It is noteworthy that among human reactions to significant events is the creation of new metaphors – think of ‘the dust bowl’ or ‘the lost generation’ – and a particular feature of the crisis of destruction facing the natural world in the 21st century, and an indicator of its scale, is that it has already prompted two.

The first is ‘the sixth great extinction’ – the idea that the Earth is entering (if not already passing through) a period of loss of species so calamitous that it can only be compared to the five mass extinctions of the planet’s life already recognised in the geological record, from that of the Ordovician period, 440 million years ago, to the one 65 million years ago, at the end of the Cretaceous, when a comet slammed into what is now the Yucatán peninsula of Mexico and ended the age of the dinosaurs.

The other novel conception also reflects the geological record: it consists of a new label for designating the epoch in which we are currently living, the Anthropocene, to replace the current one, the Holocene, which has covered the period since the end of the last ice age. The new name refers to the fact that it is the impact of human activity that is now the dominant influence on climate, the environment and the planet as a whole.

These are sweeping suggestions; but Edward O. Wilson makes free with both these ideas right from the start in Half-Earth, his new book about how to counter the accelerating obliteration of the Earth’s wildlife, since for him neither is exaggerated: they both correspond to his own sense of the apocalyptic nature of the threat. “For the first time in history,” he writes, “a conviction has developed among those who can actually think more than a decade ahead that we are playing a global endgame.” He means that half or more of the planet’s species seem doomed to disappear, through human actions, in the century to come.

Now 87, Wilson is emeritus professor in the zoology department at Harvard and is the man who begat a new age of neo-Darwinian studies by making us think afresh about how much of our personalities is genetically inherited and instinctive – and thus began a fierce row with the political left – with the publication of Sociobiology in 1975. Novel theories are his speciality – others include the theory of island biogeography and the theory of biophilia (human beings’ instinctive love of other living things) – but the role for which he is now most fêted is that of greatest living naturalist, pace our illustrious storyteller David Attenborough: he has gradually become the pre-eminent defender of the planet’s ravaged, wrecked and increasingly extinguished biodiversity.

His enormous knowledge of it – amongst much else, he is the world’s leading authority on ants – is what gives conviction and dramatic colour to his account of its disappearance, with one in five vertebrates (mammals, birds, amphibians, reptiles and fish) now facing extinction, from the frogs tumbling in numbers all around the globe because of the spread of a disease caused by a chytrid fungus, to the rhinoceroses that once roamed Africa and Asia in their millions and now, hunted remorselessly for their horns, number but 27,000 in total.

Wilson is haunted by the idea that the vast majority of the Earth’s species will have disappeared before we can know them, because although the total named and described passed 2 million last year, the full total may be as many as 8 to 10 million – yet so few are the taxonomists able to catalogue new organisms (taxonomy no longer being seen as sexy science), that it might take, he says, until the middle of the 23rd century to describe them all. In the meantime, of course, they will have vanished. Wilson is desperate to save them, and he is particularly scathing about a new group of biologists he terms “Anthropocene ideologues”, who think we should merely adjust to the different, impoverished world we are creating.

So, fittingly for a man who has spent his life coming up with Big Ideas, he endorses another one: that we should set aside half the world for wildlife. He means it: a full 50% of the terrestrial surface of the globe. There are good scientific reasons for this, to do with the mathematical rate at which species and ecosystems disappear as areas of their natural habitat are destroyed: with half an area saved you might save 85% of its representative wildlife. No wonder such grand schemes have their attraction for conservationists.

But where Half-Earth falls down is in the elaboration of the idea: there is virtually none. Wilson suggests more than tripling the 15% of the world’s land surface currently covered by protected areas such as national parks, but offers no practical prescriptions: he does not say how scores of poor nations with soaring, land-hungry human populations might be persuaded to do this in the years to come, or how such an immense programme might be funded; and he wholly fails to address the grave difficulty that in much of the world, especially in developing countries, protected areas offer protection to Nature only in name.

This is a book whose very failings vividly illustrate the crisis it seeks to counter. Wilson knows better than anyone how terrible is the threat now hanging over the world’s myriad life-forms, and offers the most graphic portrayal of it; and he realises that only a truly drastic intervention can halt the sixth great extinction. But the intervention he puts forward is so drastic as to seem impossible, once considered in practical terms; or at least, he does not sketch out a possible way to make it work – and if he cannot, who can?

This is not to be defeatist: it is merely to recognise the scale of the catastrophe facing us as we enter the new age of the Anthropocene. Ed Wilson is to be saluted in addressing it so unflinchingly; he is the true champion of the world’s wildlife. Yet even he cannot provide the answer.

Michael McCarthy is a former environment editor of The Independent and is the author of The Moth Snowstorm: Nature and Joy.