A planetary census puts humans in their place

BILLIONS of years ago a star began to die. In the process, it created something new: 65,500 billion tonnes of carbon that would later be incorporated into the nascent planet Earth. That carbon is still there, and nowadays a fair chunk of it makes up the bodies of living beings. A new study, published this week by Yinon Bar-On and others from the Weizmann Institute of Science, in Israel, provides a comprehensive estimate of how the Earth’s carbon stock is distributed among its inhabitants.

By estimating the amount of carbon stored in organisms, otherwise known as biomass, the scientists were able to compare the relative abundance of different kinds of Earth’s life, weighing both the microbes beneath the soil and the giraffes walking above it on the same scale. The mammals known as human beings like to imagine themselves the lords of the planet. But in terms of raw biomass, the results—published in Proceedings of the National Academy of Sciences—tell a different story.

No animal comes remotely close to the domination of plants, which account for 80% of the planet’s biomass (see chart). That makes sense: plants convert sunlight into food, and thus lie at the base of almost every food chain. Land plants account for the majority of that total, despite the fact that water covers almost three-quarters of the planet’s surface. Bacteria take second place, with approximately 13%. The remainder is distributed among fungi, archaea, protists, animals and viruses, in that order. Even within the animal count itself, there is little for humans to boast of. There is about as much biomass in one species of Antarctic krill, tiny shrimp-like crustaceans eaten by blue whales, as there is in all 7.6 billion human beings.

But size is not everything. Humans have had a profound impact on the prevalence of other species. Dr Bar-On’s research indicates that over the short span of human history on Earth (specifically after a large period of extinction that began 50,000 years ago) the biomass of wild mammals has decreased to a sixth of its previous value. Meanwhile, the carbon count of domesticated poultry grew to three times higher than that of every species of wild bird combined. Humans and their livestock have come to outweigh all other vertebrates on the planet with the exception of fish. That is not to say fish were spared. The biomass of fish is thought to have decreased by around 100m tonnes during humanity’s tenure. And the dominance of plants, although it is still overwhelming, was far greater before the start of human civilisation. Dr Bar-On suggests that the total biomass of plants has fallen to just half its previous level.

Of course, these numbers are estimates. Dr Bar-On and his team could not individually count each organism they reported. They relied on collating information from hundreds of other studies, public data when they were available, and their own analysis of the likelihood of a certain thing being in a certain place. They were able to be a lot more confident about visible organisms in well-explored ecosystems than they were about microscopic ones in the Earth’s deep subsurface or the ocean’s deep water, such as bacteria.

Future research may therefore change these numbers, possibly dramatically. But Dr Bar-On’s portrait of the planet is an impressive achievement—and a welcome dose of perspective.