The World Belongs to the Plants

Humans are, by consensus, the species dominating Earth (at least by their consensus). Inhabiting all continents, including Antarctica, we even reached space. Not only have we adapted to life in different and diverse habitats, we have also adapted those habitats to us, making them more convenient for us through agriculture, construction, and – of course – air conditioning. But by some measures, humans do not come in first. For example, a calculation of Earth's biomass, which is the mass of all living beings on our planet, shows that humans comprise only about one hundredth of a percent of this total mass. Actually, the mass of all animals accounts only for less than four percent – considerably less than that of bacteria, and far less than that of the true rulers of Earth: plants.

This is one of the conclusions arising from a new study conducted in Ron Milo’s lab at the Weizmann Institute’s Department of Plant and Environmental Sciences. Led by graduate student Yinon Bar-On, the study included a comprehensive survey to assess the mass of carbon in living creatures of all types and groups. Bar-On told Davidson Online that the study had actually started from a different direction – in an attempt to estimate the ubiquity of a certain protein in the world of living beings. To be able to do this, the researchers had to know the relative mass of different groups of organisms, but they found that this kind of data was not available. “To our surprise, we found no study that examines all of the groups together. We spent two years trying to assign numbers to the different groups,” said Bar-On.

The survey was based on compiling and incorporating hundreds of previous studies that approximated the biomass of different organisms using a variety of methods. Some examined the amount of fish in a certain region using nets or a sonar; others used satellite images showing the forest coverage in different regions; and yet others included research expeditions to forests to examine the density of plants there. 

This type of survey will assist scientists to better understand the structure of the biosphere – the world of living beings to which we belong – and how different groups affect one another. In addition, it can teach us about the changes induced by human intervention.

Changing nature

As mentioned earlier, Bar-On and his colleagues estimated the mass of carbon, not the actual mass of the different organisms. Carbon mass is a good approximation of the dry mass of most plants and animals, excluding any water they contain. They found that the total carbon mass of all organisms is about 550 gigatons (550 billion tons). The bulk of this mass, some 450 gigatons, comes from plants. The carbon from bacteria accounts for approximately 70 gigatons, while fungi make up about 12 gigatons. In contrast, the carbon in all animals has a mass of only two gigatons.

If only animals are taken into account, the effect of humans is even more prominent. The mass of human carbon may only be 0.06 gigatons, but this is still nearly ten times more than the mass of all wild mammals. And in contrast to the latter, livestock, mainly cows and pigs, have a combined mass that is double that of humans. Similarly, poultry, mainly chickens, have a mass that is three times that of wild birds. The combined mass of humans and farm animals far outweighs the mass of all wild mammals, birds, reptiles, and amphibians. “While our relative part of the biomass is very small, our impact on it is considerable,” said Bar-On.

Humans have affected not only mammalian and bird species in the world: our footprints can also be found on the total mass of plants. They still comprise 80 percent of the total biomass, but the study demonstrates that they have lost nearly half of their mass since the beginning of human civilization. What happened to all of the carbon that existed in those plants and disappeared is still not clear – did it go into the atmosphere, construction materials, or elsewhere, and Bar-On says that this is one of the future research directions they plan to pursue.

An upside-down pyramid under the sea

Another interesting finding arising from the study is that while most of Earth’s surface is covered with water, most of its biomass is concentrated on land. About 85 percent of the carbon mass is found on land, mostly in plants. Only a little over one percent is found in the oceans, and the rest, mostly coming from bacteria, is in the depths of the Earth.

While plants rule the land, most of the carbon mass in the sea comes from animals, mainly fish and crustaceans, and the mass of marine animals outweighs that of terrestrial animals. This means that most of the mass of autotrophs, organisms that perform photosynthesis, producing sugars from carbon dioxide while releasing oxygen, is found on land. The mass of marine autotrophs – algae, photosynthetic bacteria, and water plants – is very small compared to that of terrestrial plants. Thus, the sea demonstrates an upside-down ecological pyramid, where the mass of heterotrophs, those that eat the photosynthetic organisms or each other, is larger than that of the autotrophs.

How does such a small mass of autotrophs provide energy for the larger mass of heterotrophs? The answer most likely lies in the rapid turnover of the autotrophs. These mainly include microscopic bacteria and algae that reproduce rapidly. The animals that eat them feed on a low, but relatively constant, mass of autotrophs, while reproducing at a much lower rate. So, one generation of fish, for instance, feeds on many generations of algae.

The researchers hope that their study will provide a broad picture of the current state of the biomass, aiding scientists to look at specific studies in a wider context in the future. “Science attempts to understand the world. This work shows us what we know about the world in terms of biomass, and also what we still do not know,” said Bar-On.

Translated by Elee Shimshoni