Invasive Aliens. Dan Eatherley

of the most important of the arable weeds were rye and wild-oats. Although originating in the Middle East, both seemed better adapted to our miserable climate and harsh soils, and often outperformed wheat and barley. So tenacious were these grassy invaders that by the Early Bronze Age, about 4,000 years ago, central and northern European farmers stopped bothering to weed them out and instead harvested them as crops in their own right. Domesticated varieties of both rye and oats were soon cultivated for bread-making, for flavouring alcoholic drinks, and as animal feed. Wilder versions of the oat stuck around and remain intractable arable weeds to this day, in large part due to the similarities in appearance and lifecycle with those of crops. Selective herbicides are available but hand-weeding, or ‘rogueing’, of wild-oats is still practised on a small scale.

      When Brits took to agriculture 6,000 years ago, the door wasn’t just opened to invasive plants. Also waved through was an assortment of animal species adapted to living among people and exploiting their way of life. The house sparrow is a case in point. Remains of this small, gregarious bird have been identified in 10,000-year-old Natufian sites, suggesting sparrows long ago learned to nest in or close to buildings, purloining stored cereals and picking through the rubbish piles. By the Late Bronze Age, about 800 BCE, sparrows are known to have been present in central Sweden, so had probably reached Britain by then too. Today, they’re one of the world’s most cosmopolitan birds, outcompeting indigenous avians and proving a serious agricultural pest. In Russia alone, they’ve been accused of consuming a third of the annual grain production. During the 1950s the Chinese leader, Mao Zedong, even declared war on the sparrow, his scientists reckoning that, for every million birds killed, 60,000 extra people could be fed for a year. Chairman Mao’s scheme backfired: the removal of sparrows resulted in plagues of locusts and other insect pests, whose populations the birds had helped suppress, which in turn led to famine. The Chinese government ended up reintroducing sparrows from the Soviet Union.

      It seems therefore that house sparrows have a value in agricultural systems and in Britain, at least, we’re fond of them. The sparrow population has been falling of late: during the 1970s there were up to 12 million of them in the UK, but the population is now half that, with the worst declines in England. No one is really sure what’s killing off sparrows. Possible factors include a reduced availability of invertebrate prey, a shortage of nesting sites and increased predation by squirrels, magpies and cats. In cities, high levels of nitrogen dioxide in the air, mainly from car exhausts, also seems to be a factor, with London alone seeing a 60 per cent decline between 1994 and 2004. All this has triggered urgent conservation efforts to save the sparrow.

      Such measures won’t be contemplated any time soon for the house mouse, another accomplished non-native invader, which originated up to a million years ago somewhere between the Middle East and northern India. The rodent was first drawn to the organic waste tips of hunter-gatherer settlements in the southern Levant at least 15,000 years ago and its population was primed to explode with the invention of agriculture. Recent evidence shows the house mouse sometimes shared the more mobile of the Natufian sites with a second species, the short-tailed mouse; however when people settled down for any length of time, the house mouse soon elbowed out its wilder cousin. By the Bronze Age, the rodent had scurried into western Europe but took a while to make its mark in Britain: the earliest records date from pre-Roman Iron Age settlements at Gussage All Saints in Dorset and Danebury Hillfort in Hampshire. The mouse seems to have got established after repeated introductions as a ship stowaway; by then Britain was well connected to the continent by the maritime trade and replete with granaries. Danebury alone boasted some 4,500 pits for storing crops, making it a house mouse heaven.

      Along with rabbits, rats and grey squirrels, the house mouse shares the accolade of being among the few vertebrates to inflict both economic and social costs on a national scale. In addition to eating and fouling food stores, the rodent harbours a catalogue of unpalatable (and unpronounceable) diseases from tularaemia and typhus to leptospirosis and lymphocytic choriomeningitis. Humans have long waged a losing war against the species. These days baited traps and poisons tend to be used, but in times past barley cakes, spiked with black hellebore (a toxic variety of buttercup), would be placed at the entrance to their holes. Mice were also said to flee a censer of haematite stone and burning green tamarisk. But nature also provided a more elegant solution to the rodent problem.

