Isabella Tree, author, journalist and manager of the Knepp Wildland Project

Isabella Tree writes for publications such as National Geographic, Granta, The Sunday Times and The Observer. Her latest book Wilding – the Return of Nature to a British Farm charts the story of the pioneering rewilding project in West Sussex where she lives with her husband Charlie Burrell. It is the first lowland rewilding project of significant scale in England.

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Excerpt of Wilding by Isabella Tree

By 2008 the advance of metre-high thickets of creeping thistle in the rewilding project was jaw-dropping and by 2009 it had covered acres of the Repton Park and swathes of the Northern Block. It was the biggest challenge yet to our rewilding ethos of allowing nature to take over the driving seat. We looked out on the day of the triffids and knew what our neighbours would be saying and the threat it could pose to our funding. Less than a decade earlier, under the old intensive farming regime, we would have been out with the toppers and weed-killer for all we were worth. It took all the courage we could muster to hold our nerve and do nothing.

While we were furrowing our brows over our thorny conundrum, a kilometre up over the English Channel, another invasion was coming our way. Beyond human sight, at speeds of up to 30mph, 11 million Painted Lady butterflies were gliding towards a land they had never seen before. The Painted Lady, a famous long-distance migrant, is one of our summer-visiting butterflies. About a million individuals drift over to our islands every year but once every decade or so Britain is treated to a bonanza, the result of a population boom in their place of origin - the desert fringes of North Africa and Arabia (in this case, in 2009, the Atlas Mountains in Morocco) - and perfect weather conditions for trans-continental flight.

It was May 24, a warm, clear Sunday morning under a ridge of high pressure after the showers of the previous day and we woke to butterflies streaming past our windows at the rate of one a minute. Out in the park thousands of Painted Ladies, a shivering miasm, had descended on the swathes of creeping thistle. As we approached, the dogs ran into the prickly cover, bouncing about looking for rabbits, sending up puffs of orange and brown like autumn leaves. The butterflies, while themselves feeding on a wide variety of flowers, zone in on thistles to lay their eggs. The leaves will be food for their caterpillars.

Standing in the middle of a butterfly blizzard, eyes closed, as I did that extraordinary day, is discombobulating. The sound of a single butterfly is imperceptible but tens of thousands have a breath of their own, like the back-draught of a waterfall or an accumulating weather front. It feels as though the oscillating susurration of their wing-beats, pounding away on their supernatural wavelength, might dissolve the world into atoms. 

We walked for half an hour that morning, parting curtains of butterflies. What we were seeing at Knepp and what people were recording up and down the country was part of the greatest butterfly migration on earth - a round-trip that, in its best years, can stretch 9,000 miles from tropical Africa to the Arctic circle. It can take up to six successive generations to make the journey. We still don’t know - how can a creature, weighing less than a gram, with a wingspan of less than three inches and a brain the size of a pinhead, find its way to a land it has never known, to which even its parents and grandparents have never been? 

This was our first loudhailer lesson in the phenomenon of boom and bust, Nature’s cardiograph of explosions and cliff-falls. By autumn, after the caterpillars had wolfed down their leaves, pupated and flown, our creeping thistle fields were in tatters, purple heads nodding on skeletons easy-pickings for the ponies. The following year, our sixty acres of creeping thistle had vanished entirely. The devastation caused by the caterpillars had quite possibly weakened the plant’s immunity, opening the door to some pathogen - a virus, pest, rust or fungi - which spread through the clonal colony like wildfire. Now, when people stand shaking their heads in our fields of ragwort or – latterly – acres of the pioneer fleabane, we shrug off their concerns with a reaching-for-the-golf-clubs kind of surrender. Not even plagues of injurious weeds last forever.

We had been given a ringside seat at one of nature’s greatest spectacles thanks to sitting on our hands and keeping the glyphosate under lock and key but even without the Painted Ladies, those three years of thistles had proved a gift. The prickly cover had protected other butterflies, day-time moths and invertebrates - including an explosion of grasshoppers - from the preying beaks of birds, creating the perfect opportunity for the common lizard. Gravid females with dark stripes scuttled between the thistle stems along tracks made by field mice, hunting for insects. Though the Exmoor ponies and pigs are partial to thistles, they are reluctant to wade through robust swathes of them and tend to nibble only at the fringes. Protection from their hooves gave an added boost to anthills. Ants could build new mounds – vulnerable to being knocked down and kicked over in their early, soft-soil stage - without disturbance. By the time the thistles died back in autumn the anthills had gained height and stabilised, capped by a coating of living moss and grass like the rind of a cheese. Now, back in the post-apocalyptic expanse of the park you can tell where the outbreak of creeping thistle once was, from the density of anthills.