New research highlights dramatic decline in global biomass of wild mammals

A new study by scientists at Israel’s Weizmann Institute of Science, published in Nature, has taken on an extraordinary challenge: to weigh all the mammals on Earth — not individually, of course, but in terms of biomass, the total mass of living matter.

By tracing this back to the year 1850, the researchers have shown just how profoundly humanity has reshaped the animal world.

Their results are both fascinating and sobering. In 1850, the total biomass of wild mammals — everything from whales to elephants to mice — was estimated to be roughly equal to that of humans and our domesticated animals combined. Since then, while humans’ weight on the planet has multiplied many times over, the collective mass of wild mammals has more than halved. Today, humans and our livestock outweigh all wild mammals by about 10 to one.

A new way of seeing life

When we talk about the loss of wildlife, it’s usually in terms of species extinctions. However, extinction statistics can hide the scale of ecological change. Losing a single rare species equates to losing millions of individuals from a common species. Biomass offers a more holistic picture: not just who’s alive, but how much life exists.

The researchers, led by Lior Greenspoon, Elad Noor and Ron Milo, used a vast range of historical data, models and population records to construct a timeline of mammal biomass from 1850 to the present — the first global attempt to do so at this level of detail.

What they found was a story of human expansion, industrialisation and unintended consequences for the rest of the animal kingdom.

Expanding footprint

In 1850, the human population was about 1.2 billion. Today, it’s more than eight billion. During that time, the researchers said, the average human body also became heavier — by roughly 30% — due to improved nutrition and changing lifestyles.

Combining these two trends, they estimated that the total biomass of humanity grew from around 50 million tonnes in 1850 to 420 million tonnes today — an eightfold increase.

And we haven’t grown alone. Our domesticated mammals — mainly cattle, pigs, buffalo, sheep and goats — have exploded in number. Cattle alone make up about two-thirds of all livestock biomass, and their combined weight has quadrupled since 1850.

When the authors added up all domesticated mammals, including pets and human-associated rodents like rats and mice, they found that their total mass rose fivefold. Altogether, humans and our domesticated mammals now account for around 1.1 billion tonnes of biomass — nearly three times the estimated total for all mammals in 1850.

For marine mammals — whales, seals and dolphins — the picture is particularly dramatic. Using models based on centuries of whaling records and population data, the team estimated that the biomass of wild marine mammals fell by about 70% since 1850.

The mid-1800s marked the dawn of industrial whaling, driven by inventions such as the exploding harpoon. The largest whales — blue, humpback, fin and sperm whales — once made up 80% of marine mammal biomass. By the mid-20th century, relentless hunting had driven many populations to the brink. The decline only began to slow after 1986, when the International Whaling Commission banned commercial whaling. Some populations have partially recovered, but many remain far smaller than before.

On land, the data are patchier, but the trend is unmistakable. Wild terrestrial mammal biomass has fallen by more than half, from about 50 million tonnes in 1850 to around 20 million tonnes today. To put that in perspective, the biomass of African elephants alone in 1850 was roughly equal to the entire biomass of all wild land mammals now living on Earth.

Even though some species, such as North America’s white-tailed deer, have rebounded thanks to conservation measures, these gains are minor compared to the losses in larger species, such as elephants, bison and big cats.

What biomass reveals

The scientists estimated that global mammal biomass in 1850 was about 400 million tonnes. Today, it’s around 1.1 billion tonnes, but almost all of that increase is due to humans and our domestic animals. Wild mammals now make up just a tiny fraction.

Biomass is closely tied to energy consumption. The more we weigh collectively, the more resources we require — food, water, land and energy. The researchers noted that the need for land and water to raise livestock exerts enormous pressure on natural habitats, squeezing out space and resources for wild animals. So, while there’s no direct “zero-sum” trade-off between farm animals and wildlife, in practice our expansion has come at the expense of wild animals.

Beyond the raw statistics lies a deeper meaning. The decline in wild mammal biomass — especially in the oceans — signals a loss not only of animals but of ecological functions. Whales, for instance, are key “ecosystem engineers”. Their movements help cycle nutrients through the ocean, fertilising plankton and boosting productivity. Fewer whales mean less nutrient mixing and a poorer marine environment.

The study also highlighted a problem scientists call “shifting baseline syndrome”. Each generation tends to view the environment in which they grew up as normal, forgetting how much has already been lost. Quantitative reconstructions like this one remind us that the natural world of 1850 was already diminished — and that what we see today is only a remnant of what once was.

A difficult calculation

Estimating the biomass of thousands of wild species, past and present, is no easy feat. The researchers acknowledged the uncertainties: the further back in time, the less precise the data.

For wild land mammals, they had historical population estimates for just 37 large species out of about 6,400 known today. For smaller animals, they assumed biomass stayed constant over time, meaning their overall estimates may have understated the real decline.

Still, the results paint a clear picture: an accelerating shift from a world dominated by wild creatures to one dominated by humans and the animals that serve us.

The authors see their work as a baseline, not a final verdict. Better global monitoring of wildlife populations, especially smaller mammals, is urgently needed. Understanding biomass over time helps us grasp not only how many animals there are but also the scale of their impact — and ours.

This study invites us to rethink our place in the web of life. The sheer growth of human and livestock biomass tells a story of success from our perspective — but also one of imbalance. As Milo and his colleagues point out, having hard numbers helps us see clearly what’s been lost and what might still be saved.

It’s a reminder that even without new extinctions, we will still preside over a vast thinning of life — one that changes the planet just as surely as climate change does. The weight of the world, it turns out, has never been more human. DM