A huge area of Britain's moorlands - a habitat better represented here than in any other part of Europe - has been obliterated by plantations of conifers or by ploughing and drainage to convert it into grassland for livestock. Zealous burning and overgrazing have spelt the death knell for an even wider acreage of moorland.
Bracken - that juggernaut of a fern whose expansion is all too difficult to stop - has also taken its toll, converting vegetation of heather, heaths, bilberry and mosses to a dense stand of nothing but tough, green fronds.
In these ways, according to a new study by Dr Des Thompson of Scottish Natural Heritage and his three co-researchers, a fifth of the moorland in England and Wales was lost between the mid-Forties and Eighties. In Scotland, a little under a fifth was destroyed between the Forties and the Seventies, despite the economic value of grouse and red deer.
In a parallel study, Dr Richard Bardgett and James Marsden, of English Nature, working with Dr David Howard, of the Institute of Terrestrial Ecology, estimate that just over 14,000 square kilometres of moorland remained in England and Wales by the early Nineties. Most of the English heather is in good condition. But not so in Wales, where on over a third of the moorland area there is overgrazing by livestock and general neglect.
As a result, much of this moorland would hardly be recognisable as such. The area of moorland that is dominated by heather, crowberry, bilberry or other "healthy" plants amounts to no more than 7,000 square kilometres in England and Wales. On the rest, these typical moorland plants are in the minority.
Dr Thompson reckons that 70 per cent of what is left is "at risk of change", much of it already in a fairly poor state of growth because of overgrazing by sheep.
When heather becomes patchy and too well nibbled, burning can destroy it completely. All too easily, a heather moor redolent with insects, red grouse and the occasional pair of impressive birds of prey such as hen harriers can be transformed into a monotonous carpet of tough grasses. Insect and bird populations nosedive, while some species, such as grouse, leave for good. Of the 40 bird species totally or partly dependent on moorland, 40 per cent are declining in distribution.
In international terms, Britain's moorland is something of a star turn. "Six of its heath and mire vegetation types are virtually confined to the UK and Ireland. Another seven are better developed here than elsewhere," says Dr Thompson. Some are outstanding for the mosses, lichens and liverworts growing underneath the shrub carpet. Ground-dwelling invertebrates, a few mammals (including red deer, of course) and birds such as the golden eagle, merlin, golden plover and red grouse feed and breed almost exclusively on heather moors.
But the heather moors we have come to admire are not natural habitats at all: they are maintained by a delicate balance of management. If the land is left ungrazed, trees appear - rowan and birch in the most exposed spots. If it is overgrazed or too frequently burnt, grassland is the result. The balance tips in favour of grassland when farmers keep more than two sheep on each hectare.
There are, however, reasons to be hopeful that the balance can more easily be found so that the heather moorland that remains is retained and the condition of areas in poor nick can be improved.
For instance, several Environmentally Sensitive Areas (ESAs) have been designated by the UK government to include moorlands such as the southern uplands and Exmoor. Administered by agriculture departments, farmers in ESAs receive annual payments to stock their land appropriately and to farm in an environment-friendly way. In some ESAs there are specific measures for restoring moors in poor condition.
A Moorland Scheme - part of the Agri-Environment Programme - is also in the pipeline. It will apply to moorlands outside ESAs. Also a voluntary scheme (which has the disadvantage that farmers mismanaging their moorland are under no compunction to join), it will pay farmers for each sheep removed in order to reduce grazing levels.
None of these schemes, though, does much to address any fundamental change of thinking about how our heather moors should be managed in future. The ideal balance has, historically, been struck by most grouse moor managers who, by careful burning and light sheep grazing, create a uniform carpet of purple heather. It produces a pretty dull landscape.
What's good for grouse isn't necessarily good for all other moorland wildlife. Dr Thompson and colleagues argue for a shift in moorland management to produce a more diverse habitat. To achieve it, moors need to be managed as a mosaic; burning, grazing or cutting some parts more often than others, but leaving other areas to develop naturally with scrub and trees.
It has to be done without undermining the economic value of the moors, whether for sheep or grouse. Finding this new balance may be a challenge beyond what ESAs and the Moorland Scheme can deliver. But it is a challenge that must be addressed if we are serious about conserving one of our few habitats of international repute.