“Where the heyall did all those goddam pie-thons come from?”

Fred Warren investigates the plants and animals out of place

Every few months I am engaged in the task of raking away large, floating mats of pondweed from a friend’s pond. The plant itself is quite pretty, but the mats it forms are dense and claggy with deep, extensive roots below the surface which deprive other organisms of light and space. When one finally manages to pry a section of it away, you can almost hear the pond let out a sigh of relief. Such relief is only temporary however, as the plant can reproduce from a single fragment, meaning that one sees the mats invariably return in a matter of months. The plant behind these hours of toil is known as parrot’s feather (Myriophyllum aquaticum); a species originating from Central and South America that was introduced as an ornamental pond plant but that since the 1960s has been steadily colonising wild areas in the UK (and evidently other people’s ponds). It is now one of five similar aquatic plants that since 2013 have been banned from sale in the UK (though a quick search finds it readily available on eBay...). I expect many people have a similar invasive species story, or perhaps you just remember headlines about the £70 million removal of Japanese knotweed from the London Olympic Park prior to the 2012 games. However, you may not have considered ‘invasive species’ as a global issue with both a colourful history and present-day effects which costs countries billions and places it alongside habitat loss and climate change as a leading cause of species extinctions. This kind of awareness is opened up by two new popular science books on the subject– Invasive Aliens (2019) by British naturalist Dan Eatherley and The Aliens Among Us (2017) by Canadian science journalist Leslie Anthony. Both see their authors get down and dirty on the frontline of invasive species management, joining researchers and managers in hacking, spraying, baiting, injecting, spearing and even electrifying all manner of unwanted critters, as they try to get to grips with this ever-growing phenomenon. 

Discussion around invasive species is sometimes criticised as stoking xenophobic sentiment. While this certainly plays out in newspaper headlines – ‘Killer Asian hornets and angry 'drunk' German wasps invading UK’ ran a Daily Mail headline from 27 August – it is important to remember that invasive species are a worldwide problem. While Asian hornets might make headlines in the UK, English ivy is overrunning parks in Canada, European gorse bushes are covering large swathes of New Zealand, and one study from Singapore found that damages by invasive species cost South East Asian countries $33.5 billion each year. 

But what exactly is an invasive species? A simple definition, paraphrased from invasion scientist Daniel Simberloff, is that it is a species introduced directly or indirectly by humans which sustains itself without human assistance. This therefore excludes organisms that have been introduced but fail to reproduce in the wild, as well as native organisms that sometimes behave ‘invasively’ (despite tales of Vikings introducing them as a joke, stinging nettles do not seem to be invasive to the UK, they’re just annoying).  A way of narrowing it down further is the ‘10s rule’ sometimes used by invasion biologists; as Leslie Anthony reports: 

“For every ten species that are transported outside their range one will be introduced to a new region; for every ten introduced, one will establish itself; and for every ten that are established, one goes on to be a problem.”

There are no fixed rules about what will or won’t become invasive, though there’s a consensus that organisms which can reproduce easily stand a better chance; for instance, plants that can reproduce asexually via rhizomes (like knotweed) and animals with multiple breeding seasons (i.e. rabbits and rats). Also, there is the concept of ‘biotic resistance’ which holds that degraded, human-disturbed habitats are more susceptible to species invasions. This rings particularly true for Japanese knotweed which, as Anthony describes, found the sooty, sulphurous conditions of 19th century London a fine substitute for the lava fields where it is natively found. Once they become invasive, the ecological impacts of invasive species range from outcompeting native organisms and spreading disease to changing the entire flow of nutrients in an ecosystem, as the humble earthworm appears to be doing in the forests of eastern North America. Of course, it can be argued that the movement of organisms, and even associated extinctions, are perfectly ‘natural’. For instance, the joining of the American continents 3 million years ago meant that many North American plancental mammals moved into South America which is thought to have precipitated the extinction of South America’s own marsupial mammals. However, as with climate change, it is the unprecedented shift in the rate of change that is cause for alarm – instead of happening across geological timeframes of thousands or millions of years, introductions of new species are now happening in a matter of days as they are constantly shuffled across the world by humans either indirectly (in ballast water, on clothing, in export containers) or directly (farming, pest control, pets, hunting). As Dan Eatherley estimates, “a third of non-native species worldwide have only spread to new locations since 1970”. 

