Inheritors of the Earth: How Nature Is Thriving in an Age of Extinction. By Chris Thomas. Allen Lane; 320 pages; £20. To be published in America in September by PublicAffairs; $28.

The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth’s Past Mass Extinctions. By Peter Brannen. Ecco; 336 pages; $27.99. To be published in Britain in September by Oneworld; £18.99.

IN THE central hall of the Natural History Museum in London a dinosaur has been displaced. The skeleton, donated by Andrew Carnegie, had long dominated the nave of the imposing gothic pile intended by its founder Sir Richard Owen as a “cathedral to nature”. “Dippy”, as the Diplodocus was sometimes known, was not merely spectacular. He provided a dramatic illustration of one of the great discoveries of 19th-century science—that most of natural history is, indeed, history.

Until the end of the 18th century hardly anyone believed that the past contained creatures unknown in the present. By 1841, when Owen coined the term “dinosaur”, extinction was an accepted fact. Today it is clear that the species currently creeping, crawling, striding, swimming, photosynthesising—and sometimes just dawdling—across the face of the Earth represent a tiny fraction of those that evolution has created and discarded over the aeons.

Unfortunately, the fraction is shrinking still. Humans have been killing off species for tens of thousands of years, and continue to do so at an alarming rate. Palaeontologists have identified five horrendous “mass extinctions” in the past 500m years of the Earth’s history. Humans, it is now frequently asserted, are causing a “sixth extinction”—a phrase which, in 2014, became the title of an excellent book by Elizabeth Kolbert, a writer at the New Yorker.

It is in this context of quasi-existential dread that the Natural History Museum has replaced Dippy with an arguably even more impressive skeleton (pictured). During the age of industrial whaling, oil-fired ships with explosive harpoons cut through the blue-whale population as a knife through blubber. The whales could easily have gone the way of the dinosaurs. But half a century ago humans decided to stop the killing, and today the whales’ numbers are increasing. The blue-whale skeleton freshly suspended from the museum’s vaulted roof commemorates this turnaround. The whale’s name is Hope.

This is the attitude that Chris Thomas espouses and engenders in his thought-provoking new book, “Inheritors of the Earth”. In 2004 Mr Thomas, an ecologist at the University of York, was one of the authors of a scientific report suggesting that a relatively small amount of global warming could see almost a fifth of the species that live on land “committed to extinction”. Though that estimate came in for some stick, Mr Thomas still reckons he was broadly correct. But at the same time he believes that humans are bringing about a great new age of biological diversity.

This is most obvious locally. Most people who take an interest can tell you of species that used to grace their local scene but no longer do, even though they are still to be found elsewhere. But despite all these losses biodiversity is, almost everywhere, higher than it used to be. New species have been introduced faster than old ones have vanished. Even the islands of the Pacific, where the Polynesian peoples, and the rats that travelled with them, wiped out 10% of the world’s birds, may now boast more species per island than they did before humans arrived. In most places where there are good data, Mr Thomas writes, there are currently between 20% and 100% more plant species than there used to be.

This, to most conservationists, comes as something between cold comfort and insult added to injury. The idea that imported species make up for extirpated ones feels like biodiversity bean-counting rather than real conservation (nothing, after all, has been conserved). And invasive introduced species, like those Polynesian rats, can drive natives extinct. But through argument and anecdote Mr Thomas shows that mix-and-match “novel” ecosystems, such as the unprecedentedly cosmopolitan forests now fringing Lake Maggiore on the Swiss-Italian border, have copious charms. He also argues that the threat of invasive species is overdone. Most introduced plant species, he says, never get beyond the farms or gardens they are introduced to; most that establish themselves in the wild remain thoroughly marginal; and of the few that thrive only a small minority actually threaten the natives—a fraction of a fraction of a fraction. Invasives, he argues, are a smaller threat than either habitat loss or direct exploitation.

These points have been rehearsed in recent years in such books as “Rambunctious Garden” by Emma Marris and “The New Wild” by Fred Pearce, as well as by academics like Peter Kareiva of the University of California, Los Angeles, and Erle Ellis of the University of Maryland. They have attracted withering scorn from, among others, E.O. Wilson, a Harvard entomologist and the doyen of the belletrist arm of the conservation movement, who finds their hubristic ideas “as free of fear as they are of facts”. Where Mr Thomas goes further is in arguing that people are not merely rearranging more even-handedly a shrinking number of species. They are creating the conditions for the birth of a great many new ones.

Some scientists emphasise the scale of humankind’s redistribution of species by treating it as an undoing of the mighty work of plate tectonics. By mixing up the contents of the different continents people have recreated, in a virtual form, the world of 200m years ago—a world with but one sea, Panthalassa, and one vast continent, Pangaea.

