinct than survive far away from where we think they ‘ought’ to be? Why should we not welcome these steps in what has been called ‘the evolutionary dance’ – the creation and evolution of novel ecosystems, and adapted and new species?
Let Gaia evolve
It is surely time to abandon the ghost of nature past. How things were is not how they can ever be again. Is it not time to embrace Fred Pearce’s ‘new wild’ and accept that habitats and ecosystems evolve and fizz as a cocktail. Perhaps we should see exotics as a shot in the arm for nature, even if some are a pain in the neck for us at times. Maybe we should appreciate that all ecosystems are in a constant state of flux and there is no such thing as ‘the balance of nature’, and allow ‘invaders’ to help repair damaged habitats?
If we lift our eyes above the ecological horizon’ we will see a world which, socially and culturally, is a borderless mix. Nature strives to be like this, but we constantly try to constrain it. Instead we need to harness nature’s resilience, resourcefulness and chaos, and stop pretending that it is frozen and static. We need to create the conditions for natural processes to flourish and stop trying to impose our own ‘designer solutions’.
From outside our field here is a simple example of successfully mixing otherwise entirely separate elements. It is the famous Hovis commercial. A Czech composer (Dvorak), living in the USA, writes a haunting piece taken from a negro spiritual, which is then used to evoke Edwardian Britain in order to sell bread. That, I suggest, is the sort of eclectic thinking nature conservation needs in the 21st Century. Perhaps performing a revolutionary rather than an evolutionary dance.
I will give James Lovelock the last word:
I wholly agree with Fred Pearce’s argument for re-wilding. Life, from the smallest bacterium to the whole living planet, is dynamic. Species do not belong in a planet-sized zoo. We should let Gaia evolve”From the publisher’s review of 
1. Thompson Ken, Where do Camels Belong? – the story and science of invasive species, Profile Books 2014.
2.Pearce Fred, The New Wild – why invasive species will be nature’s salvation, Icon Books, 2015.
3. Stace Clive, and Crawley Michael, Alien Plants, Collins New Naturalist Library Book 129, 2015.
4. Plantlife website.
5. Darwin Charles, On the Origin of Species, 1859.
6. Thomas Chris D., Inheritors of the Earth – how nature is thriving in an age of extinction, Allen Lane, 2017.