Invasive Aliens

If the invaders left people alone and restricted their impacts to the degrading of natural ecosystems, that would be bad enough – not least as we ultimately depend upon these systems for our survival and wellbeing. But some non-natives harm us directly. Notwithstanding the odd pinch on the privates from a crayfish, the obvious threat is their role as agents of disease. The most notorious example in history is offered by the Black Death, inflicted by a strain of bacteria originating in Asia which, from the fourteenth century onwards, has killed tens of millions across Europe, the Middle East and North Africa. While nothing on that scale has recently been visited upon us here in Britain, new parasites and pathogens are on the radar, many transmitted by mosquitoes and other biting insects. At the moment, it’s a bit chilly for these to get a foothold here, but with climate change all bets are off.

Judging by the growing scientific literature devoted to the economic impacts of biological invaders, these species hurt our pockets too. Much of the cost arises from direct impacts such as insect pests reducing yields from agriculture and forestry, fish stocks wiped out by disease or the erosion caused when signal crayfish or Chinese mitten crabs tunnel into river banks. To the ledger we must add the eye-watering sums spent on preventing, monitoring and eradicating invasives. In excess of £5 million is spent every year in Britain removing Japanese knotweed alone. Various indirect impacts, trickier to calculate but just as real, and many times greater than the direct costs, can also be attributed to invasives. This is a complex area, but it boils down to the loss of valuable ecosystem services like nutrient cycling, pollination or flood prevention.

Overall costs incurred by invasive non-native organisms are estimated to amount to 5 per cent of the global economy. Across Europe, invasives inflict some £9-billion worth of damage every year. In the UK alone, the figure has been put at about £1.7 billion annually. Although these are ballpark estimates, resting on plenty of assumptions and subject to much debate, governments the world over are taking notice as never before. Invasive species are fast becoming public enemy number one. In 2016, the European Union banned 37 of the most problematic plants and animals from being kept or traded without a permit. These include signal crayfish, raccoons and American skunk cabbage. On this side of the Channel, the Great Britain Non-Native Species Secretariat was set up a decade ago and tasked with detecting and containing invaders, as well as helping to predict and prevent future incursions. Tackling troublesome non-natives is complex: the measures taken can be extraordinary and sometimes cause more problems than they solve, even hurting the very ecosystems they’re intended to protect.

An emerging school of thought is suggesting that the threat of invasive species has been exaggerated, that we should stop worrying about non-natives and even welcome them for the benefits they can bring. At the other extreme, a growing band of conservationists is going beyond simple calls for the eradication of non-natives to campaign for the deliberate reintroduction of a menagerie of native British plants and animals which have become extinct at the hands of humans. To its critics, the ‘re-wilding’ movement is pure eco-nostalgia.

For me though, most fascinating of all is that non-native organisms, invasive or otherwise, from rabbits to rhododendrons, mink to muntjac, hold up a mirror to our own species. Yes, the pace of invasion is higher than ever before but problematic non-natives aren’t a modern phenomenon: they’ve been with us from the outset, as unavoidable a corollary of the human way of life as cleared forests and piles of garbage. From the earliest settlement of our islands and first experiments with farming, through the Roman and medieval times, and the age of exploration by Europeans, to the current period of globalised free-for-all, the story of invasive species is the story of our own past, present and future.

2

First Invaders

‘But while men slept, his enemy came and sowed tares among the wheat, and went his way. But when the blade was sprung up, and brought forth fruit, then appeared the tares also.’

King James Bible, Matthew 13:25–26

For a million years a windswept peninsula in a corner of northwest Europe had seen various species of humans coming and going. The arrivals and departures were synchronised to the advance and retreat of continental glaciers, a dance choreographed by climatic change. They barely registered. A cluster of footprints here, a tidy pile of knapped flints there. Overwintering in caves, the people would emerge to gather shellfish from grass-fringed estuaries, pad through woodland in search of berries and nuts or pick at the carcasses left by lions and giant hyenas. The more ambitious, coveting the freshest meat, bone and fur, would rally family and friends in adrenaline-fuelled pursuits of deer, horse or mammoth.

