Life on the move

Wolves roaming the Mongolian steppes cover more than 7,000 kilometers a year. The Arctic tern flies from pole to pole in its annual migration. Compared to these long-distance travelers of land, sea and sky, humans might seem like the ultimate couch potatoes. But a new study from the Weizmann Institute of Science shows otherwise. In a scientific paper published today in Nature Ecology & Evolution, the researchers report that the total movement of humans is 40 times greater than that of all wild land mammals, birds and arthropods combined. Since the Industrial Revolution some 170 years ago, human movement has skyrocketed, while animal movement in nature has dwindled to levels that could endanger ecosystems.

Life, by its very nature, is always in motion, shaping both the natural world and human society. Yet until now, no comparison has been made among different species as to the magnitude of their overall movement. Birds, for example, cross vast distances but their overall body mass is tiny. By contrast, deep-sea fish may travel only short distances, but their combined biomass is enormous – about a thousand times greater than that of all birds. Researchers in the laboratory of Prof. Ron Milo of the Weizmann Institute’s Plant and Environmental Sciences Department have now quantified and compared the movement of different species, shedding light on the power dynamics between humans and the rest of the animal kingdom.

The index they created, called the biomass movement metric, is calculated by multiplying the total biomass of a species – that is, the combined mass of all its members – by the total distance that species travels in a year. Calculating this metric globally made it possible, for the first time, to quantify global movement across animal species – and then compare it to that of humankind.

The researchers broke down human movement by mode of transportation. They found that about 65 percent of human biomass movement is by car or motorbike, 10 percent by airplane, 5 percent by train and 20 percent by foot or on a bicycle. Remarkably, the biomass movement of humans walking is six times greater than that of all wild land mammals, birds and arthropods combined. On average, each person travels about 30 kilometers a day by various means – slightly more than wild birds. By comparison, wild land mammals (excluding bats) travel only about 4 kilometers a day. In the air, human biomass movement by airplane is ten times greater than that of all flying wildlife.

“We often marvel at the power of nature compared to how small we are,” says Milo. “But in practice, even the great migrations we see in Africa in nature documentaries – some of the largest land migrations on Earth – barely compare to the human biomass movement associated with people gathering from around the world for a single World Cup tournament.

“Animals spend a large portion of their energy on movement, and measuring their biomass movement enabled us to compare the energetic cost of transport across species. We found, for example, that a single airline burns as much energy as all wild birds combined. It may be hard to grasp just how much humankind affects nature, but the biomass movement metric does provide a quantitative measure that reveals the true balance of power on Earth.”

This power balance is dramatically changing. Humanity continues to develop and expand while nature is in decline. Most biomass movement occurs in the oceans, but even those vast habitats are severely affected by human activity.

“Since the Industrial Revolution, human biomass movement has soared by 4,000 percent, while that of marine animals has dropped by about 60 percent,” explains Dr. Yuval Rosenberg, who led the study in Milo’s lab. “Today we know that animal movement is critical for ecosystems to function properly and that ecosystems must remain connected to one another to survive. The global decline in animal movement is a warning sign to us all.”

Also participating in the study were Dr. Dominik Wiedenhofer and Dr. Doris Virág from BOKU University, Vienna; Gabriel Bar-Sella, Lior Greenspoon and Barr Herrnstadt from Weizmann’s Plant and Environmental Sciences Department; Dr. Lewis Akenji from Hot or Cool Institute, Berlin; and Prof. Rob Phillips from the California Institute of Technology, Pasadena.

170 years on the scale

Animal species rarely vanish overnight; instead, their populations usually decline gradually. But long before extinction, a species’ numbers may drop below the threshold needed to fulfill its ecological role, triggering profound shifts in nature

In an accompanying paper published today in Nature Communications, researchers from Milo’s lab have, for the first time, calculated the total biomass of all mammalian species on Earth since 1850. They found that over this period, the combined biomass of wild land and marine mammals has plummeted by about 70 percent – from about 200 million tonnes to just 60 million tonnes. In contrast, the biomass of humans has soared by roughly 700 percent, and that of domesticated animals by 400 percent, together now reaching about 1.1 billion tonnes. The study was led by Lior Greenspoon from Milo’s group in the Plant and Environmental Sciences Department.