Despite the recent telescoping of invasions, Eatherley’s book takes a broad, historical look at invasive species, with a particular focus on the UK. From this wide-ranging chronology, it is his accounts of the vogue of ‘acclimatisation’ among the upper classes of Western Europe in the 19th and early 20th centuries that stand out. This movement was premised on an “underlying philosophy that nature was there for humans to improve upon” and, in the words of one acclimatisation society chairman, aimed to populate “our fields, our forests, and our rivers with new guests; to increase and vary our food resources, and to create other […] products”. Though many of these imports, from ostriches to wildebeest, failed to thrive, some made the leap into the wild, notably in the UK the Reeve’s muntjac deer which keen acclimatiser Herbrand Russell released into the woods near his Woburn estate in 1901. Perhaps the most infamous of Herbrand’s ‘improvements’, however, was the grey squirrel which he released to “estates up and down the country”. Though not the sole spreader of this rodent that was trendy with many aristocrats, Herbrand’s releases were notable in their extent, and ultimately helped set off a chain of events that led to the extirpation of the red squirrel from much of the UK.  Easily more catastrophic were the people who spread the doctrine of acclimatisation to the New World where introductions ranged from the sporting, such as release of rabbits to Australia for hunting, to the aesthetic, as with Eugene Schieffelin’s release of 60 starlings  into New York in 1890 as “part of a scheme to populate America with every sort of bird mentioned in the plays of Shakespeare”. Though they could never have imagined the ecological and economic ramifications of their actions – from the desertification caused by rabbits in Australia [1], to the estimated $800 million in yearly crop losses and displacement of native birds caused by starlings in the US – the acclimatisers should stand as a cautionary tale of the damage that hubristic individuals with more money than sense can do to the planet. Entrepreneurs like Russ George and Bill Gates who are exploring geoengineering ‘quick fixes’ to the climate crisis, would do well to look at the acclimatisers and the dangers of naively manipulating partially-understood processes on a large scale. 

More than just the antics of the acclimatisers, Eatherley highlights just how many infamous invasions were actively encouraged by humans; from the long-flowering Himalayan balsam being cultivated by beekeepers, to rhododendrons being planted by gamekeepers as cover for pheasants. Such examples help to reverse the typical invasive species narrative; instead of insidious stowaways perniciously infecting our landscape, Eatherley’s book shows how our banks of balsam and ravines of rhododendrons are really artefacts of our own history: our colonial exploits, our desire for novelty and our gardening fashions. 

While strong in highlighting the anthropogenic aspect of our green and pleasant land, and in providing the reader with the means to ruin future walks with family and friends by proudly pointing at everything saying, "that's not native", Invasive Aliens is less clear in advancing any kind of thesis or call to action regarding the future of invasive species. How do we decide which invaders we should continue allocating resources to stop and control, and which have become so integrated that they are naturalised? Moreover, despite at times being lucid about the ecological damages wrought by invasives, Eatherley sometimes slips into a more complacent mindset, at one point stating that “we’re living through a golden era of mammals, with our furry fauna richer than at any time since the Neolithic”. This assertion, without any citation or qualification, jars with the recent announcement from the Red List that a quarter of native UK mammals are at imminent risk of extinction. Similarly incongruous is his line on the final page that “non-natives tend to increase biodiversity”. Again, a lack of citation here is unhelpful, but even if sometimes the numbers of invading species can equal or surpass the number of extinctions in an area, such reports do not take into account the loss of unique species and ecosystems. As Simberloff describes, Hawaii has had 66 bird extinctions since human colonisation and this has been nearly balanced by 53 bird introductions, but almost all those introduced were common elsewhere in the world, whereas many of the birds lost were unique to Hawaii and will never be seen again. Ultimately, Eatherley doesn’t engage with such questions and his attempts to find silver linings at the end of the book only confuse the reader, leaving them unhelpfully uncertain about how much of a problem invasive species are.    

Such ambivalence is not present in Leslie Anthony’s The Aliens Among Us, who declares that “along with climate change, habitat destruction, pollution, and ocean acidification, invasive species are now [...] a driver of the sixth great extinction”. He and the researchers he interviews have little time for the ‘invasive species aren’t so bad’ viewpoint, highlighting that the benign invasions do not offset the catastrophic ones. He moreover provides the reader with haunting images of a world where species invasions are left unchecked, leading to the homogenisation of flora and fauna with climate being the only cause of variation. With this in mind, Anthony gives primary focus to the practical management of contemporary invasions, structuring his book along ‘the invasion curve’, a model that maps the spread of an invasive species and states what response (prevention, eradication, containment, or management) is possible at which point. Unsurprisingly, Anthony emphasises that prevention is the best strategy, and he documents the painstaking measures that have been taken to keep Asian carp out of the Great Lakes, including electricity barriers, monitoring, and public education. Despite the clear economic advantages – the Great Lakes prevention costs $3.5 million annually compared to the $75 million it costs annually to manage Asian carp where they have already invaded in the US – Anthony’s interviewees detail how difficult it is to convince decision makers who “run on performance measures and outcomes” of the importance of prevention. This lack of proactivity reappears in Anthony’s account of the $8 billion expansion of the Suez Canal (already one of the world’s worst invasion corridors) which went ahead without an environmental impact assessment and with no provisions to help prevent the spread of invasives into the Mediterranean.

