‘Let it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be.’

      On the Origin of Species, Charles Darwin, 1859

      The 16 July 1898 edition of the Daily Mail devotes a single paragraph to the revelation that unusual creatures were on the loose in one of London’s better-heeled districts and defying all attempts at capture. According to the paper, ‘The wild animals on Hampstead Heath have just received an unexpected addition in the shape of two monkeys which have escaped from custody and are now enjoying a free and open life on the salubrious heights.’ A reward offered for the safe return of the simians – escaped pets from the nearby Bull and Bush tavern – proved unnecessary: a couple of days later the fugitives slunk back to the drinking house. Liberty hadn’t agreed with them. ‘They were in a deplorably dirty and woe-begone condition,’ as one account had it.

      It turns out that Hampstead Heath, a windswept expanse on a sandy ridge to the north of the city, is no stranger to the exotic. In 1944, monkey business was again reported from the Heath, the arboreal frolics of a pair dubbed Jack and Jill causing disturbance on this occasion. Things didn’t end much better that time: Jill was shot and her dejected playmate handed himself in to the authorities. Then there was the young seal fished from a pond in 1926 having alarmed nocturnal anglers by ‘barking like a dog with a sore throat’. Other tales tell of a phantom gorilla, of giant spiders, of marauding bands of wild pigs.

      For the people of London, the Heath is a little piece of countryside on their doorstep, an oasis in which to de-stress and reconnect with nature. Keats, Shelley, Wordsworth and Constable were among the many poets and artists in thrall to its bucolic charms, its wild and unspoilt landscapes. So precious is this place that the threat of quarrying and house-building prompted an 1871 Act of Parliament protecting for ever ‘the natural aspect and state of the Heath’.

      But taking a stroll on the eastern end of the Heath one chilly autumnal morning in 2017, I was struck by just how much around me wasn’t ‘natural’ in the sense of representing native British fauna and flora. The most obvious example was the resident flock of a hundred or so ring-necked parakeets with their frequent shrill calls. With eyes closed, I could have been roaming a Darjeeling tea estate. The birds originate from sub-Saharan Africa and South Asia and the stories to explain their introduction are every bit as colourful as their plumage. The most enduring legend is that they flew off the set of The African Queen during filming of the 1951 movie at Isleworth studios. Others point the finger at Jimi Hendrix for releasing a pair on Carnaby Street at the height of the Swinging Sixties. More probable is that London’s parakeet population – the current estimate is 30,000 and growing – established itself after successive escapes from pet shops and aviaries. Perhaps not as numerous as in other parts of the capital, where parakeets are accused of beating woodpeckers and nuthatches to the choicest nesting sites, the Heath’s contingent has been around for decades and seems to coexist happily with the locals. Then there were the grey squirrels. Victorians were the first to take a shine to the bushy-tailed rodents from North America and did their darnedest to spread them around the countryside. Woodland managers and red squirrel lovers alike have been gnashing their teeth ever since.

      There’s not much natural about the landscape either. Like every other part of Britain, Hampstead Heath has been managed and manipulated by people for centuries if not millennia. Cattle, sheep and goats – all first domesticated in the Middle East – have been raised here since the Neolithic period, suppressing forest regrowth and creating pasture. These days grazing duties fall to rabbits introduced from the Iberian Peninsula by the Normans. Or was it the Romans? Rabbits would give squirrels a run for their money for the title of ‘world’s worst pest’ – just ask an Aussie farmer – but on the Heath they get a pass because their incessant munching helps preserve the acid grassland, and in turn a community of rare heathland organisms. From the eighteenth century, what little was left of the Heath’s primeval woodland of pedunculate and sessile oaks, beech and birch – long exploited for timber and fuel – began to be ‘enhanced’ with Turkey oaks, horse-chestnuts, black locusts, rhododendrons, laurels and dozens more species from around the world. Early plantings were at the behest of aristocrats like Sir Thomas Maryon Wilson, Lord of Hampstead Manor, and the Mansfields of Kenwood House, whose estates constituted or bordered the Heath, but the ‘parkifying’, which included planting still more exotic trees and shrubs, continued long after the 1871 Act.

      Meanwhile, the Heath’s mineral-rich springs, dribbling out where porous Bagshot Sands meet impermeable London Clay, were exploited for drinking, laundry and their therapeutic properties. In time, the water, which had once collected in mosquito-infested swamps and bogs, was corralled into a series of ponds, which would later become a playground for bathers and anglers. Here, too, the roll-call of introductions is impressive from mandarin ducks and Canada geese to alpine newts and marsh frogs, from carp and catfish to red-eared terrapins. Some got here under their own steam. Most received a helping hand.

      Human agency is suspected in particular for the arrival of two types of crayfish, among the Heath’s more infamous aquatic denizens. The Turkish, or narrow-clawed, crayfish reached Britain in the 1930s, being joined in the 1990s by the red swamp crayfish from North America. Culinary motives are thought to have driven both introductions, with persons unknown considering Hampstead Heath the ideal place to rear them. They were spot on, for the two crayfish varieties multiplied, and are today well-entrenched in the ponds. This became painfully evident in 2012 when swimmers in the men’s pond complained of being nipped on the toes and, according to one report, ‘in altogether more sensitive places’. It gives ‘bottom-feeder’ a whole new meaning. Not long ago, the City of London Corporation, who manage the Heath, removed 500 red swamp crayfish from a single pond during routine maintenance over the course of just three days. With a single female producing up to 600 viable