Yes, the headline number is real, but it needs a little unpacking. A 2018 study in the Proceedings of the National Academy of Sciences (PNAS) estimated that the total weight of wild mammals on Earth has fallen by roughly a factor of six since humans began spreading across the planet. That is a loss of about 83 percent. The same census found that the plant kingdom now holds about half the mass it held before human civilisation, mostly because forests were cleared for farms, pasture and timber.
The striking part is the contrast. People make up only about 0.01 percent of all living matter, yet the animals we raise for food now outweigh almost every wild vertebrate group combined. Below is what the research actually measured, how the 83 percent figure is calculated, what newer work has added since 2018, and what the numbers do and do not mean.
What the 2018 biomass census measured
The study, "The biomass distribution on Earth", was written by Yinon Bar-On, Rob Phillips and Ron Milo. Milo leads a lab at the Weizmann Institute of Science in Israel, and Phillips is at Caltech. Instead of counting species or individuals, the team asked a simpler question: if you could put every living thing on a scale, how would the weight divide up?
To compare organisms as different as oak trees and soil bacteria, they used carbon as the common unit, measured in gigatonnes (billions of tonnes). Their total for all life came to about 550 gigatonnes of carbon. Each group was built up from hundreds of earlier surveys: forest inventories, ocean sampling, deep-subsurface drilling studies and livestock statistics.
The authors were open about the uncertainty. Plant biomass is known fairly well, within about 1.2-fold either way. Bacteria are much harder to pin down, with an uncertainty of around 10-fold, because so many live in deep sediments and rock where few samples exist.
The headline numbers, in plain terms
Here is how the living world breaks down by mass, according to the paper:
- Plants: about 450 gigatonnes of carbon, or roughly 80 percent of all biomass. Most of it is wood: trunks, stems and roots.
- Bacteria: about 70 gigatonnes, close to 13 percent. Most of it lives deep underground. (A later revision by the same lab cut the estimate for deep-subsurface microbes, so this share is the least certain figure in the table.)
- Fungi: about 12 gigatonnes. Archaea: about 7.
- All animals together: only about 2 gigatonnes, and more than half of that is arthropods such as insects, crustaceans and krill, followed by fish.
- Humans: about 0.06 gigatonnes, the "0.01 percent" that made the news.
Among mammals, the picture is lopsided. Livestock, mostly cattle and pigs, account for about 60 percent of mammal biomass. Humans make up about 36 percent. Every wild mammal on land and sea, from mice to blue whales, adds up to the remaining 4 percent or so. Birds show the same pattern: domestic poultry outweighs all wild birds by about three to one, so roughly 70 percent of bird biomass is farmed chickens and other poultry.
Chickens are now the most abundant bird on Earth by weight. Photo: ALP STUDIO on Unsplash.
Where the 83 percent figure comes from
The study compared today's numbers with estimates for the world before the Late Quaternary megafauna extinctions, which began roughly 50,000 years ago as modern humans reached new continents. Mammoths, giant ground sloths, cave bears and many other large animals vanished during that period. The paper notes that human activity contributed to those losses, alongside climate change at the end of the last ice age.
By the team's reckoning, the mass of wild land mammals fell about sevenfold, from roughly 0.02 gigatonnes of carbon to about 0.003. Marine mammals dropped about fivefold, which works out to around 80 percent, mainly because of industrial whaling in the nineteenth and twentieth centuries. Taken together, all wild mammals fell about sixfold. That is where "83 percent" comes from. It does not mean that 83 percent of mammal species are extinct. Most species still exist. They are just far scarcer and smaller in total weight than they once were.
The plant figure works the same way. Total plant biomass is estimated at about half of what it would be without people, a loss driven by thousands of years of land clearing for agriculture and grazing.
One common mix-up is worth correcting. Some early write-ups of this study said 36 percent of mammal biomass is pigs. The paper says 36 percent is humans. Pigs are counted within the 60 percent that is livestock.
"A cow next to a cow next to a cow"
Milo's own reaction captured the scale of the change. Speaking to reporters when the paper came out, he called it "definitely striking, our disproportionate place on Earth." He also offered an image that stuck. Children's puzzles show elephants, giraffes and rhinos, he said, but a realistic puzzle of today's world would be "a cow next to a cow next to a cow and then a chicken."
The joke works because the arithmetic is real. Humans and livestock together now outweigh all other vertebrates except fish. The biomass of a single farmed species, cattle, is far larger than that of every wild land mammal combined.
A red fox: wild land mammals of every size now make up only a few percent of mammal biomass. Photo: Katerina Bartosova on Unsplash.
What newer research has added
Milo's group has kept refining the census. In 2023 a PNAS paper led by Lior Greenspoon, "The global biomass of wild mammals", built species-by-species estimates for hundreds of wild mammals. It put the wet mass of all wild land mammals at about 20 million tonnes, roughly 3 kilograms for every person alive. Wild marine mammals came to about 40 million tonnes, more than half of it baleen whales.
Compare that with about 630 million tonnes of domesticated mammals and about 390 million tonnes of humans. The authors' summary was blunt: domesticated mammals now outweigh wild land mammals by about 30 to 1. A few wild species dominate what remains. White-tailed deer are the single largest contributor on land, followed by wild boar and the African savanna elephant. The world's pet dogs weigh about as much as all wild land mammals put together.
A separate 2020 study in Nature by Emily Elhacham and colleagues, also from Milo's lab, found that human-made materials such as concrete, asphalt, metal and plastic now outweigh all living biomass on the planet. By that measure, plastic alone outweighs all land and marine animals combined.
Why it matters, and the caveats
Weight is not the same as biodiversity. A forest of one tree species can weigh as much as a mixed rainforest. And a world with fewer large animals can still hold many species. Biomass shows something different: how much of the planet's living matter now runs through human systems. It tracks what ecologists call the footprint of agriculture.
The main drivers are well documented: conversion of forests and grasslands to farmland and pasture, overhunting and overfishing, and historical whaling. Climate change is increasingly added to that list. Researchers studying extinction rates, such as the teams behind the Living Planet Index and the IPBES global assessment, reach broadly similar conclusions using different methods.
The numbers carry real error bars. For wild mammals, the 2023 estimate has a confidence range of roughly 13 to 38 million tonnes. Still, no plausible correction changes the basic picture: today's mammal world is overwhelmingly people and the animals we keep.
What people can actually do
Because farmland is the single biggest pressure, the most direct levers involve land use and diet. Studies of food systems consistently find that beef and other ruminant meat need far more land per calorie than plant foods, so eating less of them, and wasting less food, frees up the most land. Supporting protected areas, wildlife corridors and well-run fisheries helps the animals that remain recover. Whale populations show it can work. Several species, such as humpbacks, have rebounded strongly since commercial whaling was banned in the 1980s.
No single choice fixes a planetary imbalance. But the census gives a clear, measurable baseline, and future surveys can show whether the share of wild life on Earth starts to grow again.
Sources: Bar-On, Phillips & Milo, "The biomass distribution on Earth", PNAS 2018 (PDF); Mongabay: biomass study finds people are wiping out wild mammals (2018); Greenspoon et al., "The global biomass of wild mammals", PNAS 2023; Wikipedia: Biomass (ecology); original report via Global Citizen. Photos: ALP STUDIO and Katerina Bartosova on Unsplash.
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