It was a little over 20 years ago when I saw a slender-billed curlew. I was in Morocco; the bird, lanky and reasonably slim in the beak department, was feeding on a patch of wetland decorated with wild cresses. Rather a nice sight. Not many people have shared it since then – because it’s extinct.

In all probability, anyway. The last rites haven’t been read yet and the ultimate authority on these matters, the Red Data Book compiled by the International Union for Conservation of Nature (IUCN), classifies the species as “critically endangered”. That’s scientific caution: it’s almost certainly gone. So I’ve seen an extinct bird. A rum feeling.

The baiji, or the Yangtze dolphin, evolved to live in zero visibility in the murk of the great river system it is named after. It found its way by sonar – a strange beast, like an alien life form. The baiji is also extinct: chemical pollution, noise pollution, propeller strikes and the impossibility of living among so many people combined to finish it off. An expedition in 2006 declared the animal “functionally extinct”.

According to the Living Planet Index, compiled by WWF and the Zoological Society of London, the world’s wild animals will decline in number by two-thirds by 2020. Of the 85,000 species listed by the IUCN, more than 24,000 are in danger, including lions, rhinos and giraffes, whose numbers have fallen by nearly 40 per cent since 1985. A study published in the journal Science Advances in January found that three-quarters of primate species have falling numbers, with 60 per cent threatened with extinction, among them gorillas and chimpanzees.

It’s happening in this country, too. In England, the hen harrier is close to extinction as a breeding bird: the RSPB says there was “a tiny handful” of nesting attempts last season. In the past 200 years, Britain has lost 8 per cent of its butterfly species. We know that because butterflies are easy to see and to identify. In the same time, we have lost 3 per cent of our beetles, which are harder to catalogue. If you replicate that pattern across all our invertebrate species, between 1,200 and 3,180 species will have become nationally extinct in the past couple of centuries.

It seems that we are heading for a world without animals. “The blueprint is in place,” said Matt Shardlow, the CEO of the invertebrate conservation charity Buglife. “All we have to do is carry on the way we are.”

But this is a define-your-terms situation. Despite desperate attempts across the millennia, philosophers and theologians have failed to conceal the reality that humans are a species of animal; like the Archbishop of Canterbury, we are primates. We also keep a lot of domestic animals, and there is little sign of cows and chickens going extinct.

The total vertebrate biomass – that is, the combined weight of every living backboned animal on the planet – can be divided into the wild stuff and the rest. So here’s the first killer statistic: 10,000 years ago, the biomass of humans and their domestic animals represented 0.4 per cent of the total. Right now, it’s 96 per cent and rising.

The planet, then, is going through a significant change. This is not a dire warning: it’s a current event. It is not a scare story to persuade you to adopt a dolphin: it’s a plain fact. Palaeontologists agree that there have been five major extinction episodes in the Earth’s history. The most recent did for the dinosaurs 65 million years ago, after a meteor strike. The consensus is that the sixth extinction is happening right now. The dinosaur extinction was literally the end of an era, a geological one: the Mesozoic became the Cenozoic. It is now reckoned that we are entering a new geological period: goodbye Holocene, hello Anthropocene.

We seem to have accepted the idea that the loss of wild animals is the sad but acceptable price of progress – and that progress is an incontrovertibly good thing. We recently passed the point at which more than half of the world’s human population live in cities.

The loss of animal species is not seen as a serious matter – when did you last hear a politician talk about the extinction crisis? That reflects the notion that humans come first, the domestic animals we use for

food come second and everything else is either a pest or a luxury. To care about wild animals is sentimental, childish, unrealistic. They’re expendable.

And yet in alarmingly recent history, white races believed that all other races were expendable. Genocide was wholly acceptable; the killing of Native Americans and Australian Aboriginals was considered perfectly justified. Peter Singer, the ethical philosopher, argues that our “circles of concern” have expanded since those times – beyond tribe, beyond nation and beyond race to all humanity – and should now be expanding further to include non-human species. That is happening to an extent (the worldwide ban on commercial whaling shows such thinking in action), but we are still losing both biodiversity and bio-abundance at a catastrophic rate.

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What would a world without animals be like? That is to say, a world in which the only animals were humans and their domestic animals. In a sense, that’s the wrong question. The one set by the Ethiopian famine of the 1980s was “What can we do about it?” rather than “What’s in it for me?” But let us be human chauvinists – what Singer calls “speciesists” – and ask how the loss of biodiversity will affect the surviving species.

“We won’t be able to write off every species,” said John Burton, the acting CEO of the World Land Trust, a habitat protection charity. “We’ll always have rats and cockroaches and their like for company. Which is not inappropriate.” We have always despised species that make successful adaptations to human life.

There will be no wild fisheries. There have been decades of overfishing, on the principle of “the tragedy of the commons” – “If I don’t grab it, somebody else will.” Pollution has created 405 “dead zones” on coastal waters across the world, including an area of 6,500 square miles in the Gulf of Mexico.