Make no mistake, even the earliest people were unusual. Britain had never welcomed visitors quite like them, and over the aeons these experiments in humanity forged in the evolutionary crucible of an African valley generated ever more sophisticated results: the grunts of the most obtuse of cavemen took on deeper meanings; people fashioned better weapons and perfected their hunting techniques; they got the hang of butchery and learned to tame fire. Humans would turn their new-found skills on each other from time to time. Yet, for a great sweep of history, these pioneers – Homo antecessor, Homo heidelbergensis and maybe others – were but minor players on a stage dominated by rhinoceros and sabre-toothed cat, bison and bear. A low profile was often the best strategy given the monsters with which the land was shared. People were no more masters of their destiny than were grains of pollen in the air. And, every time the cold rushed back in and the fragrance of the dwindling forest was lost once more to the bitterness of endless tundra, so would humans again retire to more hospitable refuges in southern and southeastern Europe, abandoning the briefly colonised outpost to musk ox, wolverine and ice.

In the milder periods, when permafrost meltwaters inundated what would be known as the English Channel, the peninsula became an island. On one such occasion, some 125,000 years ago, humans found themselves shut out of the party altogether: things were warming up once again and a wealth of plant and animal species had spread back into Britain. But by the time people were on the scene, the land bridge from the continent had been claimed by the rising seas. Elephant, hyena, lion, deer, hippopotamus, elk and other animals had the place to themselves, enjoying a halcyon human-free interlude lasting 65,000 years.

Even Homo sapiens, the most successful hominid – in population terms, at least – to arrive in Britain was no great shakes at first. Originating perhaps more than 200,000 years ago, modern humans took their time getting here. Not for millennia would the most substantial exodus from Africa occur, with one wave of migrants moving along the Indian Ocean coastline towards southeast Asia, and eventually Australasia; another meandering north and west across the Middle East and Europe. When, from around 40,000 years ago, small bands of nomads, each with its own distinctive material culture, started to reach our shores, perhaps in seasonal visits, they found they’d been beaten to it.

Homo neanderthalensis had been eking out a living on Britain’s cold treeless steppes for at least the previous 20,000 years hunting, with flint-tipped wooden spears, woolly mammoth, woolly rhinoceros and probably quite woolly bison. The Neanderthals, with their heavy eyebrow ridges, flared nostrils and stocky physiques, were well suited to the hostile conditions. Yet their days were numbered, the British contingent vanishing within a thousand years of the arrival of modern humans. No one knows why.

It’s tempting to equate correlation with causation and accuse Homo sapiens of behaving as the archetypal invasive species, outcompeting and eradicating a vulnerable native with cunning and violence, perhaps passing on some disease for good measure. But the story appears more complicated: the significant amount of Neanderthal DNA in the modern human genome suggests a peaceful, even romantic, coexistence between the two hominids across continental Europe dating back 100,000 years. Could it be that the two varieties of early human preferred to make love not war? In Britain, at least, it seems they had little direct contact, and in any case the argument is academic since dropping temperatures led to the most recent glacial maximum about 22,000 years ago. Any prospect of human existence was snuffed out for another ten millennia. The ice crept forward, smothering everything, wiping the sheet clean. Another fresh start.

The people who returned to a warming Britain from around 15,000 years ago could still be classed as hunter-gatherers, but there was a greater sophistication about them, judging by the plethora of artefacts and art left behind. They were dog-lovers too, grey wolves having been domesticated to mutual advantage, possibly more than once, during or even before this most recent Ice Age. Moving along the Atlantic coast, the humans tracked herds of reindeer, horse, deer and elk north from their southern European refugia. Some perhaps crossed the English Channel in boats, while others may have sauntered through Doggerland, an expanse of terrain today submerged beneath the North Sea.

