‘A wake-up call’: total weight of wild mammals less than 10% of humanity’s

Mar. 18, 2023

The total weight of Earth’s wild land mammals – from elephants to bisons and from deer to tigers – is now less than 10% of the combined tonnage of men, women and children living on the planet.

A study by scientists at Israel’s Weizmann Institute of Science, published this month, concludes that wild land mammals alive today have a total mass of 22m tonnes. By comparison, humanity now weighs in at a total of around 390m tonnes.

At the same time, the species we have domesticated, such as sheep and cattle, in addition to other hangers-on such as urban rodents, add a further 630m tonnes to the total mass of creatures that are now competing with wild mammals for Earth’s resources. The biomass of pigs alone is nearly double that of all wild land mammals.

The figures demonstrate starkly that humanity’s transformation of the planet’s wildernesses and natural habitats into a vast global plantation is now well under way – with devastating consequences for its wild creatures. As the study authors emphasise, the idea that Earth is a planet that still possesses great plains and jungles that are teeming with wild animals is now seriously out of kilter with reality. The natural world and its wild animals are vanishing as humanity’s population of almost eight billion individuals continues to grow.

“When you look at wildlife documentaries on television – for instance of wildebeest migrating – it is easy to conclude that wild mammals are doing quite well,” lead author Ron Milo told the Observer.

“But that intuition is wrong. These creatures are not doing well at all. Their total mass is around 22m tonnes which is less than 10% of humanity’s combined weight and amounts to only about 6lb of wild land mammal per person. And when you add all our cattle, sheep and other livestock, that adds another 630m tonnes. That is 30 times the total for wild animals. It is staggering. This is a wake-up call to humanity.”

The study, The Global Biomass of Wild Mammals, also reveals that those that do best – such as the white-tailed deer in the US and wild boars – are those that find it easier to adapt to the presence of humans. Both species can be found near settlements and are occasionally treated as pets. “Even within the wild, the fingerprints of humanity are obvious,” added Milo, whose team’s study is published in the US journal Proceedings of the National Academy of Sciences.

As part of the paper, researchers Lior Greenspoon and Eyal Krieger collected biomass data on about half of all known mammals and used machine-learning computational models on other zoological samples to calculate the other half.

The grim figures for land mammals were matched by those found in the oceans. The total mass of marine mammals was calculated to be around 40m tonnes. Fin whales have the largest total biomass with sperm whales and humpbacks coming into the second and third slots, respectively.

Domesticated-to-wild mass ratios emphasise the active role humans play in shaping the abundance of mammals on Earth - Study

Common pet species were also found to be major contributors to humanity’s planetary impact. Domestic dogs have a total mass of around 20m tonnes, a figure close to the combined biomass of all wild terrestrial mammals, while cats have a total biomass of around 2m tonnes, almost double that of the African savanna elephant. “These domesticated-to-wild mass ratios emphasise the active role humans play in shaping the abundance of mammals on Earth,” the researchers state in their paper.

Biomass studies are not the only way to quantify the animal world. Numbers of species are also revealing. As an example, it has been found there are 1,200 species of bats that account for a fifth of all land mammal species and two-thirds of all individual wild mammals by head count. However, they make up only 10% of the biomass of wild land mammals.

“Biomass is complementary to species richness and other diversity metrics, and can serve as an indicator of wild mammals’ abundance and ecological footprint on a global scale,” the researchers state.

Estimates made two years ago by the team suggested there were about 50m tonnes of wild mammals on Earth. The new figure, calculated using a host of techniques including AI, indicates that the crisis facing the planet’s wildlife appears to be much worse than first appreciated. Just how quickly the depletion of wild mammals is proceeding now needs to be assessed as a matter of urgency, they say, and is the focus of the study’s next phase which will assess how much of the biomass loss occurred over the past 100 years.

[Top photo: A savanna elephant in Pilanesberg National Park, South Africa. The total biomass of savanna elephants was estimated in the study to be half that of the 2m tonnes cats collectively weigh. Photograph: Arterra Picture Library/Alamy]