Yes, travel: humans are the largest migratory mammals on the planet, at least in terms of moving biomass.
The spectacular migrations of millions of wildebeest and zebras across the African savannah, or the endless journeys of Arctic terns from one pole to the other, pale in comparison to humanity’s global movements. According to a study conducted by Lior Greenspoon of the Weizmann Institute of Science in Israel, humans and their pets now represent the dominant share of the Earth’s biomass and are also the planet’s main “migrants” — at least if you measure the distance traveled in relation to body mass.
Humans dominate mammalian biomass. In recent centuries, the distribution of biomass on the planet has reversed. If once large wild mammals – bison, elephants, humpback whales – represented the largest part of the earth’s fauna, today humans and livestock bred for food constitute around 95% of the biomass of mammals, leaving wild animals with just a paltry 5%. A figure that also includes whales, once the protagonists of the most impressive migrations on Earth.
We travel more than any other species. Greenspoon and colleagues introduced a new metric to describe the impact of biological movement: “biomass movement,” expressed in ton-kilometers per year (t km/yr). The calculation – published in Nature ecology & evolution – combines the number of individuals of a species, their average mass and the distance traveled annually.
The result is astonishing: humans move about 4,000 billion ton-kilometers per year, a figure 40 times greater than that of all terrestrial wild animals combined. To give an idea, the collective movement of biomass generated by the great African migrations – millions of herbivores marching between Tanzania and Kenya – is roughly equivalent to that produced by a single human event such as the Hajj to Mecca or the football World Cup. Our bodies are smaller, but we travel much further thanks to trains, cars and especially intercontinental planes.
Comparisons with the animal kingdom. Even species known for their long migrations, such as Arctic terns – which make a journey almost from one pole to the other every year – generate a movement of total biomass (about 16 million ton-kilometers per year) lower than that of gray wolves (about 30 million).
But these numbers pale in comparison to daily human traffic: movements for work, tourism or migration far exceed any natural movement on land or in the sky. Even our foot traffic alone – people walking, climbing stairs, navigating cities and streets – equates to about six times the biomass displacement of all terrestrial wildlife, including insects.
IN the sea. In the oceans, the primacy of biomass movement belongs to fish, which with their collective movements reach around 30,000 billion ton-kilometers per year, more than seven times the human figure. The whales, despite performing epic migrations, move a biomass similar to that of the entire population of Germany. Zooplankton, despite being the largest reserve of biomass on the planet, moves very little: its total movement is approximately a quarter of that of humans.
A recent revolution. Until a few centuries ago, the picture was completely different. Around 1850, humans made up only one-seventh of today’s population, and most people never strayed more than a few miles from their place of birth. During the same period, wild land animals moved about twice as fast as they do today.
The whales fertilized large portions of the ocean with nutrients from their feces, encouraging the growth of phytoplankton – the main sink of atmospheric carbon. But intensive hunting and overfishing have reduced the movement of marine biomass by more than 70% since pre-industrial times. Even further back in time, in the late Pleistocene, before the disappearance of mammoths, megacerans and other giants of the megafauna, the biomass of terrestrial mammals was ten times higher than today. The loss of these animals has not only altered ecosystems, but also fundamentally changed the physical flow of living matter on Earth.
moving “steel” bodies. Curiously, the study only considers the biological mass of humans, ignoring the weight of the vehicles we use. If airplanes, ships, cars, and trains were included, the movement of matter related to our activities would far exceed any biological scale that has ever existed on the planet. As the authors note, today a single power plant produces as much energy as is consumed for locomotion by all wild terrestrial mammals combined.
A fact that perfectly summarizes our time: an era in which the most mobile species on Earth moves thanks to the energy of an increasingly immobile planet.
