Very few parts of the British landscape have eluded great change in the past 1,000 years, but none of them have suffered a loss which can remotely compare to that of the fens of eastern England. These vast wetlands – at times desolate but always teeming with wildlife – once stretched from just above Cambridge to north Yorkshire.

But, at first gradually, and then systematically, they have been all but destroyed, the loss amounting, if you can credit such a figure, to around 3,500 square miles. This is equivalent to the combined acres of all of Hertfordshire, Herefordshire, Hampshire, the Isle of Wight, and Berkshire, or, if you think in these terms, more than three-quarters of a million football pitches.

The story of this, the biggest transformation of nature in British history, is told in a new book, ‘The Lost Fens’, which reveals a unique habitat, and its fish, fowl, insects, birds and plants, which has now almost gone. Its author, Ian D. Rotherham, calls what has happened “the greatest single loss of wildlife habitat in Britain and maybe Europe. This was an ecological catastrophe almost beyond comprehension.”

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Instead of this rich wilderness, we now have sanitised, chemicalised agricultural acres which stretch for mile upon mile, and are more inhospitable to wildlife than the most decked, patioed, and over-tended suburban garden. Thus we have traded in what Rotherham calls the richest landscape for nature in all of Britain for food-growing on an industrial scale – an unromantic swap, regrettable in its extent, but perhaps inevitable given our population growth and expectations. Without it, we’d be eating more expensively, and less well.

The process of human interference in the fenlands was very small scale until the beginning of the seventeenth century. The Romans may have brought to Britain the metal spade, but they and their successors made very little impact. This was a wild place, where lived unruly, rebellious people – the sort of landscape which could harbour the English renegade, Hereward the Wake, and centuries of bolshiness thereafter. It was a vast floodplain with reedbeds, swamps, slow dawdling rivers, damp woods of alder and willow, bogs, pools, and the whole often covered with a low moody mist – the sort of miasma that gave rise to the name of that common disease of wetlands, malaria (the bad air).

Most of all, it remained for longer than any other part of lowland Britain, a primeval place, where wildlife fared better than humans. In its sluggish waterways, for example, eels thrived in such quantity that the monks of Ramsey in Cambridgeshire paid for the use of a stone quarry with a rent of 3,000 eels, and could, apparently, have made it ten times that amount without depleting their own supplies. There were perch, pike, lampreys, turbot, tench, and many other fish, all of whom could be taken for the high tables of medieval England without any risk of denting the stocks.

The same applied to birds and waterfowl. Rotherham reports that the feast for the enthronement of George Neville as Archbishop of York in 1466 included 204 cranes, 200 bitterns, 400 young heron, hundreds of ruff, woodcock and curlew, and a thousand egrets, all harvested on the Derwent Washlands south of the city, and all of them snaffled, trapped, and netted before the gun was invented.

But it was the arrival of Dutch experts in the seventeenth century, the use of windmills to drive pumps, and then the use of steam power in the nineteenth century, that sucked the water and the life out of the fens. Rivers were straightened, dykes built, water tables lowered, and peat cut on a large scale. The change was immense. Take Whittlesea Mere, Cambridgeshire, the largest natural lake in southern England. It was mapped in 1786 and then measured 3.5 miles by 2.5 miles with a depth varying from two to seven foot. Drained in 1851, it is now beyond recall or restoration.

Of the large birds that once lived on the fens, spoonbills and egrets were extinct for some 300 years before returning recently; but the great bustard, once sufficiently plentiful to be taken for lordly banquets, has never returned, and neither has the purple heron, except as a passing rarity. Cranes were gone by 1600 and, with a seven-foot wingspan, are unlikely to have been overlooked. They have recently come back, colonising the newly created Lakenheath Fen in Suffolk, and are starting to spread, even being seen flying over Sheffield. Black terns, once a feature of the landscape, were reduced to a few pairs by 1800s, and, since 1930s, have been only an occasional visitor. Ruffs, capable of being caught by the dozen in the 1760s, were sparse by mid-Victorian times, and the spotted crake, snipe, and water rail all went the same way, as did the dotterel, a plover hunted to the point where it became a passing migrant.

Large mammals, of course, fared even worse. The beaver had been hunted to extinction by 1300, and the wild boar disappeared 300 years later. Conspicuous and striking plants were much reduced or eradicated: the marsh sowthistle and bog orchid were thought gone by 1870, and the populations of bog myrtle, water germander, marsh violet, and bog pimpernel shrank. The fen ragwort, which can thrust its yellow flowerheads to a height of more than six feet, probably once grew all over the region, but died out and was believed extinct until a lone colony was found near Ely. It has now been re-introduced to Wicken Fen, Cambridgeshire.

Insects were largely unstudied until much of the fens had gone, and, since we don’t know what we once had, we don’t know precisely what we’ve lost. The lack of any reliable stock-taking is exemplified by the discovery, as late as 1956, of the fen raft spider, Britain’s largest native arachnid, and almost certainly a species more plentiful in the past. A few hundred hang on. And then there is the absurd story of the pool frog, an amphibian long thought to be an introduction and therefore, as its population dwindled, no efforts were made to conserve it. Only after the last one died was it established, from studying the bones of frogs found at archaeological digs, that the species was indeed a native. It has now been re-introduced, using animals from Sweden.

Other re-introductions include the large copper butterfly, still an everyday summer sight in the 1800s, gradually reduced to nothing, and now a small imported colony of the Dutch sub-species is maintained at Woodwalton Fen. The marsh fritillary, however, one of the most exquisite butterflies of Britain, was gone by the 1940s and has not returned, although it is found in the south-west and Wales.

Today, only Wicken Fen represents how the old fens might have looked, and has 5,000 insect species including 700 butterflies and moths, and 200 spiders, of which six are unique to the site, plus 300 flowering plant species. It and Woodwalton Fen were saved by Charles Rothschild at beginning of twentieth century. They now consist of about an 800th of the original fen area. The ambitious Great Fen Project, involving the National Trust, Environment Agency, Natural England, and the Wildlife Trusts, aims to create a 9,142 acre wetland connecting Woodwalton Fen with Holme Fen and will reproduce a small part of this once great wet wilderness, but cannot restore it. Food may now be produced here on an heroic scale, but that, in Rotherham’s view, cannot change the fact that the loss of the fens is “the greatest single ecological catastrophe that ever occurred in England”.

'The Lost Fens: England's Greatest Ecological Disaster' by Ian D Rotherham is published by The History Press at £17.99