Invasive Aliens. Dan Eatherley

downstream. The younger specimens were grown in stock ponds, either purpose-built or adapted from existing millponds, moats and former river channels.

      Meanwhile in Britain, the eating of fish also grew in popularity in the centuries following the Norman Conquest. The people of this island nation had always enjoyed access to seafish, but the continental pond culture centring on freshwater varieties was nevertheless imported, as much for its prestige as for its nutritional benefits. Like deer parks and coneygarths, Britain’s fishponds were first associated with the wealthy, and by 1300 could be found on the estates of clergy, aristocratic landowners and the Crown from Wiltshire to Yorkshire – the ones at Bruton being surviving examples. In time, husbandry techniques advanced enough to provide a steady food source and aquaculture became a widespread commercial enterprise, although freshwater fish retained its cachet. The sorts of fish best able to endure the warm, slow-moving, turbid and oxygen-poor water of medieval ponds were favoured. That meant roach, tench, chub, dace, perch, and especially pike and bream. To this roll-call of hardy natives would later be added a foreign fish whose origins are as murky as the waters it frequents.

      The common carp is today among the world’s most important food fishes. Three million metric tonnes are grown annually across 100 countries, equivalent to a tenth of all freshwater aquaculture production. It’s easy to see why: the fish breeds and grows fast, tolerates a wide variety of environmental conditions and eats pretty much anything that can be sucked up by its telescopic mouthparts. The koi carp, a colourful variant developed in a mountainous region of Japan, is perhaps the world’s most popular outdoor ornamental fish and almost as well travelled as its edible cousin. Meanwhile, the species supports an angling market which in Britain alone is worth close to a quarter of a billion pounds each year.

      The ancestral common carp evolved close to the Caspian Sea around 2.5 million years ago and, taking advantage of the proliferation of waterways during warmer interglacial periods, expanded its range east into mainland Asia and west to the basins of the Black and Aral Seas. The European version of the common carp appeared in the Danube river some 10,000 years ago. And there the fish might have stayed were it not for its discovery by the Romans – keen aquarists – sometime in the first or second century CE. (Carp bones dated to that period have been identified at the site of a former Roman frontier fort near Iža in Slovakia.) Able to survive out of water and without food for prolonged periods, the carp were transported, possibly wrapped in wet moss or sacking, to the piscinae (reservoirs) of Italy as gourmet items and pets.

      Carp however, live specimens at least, weren’t present in Roman Britain, and don’t feature in European pond culture until around the twelfth century. The first written reference on this side of the Channel comes from the kitchen accounts of King Edward III at Canterbury, dated to 1346, which show a carp and eight pike costing 22 shillings. The carp in this case was probably an imported specimen, because the fish doesn’t seem to have been stocked in this country for another century. In 1496, The Boke of St Albans – attributed to Dame Juliana Berners, prioress of Sopwell nunnery – describes the carp as a ‘deyntous [delicious] fisshe’, and then in 1532 ‘Carpes to the King’ appears in Henry VIII’s Privy Purse expenses for that year.

      Despite this, carp was historically less important in Britain than elsewhere, perhaps because by the sixteenth century improvements in navigation and ship technology were, for the first time, allowing exploitation of vast new shoals of marine fish from offshore Atlantic waters. The common carp nevertheless qualifies as among the first non-native fish to have naturalised in Britain and remains abundant in still and slow-flowing waters across England, with scattered populations in Wales and Scotland. The species is often accused of muddying the water as it ploughs river and lakebeds for invertebrates, fish eggs and other buried morsels. The resulting high water-turbidity stops light penetrating and interferes with photosynthesis, messing up food webs. But carp enthusiasts, of which there is a growing army in Britain, argue that recreational boating and other human activities are as much to blame.

      A fascination for all things botanical, both native and exotic, also germinated within the monasteries and aristocratic households of Britain during medieval times; commercial horticulture can be traced to the thirteenth century, with enterprises in London and Oxford selling seeds in large numbers. Husbandry, a set of rules for estate management by Sir Walter of Henley published in 1280, states that imported corn-seed often outperforms home-grown counterparts, and this influential work may have encouraged the acquisition of foreign plants. By the late 1300s the Dominican friar and herbalist Henry Daniel was nurturing 252 sorts of herb in his garden in Stepney, London, of which 100 were non-native.

      Plants were cultivated primarily for function not aesthetics, although the beauty of snapdragons, snowdrops and snake’s head fritillaries – all of them apparently introduced during this period – is undeniable. Dill, coriander, summer savory, black mustard, fennel, caraway and parsley were all condiments whose use had declined after the Romans left but which made a big comeback during medieval times. Hitherto unknown species also arrived including saffron, a luxurious yellow spice made from the dried stigmas of a crocus flower. The plant originated in western Asia and is first recorded in England in the fourteenth century. Used as culinary ingredient, dye, perfume and aphrodisiac, saffron was famously grown in East Anglia, its economic significance such that a major centre of production, the Essex town of Walden, adopted it as a prefix in the sixteenth century.

      Horticultural introductions served other purposes. As its name suggests, the leaves and roots of soapwort, a member of the pink family native to the Middle East, contain natural detergents. Appearing in Britain from medieval times, soapwort found use in the wool trade, washing not just woollen products but the sheep from which they were derived; as recently as the 1970s, extracts were employed to clean fragile tapestries. Chasteberry, a type of vervain with purple flower cones which originates in the eastern Mediterranean, was used in monasteries to suppress libido among the acolytes, and nuns stuffed their bedding with the aromatic leaves to quash wicked urges. (The ancient Greeks prized the plant for the reverse effect: women slept on it to enhance their fertility.) The Aegean wallflower, meanwhile, was esteemed for the fragrance of its vivid golden blooms, reminiscent of violets. In its home range, the plant spreads over cliffs, and may have first reached Britain stuck to building stone imported by the Normans. It’s still found clinging to ancient edifices from Bury St Edmunds Abbey in Suffolk to Northumberland’s Lindisfarne Priory.

      Almost every introduced plant offered some or other kind of therapeutic function. Gout was treated with wall germander, a variety of mint; feverfew, in the daisy family, was a traditional painkiller; hollyhock, a laxative. Many plants were considered panaceas. Sweet cicely, a celery relative whose strong scent called to mind myrrh, was one of countless such ‘cure-alls’ and was used to remedy rheumatism, cleanse cuts and salve sore throats. It could relieve asthma, cure snakebite and promote sleep. Sweet cicely even stopped you farting. From time to time, serious mistakes could be made: to medieval midwives, the pretty yellow flowers of birthwort, a variety of clematis, resembled wombs – one shudders to imagine how they would know that – and they would administer its sap during labour to expel the placenta. It turns out that birthwort extracts are carcinogenic and may have killed thousands of women over centuries of misuse.

      Such cases were rare however, and did little to disillusion medieval herbalists. Yet, in the fourteenth century there arrived in Britain a disease – caused by one non-native and apparently carried by others – which even sweet cicely would be powerless to prevent (although people gave it a go). It would help change the course of human history, disrupting existing power structures and kick-starting an era of empire building and world exploration to dwarf anything achieved by the Romans and Normans. And the unprecedented globalised trade and migration that resulted would turn a trickle of non-native species into a deluge.