Imagine sitting on the beach hunched over your fish and chips as greedy seagulls circle above, when suddenly hundreds, if not thousands, of whales swim past.

Whales everywhere, as far as the eye can see. Some majestically spring from the water as they migrate from north to south. For hours on end.

Can you picture it?

This was once a recurring event, according to the notes of 16th century Once or twice a year, a vast number of whales would pass the Netherlands. Even Prince William of Orange visited the beaches of the North Sea to experience the beauty of whales migrating, Coenen wrote, as if to indicate the

Illustration of whales

Paradise lost

When it comes to natural observations of the past, the story is always the same: we read descriptions of a natural richness that we couldn’t begin to imagine. Seas teeming with fish, skies dark with birds. Trees filled with kakapos. Vast coral reefs that were the scourge of many a seagoing vessel.

But the notion that nature, or rather "biodiversity", is not faring well has only recently taken root in the collective consciousness. 

Last May, the – something like the United Nations for biodiversity and ecosystems – stated: " are threatened with extinction". The biodiversity crisis immediately became our second major problem, Some, in fact, consider it an even greater problem. The jury’s still out on that one. 

Illustration of an insect.

All this attention is wonderful. But the reporting on biodiversity and species extinction has two problems.

One: the definition of "biodiversity" It’s often explained in terms of many different species, but how many species really exist? What exactly is a species? And how is the earth’s living cover organised anyway? 

Two: by merely focusing on species that may or may not become extinct, we overlook a whole range of other important issues.

The new GBP

Unlike 16th century fishermen, when we think of beaches, we would never picture a horizon filled with whales. Marine biologist Daniel Pauly calls this "the shifting baseline syndrome". 

Nobody remembers how abundant nature was before their time. The frame of reference for each new generation is the natural riches from their own youth.

Yet every organism that was once very commonplace first becomes rare before it goes extinct. "You don’t lose abundant animals," "You always lose rare animals. Therefore, they’re not perceived as a big loss." But our memory is too short to know what has actually disappeared. 

For extinct species, it’s definitely too late. But before a species is threatened with extinction, the greatest damage has already been done. How do we express this aspect of biodiversity?

The answer: in biomass. In kilogrammes of living matter. How many kilos of creature are there today? How many kilos of creature were there in the past? What if we could map this visually? Would newspapers devote graphs to shrinking biomass, next to pages on economic growth?

Think of it as our Gross Biomass Product or GBP.

What choices would you make if such a concept existed? Would you walk into an H&M thinking about the destruction of natural wealth behind the I’m curious. So here’s a biodiversity crash course in species and biomass.

How many species exist?

"One of the most important questions in biology, perhaps even in society, is: how many species exist?" Tree biologist Olaf Bánki of Naturalis Biodiversity, a national museum of natural history in the Netherlands, says with a deep sigh. “We just don’t know." Bánki has devoted his working life to a mega international project: digitising every known species and associated data in an online database called

We don’t know for three reasons.

One: we simply haven’t discovered all life on earth yet. The tropics and the deep sea alone house all kinds of organisms that no one And some organisms we literally can’t see – they’re too tiny. Biologists call this diminutive stuff "dark matter".

Two: we don’t know what we have already discovered – or even if we’ve in fact discovered some species Natural history museums are like closed silos of species information, non-digitised, full of data no one knows exists. Bánki is trying to fix this with his catalogue. 

And three: biologists disagree about what a species even is. Over 20 definitions

So when newspapers – taking a cue from the IPBES – warn that a million species are about to go extinct, they are relying on an estimate of the total number of species on Earth. An estimate from 2011, which claims there are 8.7 million species, give or

Estimates from previous decades range from two to although most biologists currently believe there are probably several million. No more than are listed in the Catalogue of Life.

If we look at the number of species per kingdom, on land and in the sea, the most recent estimate looks like this. (A “kingdom” is a biological rank in the great order of life

The urge to organise the many manifestations of life in clear groups and subgroups is timeless. Even Aristotle took a shot. But the current taxonomic system builds on the work of Swedish biologist and physician Carl Linnaeus (1707-1778), especially his Systema Naturae.

