A diverse range of mammals once roamed the planet. This changed dramatically with the arrival of humans, who have become the dominant species through our own populations, as well as the animals we breed and raise for food.
There are various ways to compare the distribution or abundance of different types of mammals. One way is to compare them based on the number of individuals. In these terms, very small animals vastly outnumber larger animals, but this doesn't necessarily give us an idea of how much ecological and biological resources different animals use.
Another metric that ecologists often use is biomass -- the total weight of all animals of a given species. This not only takes into account the number of animals but also factors in their size. It gives more weight to larger animals at higher levels of the ecological "pyramid": these rely on well-functioning bases below them.
Let's then look at the breakdown of the global mammal kingdom in these terms. It's shown in the chart below. This data is sourced from the study by Lior Greenspoon and colleagues.
Each square represents one percent of the world's mammal biomass, including both land and marine animals. For context, that 1% is equal to around 11 million tonnes.
The dominance of humans is clear. We account for more than one-third of mammal biomass. Our biomass is more than seven times greater than all wild mammals combined.
Our livestock and pets, which are primarily cattle, account for 59%.
That leaves just 5% as wild mammals, which includes thousands of different species, from elephants and deer to lions and whales.
Beyond the totals for humans, livestock, and wild animals, there are a few striking comparisons that we found surprising. Farmed pigs weigh as much as all of the world's whales, orcas, sea otters, seals, and dolphins combined. All the dogs in the world, including pets and feral dogs, weigh as much as all wild mammals on land.
When we show people the chart above, one question often comes up: what about chickens? Of course, chickens are not mammals. But we can make a similar comparison between poultry and wild birds.
Like mammals, poultry livestock collectively weigh much more than all the world's wild birds. You can see this in the next chart.
The size of the difference between poultry and wild birds, though, is much less certain than it is for humans and livestock versus wild mammals. That's because estimates for the number of wild birds vary a lot.
Using any method, the overall direction of the result is the same: chickens and other livestock birds weigh more than their wild cousins.
How did humans come to dominate the mammal kingdom?
A huge decline in the number and size of wild mammals has played a major role. Estimates suggest that the biomass of wild mammals has declined by roughly 85% over the last 100,000 years, and particularly since the migration of human populations across the planet.
But that's not the only reason. It's not that the abundance of wild mammals was replaced one by one by humans and livestock. In fact, human activity has dramatically increased the total amount of mammal biomass on the planet.
Around 100,000 years ago, the total biomass of land mammals summed up to approximately 120 million tonnes, essentially all of it in the form of wild animals. By 10,000 years ago, this had fallen to 90 million tonnes. But the most dramatic changes followed the advent of agriculture. Wild mammal populations and biomass continued to decline, while human and livestock populations gradually increased.
By 1850, the total mammal biomass on land -- including wild animals, humans, and livestock -- had increased to an estimated 250 million tonnes. Since then, this has continued to increase rapidly. Today, mammals weigh roughly 1100 million tonnes, which represents a quadrupling since 1850. Wild mammals declined, but this was more than offset by the huge rise in biomass of humans and farmed mammals.
In the sections below, we detail the methods and figures that the authors of the scientific papers used in producing the statistics shown in this article. We do this to help make the work and methodology more transparent and understandable.
Unless otherwise stated, the methods we're describing are those of the paper by Lior Greenspoon et al. (2023).