The reading gets grimmer as Anthony moves onto some of the Sisyphean attempts to manage invasions; as he says, “the establishment and spread of successful invasive species almost always takes place unnoticed” meaning that by the time the public becomes aware of an invasion, it’s likely at a stage where any management is incredibly difficult, if not impossible. Such has been the case with the Burmese pythons that are literally squeezing the life out native mammals and amphibians in the Florida everglades. Thought to be caused by releases of exotic pets, the pythons have so far resisted any form of effective control; even a month-long bounty hunt called Python Challenge™, which saw 1600 enthusiastic python hunters sign up, resulted in “only sixty-eight pythons turned in”. Anthony’s account of Florida generally is fairly damning with the pythons being just one of a litany of invasive plants and animals – from Brazilian pepper trees to the Nile monitor lizards - that are irrevocably changing the distribution of life in the Everglades.

Anthony’s accounts of management are not all as ill-conceived as the Python Challenge™ and, in a rare success story, he describes the immense effort in 2011 to remove invasive rats from Arichika and the Bischoffs – tiny uninhabited islands in the Haida Gwaii archipelago off the coast of Canada – in a bid to restore the populations of rare, native seabirds. This 8-week military operation, complete with a command station stocked with cinnamon buns, involved covering the islands in bait stations and took over 822 person-hours to carry out. Though the initial operation succeeded, when Anthony checks up on the team a couple of years later, the rats have inexplicably been found again on the Bischoffs leading the project manager to wonder: “if we can’t keep rats off […] do we try to figure out a control regimen […] or do we just let those islands go?” This battle-weary tone comes across in many of Anthony’s interviews which are filled with sighs, knowing looks and ironic smiles from invasive species researchers and managers who at times seem to be fighting a losing battle on their own. Particularly affecting is a conversation with Carita Bergman at Parks Canada, who says of her work on invasive species “I see everything through a completely different lens now, and a pretty depressing one […] it kind of upsets me”. Indeed, despite Anthony’s lively writing style, The Aliens Among Us is at times a tough and unrelenting read. So much so that one can see how, among such gloom, a market has emerged for counterfactual voices preaching a gospel that the threat of invasive species is overblown, and that invasion scientists should stop trying to turn the clock back to an impossible past. Regardless of the falsity and speciousness of such claims – as Anthony says, “no one I’d spoke with advocated returning any ecosystem to its pre-human condition – only to functionality” – what the popularity of this vocal minority conveys is a desire for optimism often lacking from the language of invasive species management. While reports of plagues of pythons and toxic hogweed might galvanise a short-term public response, any initial bloodlust (sap-lust?) rarely lasts long enough to effect meaningful change. Instead, the management of invasive species is perhaps better framed alongside projects that are actively working not just to preserve biodiversity and habitats, but to increase them. Such a perspective is touched on towards the end of Anthony’s book when he describes how carefully considered rewilding projects are in keeping with the restorative aims that guide most attempts to manage invasive species. Rewilding projects, from the reintroduction of wolves at Yellowstone, to the de-domesticated ungulate herds at Oostvaardersplassen in the Netherlands aim to significantly improve biodiversity through the return of ecosystem services that these animals once provided. 

There have been some notable successes and last month a court ruling meant that beavers, whose actions help create aquatic habitats, in the river Otter in Devon are now the first extinct native animal to have been legally reintroduced into England. Of course, it’s important not to become complacent about invasive species whose prevention and management remain crucial, and likewise not to be too gung ho with rewilding projects (lest the mistakes of the acclimatisers be repeated). Yet by viewing ‘invasive species’ not as a separate issue, but as part of a broader project of ecosystem restoration, the emphasis is shifted away from simply ‘managing loss’ towards the creation of ecosystems that are healthier, more biodiverse and more resilient. I recently even came across a paper claiming that beavers reduced the abundance of parrot’s feather in a wetland by 90%, so who knows, should Devon’s population paddle across to Dorset, I may not be raking pondweed in perpetuity after all… 

[1] Not all the blame for rabbits in Australia goes to deliberate acclimatisers, but Eatherley and others point to a release by acclimatiser Thomas Austin in Barwon Park in 1859 as a key event in the story of their invasion. 

Fred Warren is a writer, filmmaker, and founding member of Chasing Cow Productions who graduated from the University of Exeter in English.