But when we talk of extinction, it’s the potential loss of the great beasts – the charismatic megafauna – that reaches people: lions, rhinos, gorillas, elephants, tigers, whales. Their loss wouldn’t affect many humans materially, but the idea of losing them is distressing. We seem to be moving towards the idea of tokenism: the survival of a handful of wild tigers tells us that the world is still OK, and we can watch them whenever we like on the ever-more-dramatic wildlife documentaries. But a world without any wild animals at all is a more complex notion.

“There’ll be very few flowering plants,” Shardlow said, “but plenty of dandelions. They don’t need insects to pollinate them.” The impact of the loss of wild pollinators will be considerable, as most crops depend on pollination by animal species. It has been estimated that the annual value of wild pollinators to the global economy is $190bn. Modern conservationists talk about “natural capital” and estimate a fiscal value for “ecosystem services”.

The loss of pollinators has led to an industry that supplies domesticated bees to do the work that was once done for free. In some places, notably Sichuan in China, the pollination of fruit is performed by

humans with paintbrushes or the filter tips of cigarettes.

Lynn Dicks, an ecologist at the University of Cambridge’s zoology department, estimates that the loss of wild pollinators will reduce global production by 5 to 8 per cent, which is more serious than it sounds, when we consider that the human population is increasing by 75 million a year.

It’s also possible that species diversity is the structure that underpins all life on Earth. Natural systems have a “redundancy” – they contain more species than are necessary to make the system function. “The argument in ecology is that the redundancy is needed for the long-term resilience of the system,” Dicks said.

A monoculture is more prone to collapse than a diverse system: we have the example of the Irish potato famine of the 19th century. Modern farmed monocultures require a considerable chemical back-up to make them work. It’s possible that the end of biodiversity – and with it bio-abundance – will create a series of ecosystem collapses.

James Lovelock, who gave us the Gaia theory – that the Earth is best considered as a single living organism – has suggested a hideous future of small, scattered human populations perpetually at war with each other. Others believe that the startling ingenuity of humankind will find a way to survive. Nobody knows, but as the great American scientist and writer Edward O Wilson said: “One planet, one experiment.”

There are other forms of loss associated with the divorce of humans from nature. The loss of birdsong and flowering plants is not like the absence of wallpaper and ambient music. Recent research has shown that the physical and mental health of humans is closely associated with access to nature. It has been demonstrated that people in hospitals recover better from surgical operations if they have a window – and better still if they can see a tree. Those with depression show improvement if they spend time in natural surroundings. Children with learning and behavioural difficulties do better – sometimes astoundingly so – when they are in touch with the natural world.

Professor Andrew Balmford, also of Cambridge University’s zoology department, quoted a series of experiments on the effects of the natural world on human behaviour. One required people to pass notional judgement on offenders, one group doing so before images of skyscrapers, the other before images of trees. Those who saw only buildings gave harsher sentences, especially to offenders from minority groups.

In another experiment, people were asked about their core values. One group said that what mattered to them was fame and money; a second group said it was family and friends. This second group had been questioned after three pot plants had been added to the room.

You get the idea: we are nicer people – more humane, more truly human – when we have access to non-human life. If we complete our divorce from nature, it seems we will have a much less pleasant society.

Now all of this is very fine and true and important, and not to be set aside. But the extinction crisis is not happening by itself. You can regard the extinction of animal species as the ultimate disaster, or you can take a smaller view and see it as a symptom of the crisis facing the human species – but either way, there are terrible things going on.

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We are in the process of killing off our planet: or, at any rate, changing it beyond recognition. We have already done the latter, but the process is nowhere near completion. We destroy forests. That contributes to the rise in global temperatures, but we need the land for agriculture or grazing. As a result, the land no longer holds water when it rains, so there are catastrophic floods that destroy crops and create famine. You can mourn the extinction of the bird species that lived only in that forest or you can mourn the human cost – but it’s all part of the same disaster.

The global temperature continues to rise. Climate change deniers will be regarded like today’s Holocaust deniers in times to come. We are living with a global rise of 1.2° C and climbing. It’s suggested that 2° C will be a tipping point and will lead to more extinctions – perhaps of the polar bear. It will also have a considerable impact on human lives.

It all comes back to population, the problem that dare not speak its name. Since 1950, the world’s human population has tripled; in 2016, we reached 7.4 billion. Energy use has increased by five times; so has fresh water use. You can argue that many of the recent events in politics and world affairs have been driven by the increasing pressures and proximity of human existence. “Even if we had a couple of extra planets, that wouldn’t solve the long-term problem,” said John Burton of the World Land Trust.

Human population growth is the principal driver of the global extinction crisis. There are not separate crises going on: it’s all linked. The loss of biodiversity and bio-abundance inevitably ensues. The long-time campaigner and environmentalist Tony Juniper said: “It follows that solutions are linked. It’s about sustainable economies – if we continue with economic growth, we will trash ecosystems and the soil. We need to end the extinction, reduce CO2 emissions and protect soils.”

Gerald Durrell, the pioneer conservationist, summed up the extinction crisis a generation ago: “People think that I’m just trying to look after nice, fluffy animals. What I’m really trying to do is stop the human race from committing suicide.”

All unattributed statistics are from Tony Juniper’s book “What’s Really Happening to Our Planet?” (Dorling Kindersley)