What wild mammal treads most heavily on the land? Not elephants, according to a new global estimate of the total masses of mammal species. Not wild mice, despite their numbers. The heavyweight champion is that furtive denizen of parks, meadows, and forests throughout the Americas, the white-tailed deer. It accounts for almost 10% of the total biomass of wild land mammals.
The study, which honed its numbers using artificial intelligence, “is the first that provides quite convincing values for mammals,” says Patrick Schultheiss, a behavioral ecologist at the University of Würzburg. Published this week in the Proceedings of the National Academy of Sciences, it concludes that wild land mammals alive now have a total biomass of 22 million tons, and marine mammals account for another 40 million tons.
Those numbers are relatively puny: Ants alone amount to 80 million tons, Schultheiss has estimated. But the comparison the team hopes will capture attention is with humans, who weigh in at 390 million tons, with their livestock and other hangers-on such as urban rats adding another 630 million tons. It is stark evidence of how the natural world is being overrun, researchers say. “I hope it will be a wake-up call to humanity that we should do all we can do to conserve wild mammals,” says lead author Ron Milo, a quantitative biologist at the Weizmann Institute of Science.
Milo says he and his colleagues have long believed numbers can “provide a sixth sense of the world.” In 2018, they grabbed headlines by estimating the global weight of all life; 2 years later, they added the global weight of all humanmade objects and infrastructure, from cars to buildings. They also made a rough estimate of 50 million tons for wild mammals—“a shockingly tiny fraction of the mass of life on Earth,” recalls Shahid Naeem, a biodiversity ecologist at Columbia University. Since then, the team has worked to sharpen its estimate of this fraction.
At Milo’s lab, Lior Greenspoon and Eyal Krieger were able to find detailed data on the global number, body weight, range, and other measures for 392 wild mammal species, enough to calculate their total biomass directly. To predict the total mass for less-studied mammals, they used the data for half of the 392 species to train a machine learning system. They tested and refined the model until it could accurately predict the biomass of the other half of these species. They then fed whatever data they could find—ranges, body sizes, abundance, diets—for each of about 4400 additional mammal species into the model to estimate their biomasses and abundance. The work “was a major undertaking,” says Renata Ivanek, a veterinarian and epidemiologist at Cornell University who has evaluated ways to estimate the biomass of livestock.
On land, much of the wild mammalian biomass is concentrated in a few large-bodied species, including boar, elephants, kangaroos, and several kinds of deer. The top 10 species account for 8.8 million tons—40% of the estimated global wild land mammal biomass, Milo’s team reported. Rodents—not counting human-associated rats and mice—make up 16% and carnivores account for 3% of that biomass. Among marine mammals, baleen whales account for more than half of the biomass. But in sheer numbers, bats rule the mammalian world: They constitute two-thirds of individual wild mammals, though only 7% of the total terrestrial mass.
In contrast, on the domesticated front, cows collectively weigh 420 million tons and dogs about as much as all wild land mammals, the new study reports. The biomass of housecats is about double that of African elephants and four times that of moose.
“There is much uncertainty around the estimated [biomasses],” Ivanek adds, “but it is a start.” The contrast with the masses of wild animals is having Milo’s desired effect. These results “changed my notion about the seemingly endless ubiquity of wildlife and provided insight into the extent to which our activity as humans has impacted our world,” says Ece Bulut, a Cornell food scientist who collaborates with Ivanek.
But so what? It is an “impressive effort to provide a snapshot of the state of the mammalian world,” Naeem says, yet he contends that this snapshot “won’t drive conservation nor transform our way of thinking about the issues.”
Not true, counters Sabine Nooten, an insect ecologist and Schultheiss’s collaborator at Würzburg. “We can only conserve what we understand, and we can only truly understand what we can quantify.”
Correction, 28 February, 10:25 a.m.: Several numbers in this story, such as previous biomass estimates for ants and wild animals and how many species were used in the machine learning training, have been corrected.