      The African wildcat’s mouse-destroying prowess, along with its skill as a bird and fish catcher, may have been what recommended the species as the perfect household pet to the Egyptians more than 4,000 years ago. If true, that would make its tame version, the domestic cat, an early agent of biological control (the use of one organism to reduce populations of another). The sacred importance of cats in ancient Egypt is the stuff of legend with the feline deity Bastet worshipped as a goddess of fertility and the moon. The Greek historian Herodotus famously – but perhaps not altogether reliably – reported that the death of a cat prompted all those in the household to shave their eyebrows. The pet would then be embalmed. One cemetery unearthed at Beni Hasan in 1888 was said to contain the remains of 80,000 cats. A 20-tonne consignment of the corpses was later exported to Liverpool as fertiliser. One or two of the mummified moggies were saved for posterity by the city’s museum. The human relationship with cats may predate ancient Egypt, with the suggestion that the felines began domesticating themselves during the Early Neolithic period; as sparrows and mice were drawn to Natufian grain stores and spoil heaps over 10,000 years ago, so cats were drawn to the sparrows and mice. A rise in the feline population may have been further sustained on proffered titbits from people, as well as rummaging through our mounting piles of rubbish.

      Like the house mouse, the domestic cat first appeared in Britain towards the end of the Neolithic, with signs of the species at Gussage All Saints and Danebury Hillfort – just like those of its famous rodent quarry. Could it be that the cat’s pest control qualities were appreciated in Iron Age Britain? Cats were, however, rare until medieval times. The earliest written record dates to the reign of the Welsh king Howell the Good (880–950 CE), who issued the edict that anyone slaying or stealing a cat was liable for a financial penalty calculated in terms of the equivalent cost in grain: ‘The worth of a cat that is killed or stolen; its head is to be put downwards upon a clean even floor, with its tail lifted upwards, and thus suspended, whilst wheat is poured about it, until the tip of its tail be covered.’ Today, an estimated nine million cats prowl Britain’s towns and countryside, each year snaffling some 100 million prey items, including mammals, birds, reptiles and amphibians. One 1987 study from the village of Felmersham in Bedfordshire implicated cats in almost a third of house sparrow deaths. It seems old habits die hard.

      Perhaps the greatest feline felony is a crime of passion. As with cows and pigs, keeping apart wild and domesticated versions often proves futile. The same seems true of pet pussies and Britain’s own native wildcat, an endangered beast confined to the forested margins of Scottish moorland. The two versions have interbred so often that hybrids now dominate the wildcat population. Conservationists worry that too much domestic cat in the genome of the wildcat weakens it and leaves an animal which is already threatened by habitat loss and persecution close to extinction.

      The arrival in Britain of a tabby of a different sort is also linked with the advent of Neolithic agriculture. Also known as the grease moth, the large tabby gets its name from the uncanny resemblance that its forewings bear to cat fur. With an appetite for dried dung, dead skin, old feathers, bits of straw and other unmentionable detritus, tabby larvae probably first hitched a ride here ensconced in livestock bedding. Suggesting that its natural habitat might once have been caves, the insect lurks in the gloomy recesses of stables and outhouses, where the larvae spin protective silken tubes about themselves then munch away undisturbed on their rarefied diet for up to two years before turning into adults.

      A similar niche is exploited by dermestid beetles, many of whose 1,000 species and subspecies are spread by human migrations and globalised trade. Some are specialist scavengers on desiccated animal remains including hides, furs, feathers, tendons and bone, and a few are associated with Egyptian mummies, as well as with human remains from Middle Bronze Age sites in the southern Levant where the larvae drilled tunnels into the bone. Museum taxidermists still use these insects to nibble flesh from animal skeletons prior to display. Some dermestids could have reached Britain as early as the Neolithic period in the same way as the large tabby moth.

      Among