These people exploited natural resources with unprecedented intelligence: flint-tipped arrows of hazel, fired from bows of elm, felled aurochs (wild ox), red deer and wild boar with accuracy; the slipperiest of fish were trapped in river weirs purpose-built from willow; birds and smaller mammals were noosed and snared; a wider range of plants was collected, stored and cooked than ever before. People were thinking ahead. Fire was used to manage woodland. Freshly burnt clearings, the ash festooned with appetising plant regrowth, could be used to lure hungry game, which was much easier than tracking a deer or boar through dense forest. Nevertheless, impacts on the landscape were minimal. As in previous migrations people travelled light and, save for the plant seeds brought as food or stuck to clothing and bedding, few in the way of new species were conveyed to Britain during this period. Things though were about to change.

Danger. Tree felling in progress. A yellow warning sign greeted us as we approached the kissing gate. I had expected this: Hembury Hillfort’s website requested visitors to ‘observe cordoned off areas with red and white tapes’, and please to ‘not climb on timber stacks’. Thankfully, given my five-year-old daughter’s enthusiasm for outdoors rampaging, neither woodpiles nor tape were in evidence today. The works programme, aimed at reducing root damage to the site’s archaeology, was finished for the season. The tree clearance had a secondary function, to open up the view: that’s what partly drew us here. Hembury didn’t disappoint.

Twenty minutes later saw us picnicking amid bluebells at its southernmost tip. From the 240-metre-high bluff we were offered stupendous views across the Otter river valley towards the coast at Budleigh (the sea itself was lost in haze). The landscape was a hodgepodge of greens, interrupted here and there with the dull copper of a newly ploughed field, a yellow patch of oilseed rape, and the occasional pale minaret of wood smoke. Just visible to the west was Exeter, and beyond the grey eastern tors of Dartmoor from whose direction a brisk wind blew. Birds sang and robber flies buzzed. There was the faint drone of distant air traffic. Above us circled a pair of buzzards.

‘What can you see?’ I asked my daughter.

‘Cows,’ she replied, mouth stuffed with cheese-and-onion crisps.

Today’s miscellany of embankments, trenches, mounds and other vestiges of Hembury’s convoluted history confounds those wishing to understand it. The modern visitor is further disorientated by colossal beech trees which have erupted from the earthworks, clinging on with tentacular moss-covered roots. Yet its secrets are yielding to the archaeologist’s trowel.

Hembury’s strategic location and defensive qualities have long been recognised by those keen to defend themselves and command the region. It’s a real Russian doll of a place: ostentatious double-ditched ramparts dug in the Iron Age, some 3,000 years ago, surround the entire three-hectare monument, which is perched at the edge of the Blackdown Hills in East Devon. Easy access to nearby iron ores and smelting works perhaps justified the investment in time and effort to shift the countless tonnes of earth by hand. Members of the Belgae tribe, from northern France and the Low Countries, subsequently laid claim to Hembury, making their own mark in about 50 BCE with additional defensive ditches and ridges across the centre of the fort. Then, in the middle of the first century CE, the Roman military too added Hembury to its network of forts – apparently taking it without a fight.

More fascinating still was Hembury’s much earlier, Neolithic, incarnation, dating to around 6,000 years ago. This period was the focus of a pioneering series of digs in the early 1930s undertaken by the Devon Archaeological Exploration Society. The work was led by Dorothy M Liddell, a formidable and inspirational personality, and one of an emerging breed of female archaeologists. (A 17-year-old illustrator called Mary Nicol was one of Liddell’s protégés at Hembury. Later, as Mary Douglas Leakey, she would make her own name with palaeontological discoveries in Africa.) Through meticulous excavations, Liddell detected signs of earlier inhabitation at Hembury, including a causewayed (or interrupted) enclosure; post-holes denoting a once-grand timber gateway; the remnants of daub huts; shallow cooking pits, a metre and a half in diameter; and traces of a circular wooden building, possibly a guard house. Her team also recovered flint arrowheads and axes, and other stone implements, along with jet and greyish steatite beads and some of the earliest pieces of southern English pottery. Known as ‘Hembury ware’, the latter included simple round-bottomed bowls with lug handles, made using gabbroic clay, an orange-coloured mineral naturally occurring around the Lizard in Cornwall, 200 kilometres to the west. The finds hinted at a connection to an ancient and extensive commercial network stretching across the region and beyond.