The infographic above lists seven realms: bacteria,   plants, animals, fungi and (It is unclear whether chromists

So while kingdom is the second highest taxonomic rank, species is the lowest, or, better put, the Humans belong to the kingdom of animals and the species Homo sapiens. In between are all kinds of different ranks. For example, we belong to the phylum Chordata – animals with the rudiments of a backbone. Below that, we have the class of mammals – animals with the rudiments of a backbone that suckle their young. (For the enthusiast, the full

Taxonomy and its ranks work like an address book. So, The Correspondent HQ can be found on Planet Earth, on the continent of Europe, in the country of the Netherlands, in the province of North Holland, the city of Amsterdam, the street Barentszplein, and so on. But the taxonomic street guide isn’t called Google Maps but "phylogenetic tree". Or in everyday language, the tree of life.

The tree of life maps which organisms are related to which other organisms. Or when creatures embarked on separate evolutionary paths because populations of the same species encountered different challenges in life. In short, which route a species has travelled through evolutionary history.

Think of it this way: life started 3.7bn years ago with a handful of cells floating around in the primordial soup. And now we have everything from whales and humans to lichen. It is precisely this evolution that makes defining what a species is so complicated. 

What is a species?

The problem with the concept of "species" is perhaps best illustrated by an anecdote from evolutionary biologist Richard Dawkins. 

Dawkins once ended up debating with a lawyer after a lecture.

The lawyer was troubled by the concept of evolution: "If species A evolves into a later species B … there must come a point when a mother belongs to the old species A and her child belongs to the new species B. Members of different species cannot interbreed with one another. I put it to you (…), that a child could hardly be so different from its parents that it could not interbreed with their kind.

Isn’t this a fatal flaw in the theory of evolution?"

To Dawkins, asking this question is as bizarre as claiming that when you heat a kettle full of cold water, there is no specific moment when the water stops being cold and becomes hot, so making a cup of tea is impossible. 

The lawyer was clearly using caged thinking: animals can only be species A or species B, and not halfway between species A and B, or one-tenth of the way on the path from A to B. 

Taxonomists also use caged thinking; they have to. Without convenient labels like species, they wouldn’t be able to discuss a single animal. But the size of the cage – the degree of variation it allows – is often based on the individual preferences

In 1940, biologist Ernst Mayr proposed the most widely accepted definition of species: a collection of organisms that are mutually crossable and have fertile offspring. Tigers and lions can have cubs together, but the cubs themselves are infertile. So tigers and lions are different species. 

Illustration of a beetle.

But polar bears and grizzly bears – also two different species – may beget fertile brown-white bears (grizzly polar bears). What’s more, many animals, such as jellyfish, corals and plants, do not require sex for reproduction. And with bacteria, who knows what’s going on. Are they having sex or eating each other?

That is to say, life doesn’t really care about our species boundaries. Charles Darwin found the whole enterprise of defining boundaries pretty nonsensical. 

In 1856, he wrote to his best friend, a botanist: "It is really laughable to see what different ideas are prominent in various naturalists’ minds, when they speak of ‘species’. It all comes, I believe, from trying to define  

Species A and species B do become separated after a sufficient number of generations, says biologist Michael Benton. "It’s not as though every species we see around us are all blending into each other in front of our eyes." At any time, there may be a few hundred species undergoing splitting, and so in this intermediate state.

But regardless of how many species we think exist, there is no question life has become increasingly diverse. In 2016, Benton made the following assessment of how diversification took off in

Earth came into being 4.6 billion years ago, and life arrived one billion years later. That life would remain single-celled for three billion years, but around 900 to 700 million years ago, something changed. A few single-celled species discovered the added value of clustering and dividing labour, and so the first real animal gradually emerged.

That first animal was a sponge or a comb jelly; biologists have been discussing this for a decade now. Anyway, multi-cell life began under water. Benton’s graph shows a small explosion of species around 500 million years ago: the Cambrian explosion. And a little later, around 430 million years ago, the pioneering plant kingdom gradually began colonising the world above water.

It clearly shows that when plants invented – a unprecedented sexual organ – around 140 million years ago, diversification on land went wild . The variation in flying, pollinating insects exploded. With that, the land overtook the sea for good.

Life in biomass

Looking at the species distribution on Earth, animals, particularly insects, would seem the dominant life form. We would live on Planet Insect. But viewed in terms of biomass, it’s plants that have undisputed dominion. In fact, the presence of plants pretty much dwarfs all other forms of life.