Using observational data and complex statistical analysis, a team of scientists calculated the number of mammals in the world and, more importantly, the total mass of mammals. Among other things, this work shows that almost half of the total biomass of wild mammals can be attributed to even-toed ungulates. This group of animals includes the white-tailed deer, which is widespread in North America, and the wild boar, whose natural range extends from western Europe to eastern Asia and which benefited from the displacement of predators such as wolves.
Ron Milo of the Weizmann Institute of Science in Rehovot, Israel, and his team collected global population data for 392 species of land mammals and recently published a analysis of this information in the journal Proceedings of the National Academy of Sciences of the United States. These 392 species for which data are available represent approximately 6% of all wild terrestrial mammal species. Using a machine learning model, the researchers then estimated the size of global populations of mammalian species that do not have precisely known distributions. The team found that the wild land mammals weigh in total around 22 million metric tons. Collectively, wild marine mammals weigh twice as much as wild land mammals. The lion’s share of this category is made up of baleen whales such as the fin whale, which has a biomass of eight million metric tons, or about 60% of aquatic mammals. The sperm whale, a type of toothed whale, also contributes a significant seven million metric tons.
Researchers have found that the number of individual animals that make up a particular species provides no indication of the overall mass of that species. Bats have the highest estimated head count with nearly 56 billion animals. Rodents, Milo’s team estimates, total 25 billion. At 0.5 million metric tons for bats and 1.1 million metric tons for rodents, however, the biomass of these mammals contributes next to nothing to the total mass count, the researchers found.
The team writes that biomass can serve as an indicator of the abundance and ecological footprint of wild mammals on a global scale. Collecting global data also helps identify trends that can answer researchers’ pressing questions: Are population sizes and species diversity changing? And if so, how much do they change?
The research team also compared their estimates with the total number and mass of all mammals on Earth. Of this total mass, wild mammals represent only 6%. People, livestock, and pets dominate the overall picture in every way. Homo sapiens contributes a total weight of 390 million metric tons, which is slightly less than the weight of domestic livestock at 420 million metric tons. Dogs rank fifth at 21 million metric tons, but still make up about as much mass as all wild land mammals combined.
When people observe the natural world, write Milo and his team, they tend to be guided by intuitive ideas – that the world is huge and its natural diversity exceeds anything that humanity has created. With these estimates, the researchers hope to correct this erroneous image.
This article originally appeared in Spektrum der Wissenschaft and has been reproduced with permission.