But, for me, Liddell’s most important discovery at Hembury were some charred grains of spelt, an ancient form of wheat. Carbon dated at roughly 5,000 years old, these represent some of the earliest archaeological evidence for the cereal anywhere in Britain. Liddell also turned up stone querns for grinding the crop into flour. Evidence of the importance of cereals in the diet of Hembury’s Neolithic occupants was bolstered by the later discovery of 13 impressions of wheat grains embedded within some of the Neolithic ceramics.

How and why did a food plant native to the Middle East – 3,500 kilometres distant – come to be eaten atop a windy promontory in southwest England? The answer lies much further back in time.

Some 23,000 years ago, while Britain and the rest of northern Europe was gripped in an endless winter, people basking in the more benign climate of the eastern Mediterranean were gathering, grinding and cooking the grains of wild wheat, barley, oats and other grasses. It’s possible that the most far-sighted and patient among them may have planted out some of their seeds and waited to harvest a crop. The evidence for such an innovation back then is patchy, but certainly by around 12,500 years ago farming communities had materialised across the region.

The specifics of the transition from restless nomadism to a sedentary way of life based on cereal cultivation are still to be understood, but the shift is remarkably well documented in the Natufians, a people whose settlements are scattered across what is today Israel, Palestine, Jordan, northern Syria and southeastern Turkey. From about 14,500 years ago they started exploiting wild grasses such as emmer wheat and barley to make flatbread, beer and, later, animal feed. The transition from hunter-gatherer to settled farmer was by no means simple and direct. For some reason, the Natufians, having earlier taken up agriculture based on the intensive harvesting of wild grains, decided to resume a more mobile existence around 12,800 years ago. This about-turn has been linked to a colder period known as the Younger Dryas that reduced the natural availability of wild cereals in the Mediterranean region, forcing people to keep moving to fill their bellies.

Eventually, the Natufians and others returned to the cultivation of cereals. By selecting varieties with the greatest yields, or those which thrived in diverse conditions, crops were gradually domesticated. Early agriculturalists benefited from a common mutation in wild wheat and barley that causes the grain-carrying spikelets to be more tightly gripped to the plant after ripening – just when they should be releasing them. In wild conditions, these ‘non-shattering’ mutants are at a competitive disadvantage compared to normal grasses which can spread their seed far and wide, but they lend themselves to being harvested and cultivated by humans. People learned to exploit other plants too, including flax, pea, chickpea, lentil and bitter vetch, intentionally planting, tending and harvesting them.

Scientists wonder whether the timing of this shift to crop domestication, which probably occurred independently in different places across the Fertile Crescent – as well as in parts of eastern Asia where wild varieties of millet and rice were the grains of choice – might not be a coincidence. One suggestion is that as the climate warmed at the end of the last Ice Age, and sea levels rose, so people were forced to higher ground where they would have encountered wild wheat and barley growing naturally. Levels of carbon dioxide were also increasing in the atmosphere – possibly due to its release from warming oceans – boosting worldwide plant production, including grasses such as cereals, and kick-starting what is often called the Neolithic revolution.

With the right kind of seeds, well-prepared soil and a favourable climate, the pioneer farmers soon found themselves amassing more food than they needed. This calorie boost, combined with a reduction in energy spent moving around, is thought to have ramped up human reproductive rates. A population boom led to civilisations across the Fertile Crescent, an 800-kilometre arc of territory encompassing the floodplains of the Nile, Tigris and Euphrates. But the discovery of farming may have set off a vicious cycle: the more people bred, the more food was needed and the harder everyone had to work. If they didn’t want to, or couldn’t, they might cheat or steal, requiring strong laws and even stronger rulers to keep the peace. Of course, there was nothing stopping rulers themselves from hoarding food and growing their own power in the process. At the same time, more and more of the landscape was turned over to crops which meant an acceleration in deforestation, erosion and other varieties of environmental degradation.

