If human beings were to vanish from the Earth, what would their effect on wildlife have been? A rash of extinctions, a lot of mixing up so that wallabies and parakeets live in England and rabbits and sparrows in Australia, but also — according to Chris Thomas — an eventual doubling in the number of species on the planet: a “sixth genesis”, as he calls it in reference to the five previous times that biodiversity has expanded rapidly after a mass extinction. We are causing a mass speciation.

At a local scale diversity has increased a lot: “The number of species living in virtually every country or island has already increased during the period of human influence, and numbers continue to increase.” The fauna and flora of Britain are much richer today than 10,000 years ago as a result of farming, towns, gardening, climate change and the deliberate introduction of exotic species. Thomas finds the same to be true in tropical forests in Cameroon, Costa Rica and Brazil: the net effect of some human disturbance can be more biodiversity.

You can resent some of the exotic species (I do) but you should pause to recognise that in terms of the functioning of ecosystems, there has been mostly improvement. In an extreme case, Ascension Island was a barren volcanic rock with a few ferns on its summit. It is now a semi-green island capturing more moisture from the wind, thanks to a deliberate effort, begun by Charles Darwin, to enrich its ecosystem.

Professor Thomas, an ecologist and evolutionary biologist from York University, has produced an immensely significant book. It is fluently written, carefully thought through, ruthlessly argued, neatly illustrated with case studies — and shockingly contrarian. He shows the upside for wildlife in the Anthropocene. He does not deny that human beings also cause problems for wildlife, far from it, but he does think we have almost entirely overlooked the gains for wildlife that our presence is also creating.

I have for some time been thinking that while human beings have caused many species extinctions, they must also be causing many speciations. I have not quite had the courage to say so, for fear of being accused by the green thought police of going too far. While watching sparrows on a recent trip to Hawaii, it occurred to me that, though they were little different from the ones I see in London, they must, through isolation, be on the way to becoming a new species of sparrow. Just as a flock of Asian rosefinches shipwrecked on one of the Hawaiian islands six million years ago have turned into scores of species of honeycreeper, half of which are now sadly extinct.

Thomas has the courage I lack. He begins his book with sparrows, as it happens. Sparrows are not native to Britain at all. They spread from central Asia with people, the first of many birds to exploit the urban habitat. In Italy they hybridised with Spanish sparrows to produce a new true-breeding species, the Italian sparrow, already almost reproductively isolated from its parent species. Add one to the list of bird species. Sparrows are persecuted in America for stealing nest sites from native bluebirds, but protected and encouraged in Britain, where a short-lived (and now reversed) decline in the 1980s and 1990s led to concern that we might lose them. This contradictory attitude makes no sense: they are man-assisted exotics in both places.

Thomas documents the way new species are evolving. There is an ex-Australian cricket on a Hawaiian island that has fallen silent in the past few decades because of a mutation. This has been caused by the predation of a parasitic fly from North America that hunts down its mating call. And there is a species of fly that eats hawthorn berries, some of whose members have switched to eating apples. They are evolving towards a distinct species, together with their three species of parasite wasp — turning four species into eight.

The old idea that evolution happens only very slowly is being cast aside. “The biological processes of evolutionary divergence and speciation have not been broken in the Anthropocene. They have gone into overdrive. We have created a global archipelago, a species generator.”

And then there is hybridisation. In America a blueberry fly and a snowberry fly, separate species, have hybridised to form a honeysuckle fly to eat non-native honeysuckle, itself a hybrid of various Asian species. In the city of York, growing on a roundabout, there is a unique species of flower called Yorkwort, recently rescued from extinction by using saved seeds. But Yorkwort was only born as a species in 1979 when the railways allowed Oxford ragwort — itself a natural hybrid from Mount Etna, collected by a botanist in the 18th century — to spread around the country and hybridise with groundsel in, for some reason, York.

“More new plant species have come into hybrid existence in Britain in the last 300 years than are listed as having died out in the whole of Europe,” writes Thomas. If you are sniffy about hybrids, remember you probably are one: all people with any non-African ancestry have genes from Neanderthals (Europeans) or Denisovans (Asians) in them.

Thomas’s argument is that “humans must adapt and help direct change, rather than attempt to preserve the world in aspic”. Nature is much more dynamic than we generally admit. Species move about, become rare, become common again. In time and space nature is constantly on the move. In Ice Age deposits in Britain you find frequent remains of a dung beetle species today known only from the high Tibetan plateau — how did that happen? The Monterey pine is barely clinging on to a few tiny refuges on the California coast, but is the mainstay of a vast timber industry in New Zealand, Chile, Australia and elsewhere. Blue gums from Australia are found throughout California, thickets of Chinese palms blanket the shores of Lake Maggiore in the Alps and Himalayan balsam invades British river valleys.

It is only here that I begin to part company with Thomas. While he accepts that the eradication of rats from South Georgia, to save the seabirds, was a good thing, he is not convinced that New Zealand should, let alone could, eradicate its mammals — none of which is native — for the sake of its birds. Just let them evolve instead. He is right that not all non-natives are bad, but in conceding that human beings should actively manage the process of natural change, he should perhaps look on pest eradication more favourably as an example of just such management.

For instance, I cannot stand idly by and watch three species disappear from my farm, all as a result of American invasive species: the water vole (eradicated by mink); the native crayfish (eradicated by signal crayfish); and the red squirrel (being eradicated by grey squirrels). So I trap mink, signal crayfish and grey squirrels whenever possible. It may be a futile, Sisyphean task for the moment, but in the long run new genetic techniques may make it easier. Invasive species are by far the greatest cause of local and global extinction, not habitat loss or hunting.

Thomas briefly blames the extinction of harlequin frogs in Central America partly on climate change before correcting himself later to agree that the cause was a fungus spread around the world from Africa, partly in pregnancy-testing kits. He is far more balanced than most academics on the topic of global warming, conceding, for example, that “the basic expectation of a warmer and slightly wetter world is that the diversity of many – and perhaps most – regions in the world will increase”.

This is certainly true in Britain, where more warm-loving species are arriving than cold-loving species are leaving. Oddly, he omits any mention of global greening, the phenomenon by which the global vegetation has grown greener over 33 years by the equivalent of a continent twice the size of the United States, 70 per cent of the cause of which is extra carbon dioxide in the atmosphere. That surely rates a mention in a book celebrating the gains from man-made interference with nature.

When he writes that “it is difficult to understand why any particular moment in the continuous passage of time should have special significance”, this surely applies to climate change too. The world was much warmer than today in times past and much colder at others. The rate of change was much faster at the end of the last Ice Age.

We think of human beings as unnatural, as separate from nature, and a “separation myth” permeates our writing about the natural world, but it is nonsense. Thomas writes: “We may not be happy about some of the changes that are taking place as a consequence of our existence, but they are still natural.” We caused many extinctions, especially of large mammals and birds, when we were hunter-gatherers fully embedded in natural ecosystems. Wherever human beings appeared 50,000 years ago, there followed a disappearance of mammoths and rhinos, of diprotodons and giant kangaroos, of moas and rocs, of giant elks and great auks. Modern technology is not the problem: we caused twice as many extinctions of birds and mammals before 1700 as we have caused since.

Full review