Topsy-turvy

But, as Mr Thomas points out, the new Pangaea is best seen not as a continuous land mass, but as an all-encompassing archipelago—a mosaic of ecosystems in intermittent contact with each other. Charles Darwin realised, when looking at the finches of the Galapagos, that as island populations go their separate ways, new species spring up. (This is why the Pacific islands, which make up only 0.25% of the Earth’s dry land, were able to sport 10% of its bird species before the great day of the rat.) The world full of new contexts that humans have provided for old species, Mr Thomas argues, will see similar diversification.

They come and go

Some examples: sparrows, which have spread from their Central Asian point of origin by adapting to life among people, are turning into different sorts of bird in different sorts of places; flies which until recently dined on the fruit of the American hawthorn have, since the arrival of European apples, split into spinophilic and pomophilic populations—forcing the parasitic wasps that prey upon them to do the same; a pretty ragwort brought to Oxford from the slopes of Mt Etna has, through a liking for the gravel beds of railways, spread from city to city in England, creating new varieties as it did so.

All around the world there is similar evidence of hybridisation and incipient speciation as separated populations go their different ways and get it on with different far-flung relatives. “I find it difficult”, Mr Thomas writes, “to imagine a period in the entire history of terrestrial life on Earth when the speed of origination of new evolutionary lineages could have been faster.”

The idea of new species springing up at such a pace will seem incredible to readers taught, by childhood outings to visit dinosaurs such as Dippy, that evolution takes a million years or so just to get out of bed, let alone do anything constructive. But evolution takes its tempo from the world around it. When driven by slow geological change, evolution is slow. Faced with sudden change, such as that which comes from being plonked down in novel but not inimical situations, it ups its game accordingly.

This creative aspect to the current crisis calls into question the equation of today’s depredations with the mass extinctions of the past. In “The Ends of the World” Peter Brannen, a journalist, provides a fascinating account of these planetary paroxysms, in each of which more than half the species living at the time were wiped out. Though there are common themes, such as changes in sea level and upsets in the carbon cycle, which resonate with what humans are doing to the Earth today, no two of these disasters were quite alike. And looked at in the round they leave a strong impression that today’s threat to species is, as yet, quite unlike those of the past.

In the “Big Five” mass extinctions whole chunks of the planet became deadly to most of their inhabitants as severe shifts in sea level, climate and ocean chemistry (along with, in one case, an asteroid impact) caused ecosystems to topple like dominoes. That is not what is happening now. Despite the fact that humans have contributed to the extinction of an estimated 177 species of large mammals—almost all of which were done in by Stone Age hunters—there are still vast numbers of large mammals wandering around. It is just that they are mostly pigs and cows.

In terms of biodiversity, that is bad. But it is nothing like the near uninhabitability that characterised large parts of the planet during past mass extinctions. Ecosystems have been disturbed, diced up and in some cases badly damaged—but few have failed utterly. Humans are changing the climate, the carbon and nitrogen cycles and more, and those changes pose mortal risks to hundreds of millions of humans and existential risks to many other species. But they do not look like the cascading ecosystem collapses seen in mass extinctions. As Doug Erwin, a palaeontologist, explains to Mr Brannen, “If it’s actually true that we’re in a sixth mass extinction, then there’s no point in conservation”—because if things were that far gone the whole world would, in effect, be coming to an end. Again.

The idea that today’s decimation is qualitatively different to the mass extinctions of yore, and that a compensating mass speciation is already under way, seems at some level to be quite encouraging. But what does it mean in terms of practical action? For action is clearly needed; things are not fine as they are. Many extinctions do not have to add up to a certified mass extinction in order to be a parlous loss. The world needs to get its biogeochemical cycles back in balance and to halt the warming, the rising and the acidification of its oceans. The needs of biodiversity and human welfare are, in this respect, well aligned.

When it comes to coping with the damage already done, and that which cannot be averted, Mr Thomas suggests that people must get over their desire to conserve particular ecosystems in the form to which they are accustomed. He does not see much point in throwing resources at all endangered species—especially those, such as many of New Zealand’s predatory-mammal-beset flightless birds, that he deems unlikely to make it without constant assistance. Instead, he says, concentrate not on familiar landscapes but on their living components; create and embrace novel ecosystems in which creatures expelled or emancipated from their ancestral homes can find new life.

Such creative utilitarianism may inspire some. But it will make conservation a more fractious field than it was when it was mostly a matter of whales and hope. Who gets to create the new, and in whose name? Who gets to reject it, and by what right? Much of what people get from conservation is a sense of commitment to both the future and the past. That commitment does not have to be a rational allocation of resources for it to feel worthwhile; it is not necessarily diminished by scant chances of success. Indeed, its likely failure can give it a Sisyphean depth. If fighting for in situ species that Mr Thomas sees as doomed adds value to people’s lives, it is hard to begrudge them that boon.

Correction (August 4th): An earlier version of this piece referred to Peter Brannen's book as “The Ends of the Earth”. The correct title is “The Ends of the World”. Sorry.