Unsurprisingly, given its peripheral location and challenging climate, Britain wasn’t an early adopter of agriculture. By the time wheat and barley made their appearance here some 6,000 years ago – and those precious spelt grains were being hoarded in a primitive hut on Hembury hill – the world’s first city of Uruk was already rising from the Mesopotamian floodplain. The farming of livestock also appeared in Britain, and the rest of northern Europe, around this time, again having been pioneered long before in the Middle East.

Goats and sheep are believed to have been domesticated from their wild ancestors – bezoar and mouflon, respectively – across southwest Asia from about 11,000 years ago. These low-maintenance creatures, compatible with a semi-nomadic lifestyle, were probably first kept for their flesh alone, and only later used for milk, wool and other secondary products. The fertilising properties of livestock manure was also noticed and exploited. Despite their benefits, sheep and goats would go on to become among the world’s most destructive invaders, especially on islands where their relentless chomping wipes out rare plants and degrades ecosystems. Indeed, their unfussy diet, their rapid reproductive rate, their tolerance of a breadth of environmental conditions – the very traits which first drew us to them and of course to so many other problematic species – go a long way to explaining their world domination. At the last count, two billion sheep and goats roamed the planet.

Around the time that people first domesticated sheep and goats, cattle also joined the ranks of tamed ruminants. Cows were descended from the extinct wild ox, or aurochs. This was a spectacular beast, particularly the bull which stood nearly two metres high at the shoulder and sported fearsomely curved horns. Unlike the bezoar and mouflon, aurochs were already present in post-glacial Britain – indeed, they roamed the entire Eurasian landmass; however, domestication probably occurred in the Middle East. That’s because early cattle were much smaller than our native aurochs, and DNA studies show that modern cows, including British ones, are genetically closer to Syrian aurochs than home-grown ones. In fact, today’s entire global cattle herd – numbering some 1.5 billion cows – is believed to be descended from a founding stock of just 80 animals, likely to have originated in the Middle East. There’s a good chance, however, that hybridisation would have occurred between local British aurochs and the smaller incoming cattle. Neolithic farmers may not have been thrilled about this: their petite cows, bred for milking, may have risked serious injury when attempting to birth an outsized hybrid calf.

Other modern domesticates with native British versions also seem to have derived from imported stock. These include the pig, whose ancestor, the wild boar, was widespread here before the advent of agriculture. Porkers are thought to have been first farmed in the eastern Anatolian region of modern-day Turkey about 10,000 years ago – along with a later independent domestication event in central China – and descendants of these Anatolian versions were subsequently brought to Britain. As with cattle, the amount of wild boar DNA in the genome of domestic pigs suggests frequent hybridisation between the two. To an extent, this may have benefited pig farmers, as crossbred versions may have been better suited to the more bracing local conditions in Britain, although too much of the ‘wild’ in a pig could make it a handful. A balance had to be struck.

Many of our supposedly native crops may also have come from elsewhere too. For instance, Britain’s blackberries, raspberries, carrots and parsnips, as well as the perennial ryegrass, red clover and common vetch traditionally used as animal fodder, all probably derive from southern European strains.

Whether there’s the whiff of the exotic about other domesticated species is less certain. For instance, the honeybee is thought to have originated in Asia, or maybe Africa, around 300,000 years ago, later spreading naturally across Europe, so the likely presence of this woodland insect in Britain before the most recent Ice Age would qualify it as native. Yet, the earliest known archaeological evidence for honeybee exploitation by humans in this country – as suggested by beeswax residues on seven pieces of Neolithic pottery found in southern England – dates to as recently as 4,000 years ago. That’s several millennia after sweet-toothed pioneer farmers in Turkey, and later in central Europe, began gathering honey and wax from the insects, and possibly even domesticating them. So, we’re left to wonder if Britain’s first apiarists collected honey from wild bees or perhaps were using a tamer, introduced, variety that had been bred on the continent. In a sense, this discussion is somewhat academic, since pretty much all of our honeybees are today derived from southern European stock after parasitic mites devastated Britain’s existing honeybee population in the early twentieth century.