If you were writing to an alien pen pal in a distant galaxy, how would you describe life on Earth? Would you describe yourself? Your dog? Maybe birds or fish or snakes? The planet is teeming with diversity, but what would be most representative?
According to a new study, you’d be better off describing something green.
Most of life doesn’t look like me or you — or even Fido. A recent census of all living things found that plants are most common, at least by mass. They weigh in at about 83 percent of the total. So how much do other life forms contribute? And where do humans fit in? The answer is a bit complex. It also depends on how you measure.
Over the years, scientists around the world have conducted surveys of individual species and ecosystems. However, no one had surveyed all of Earth’s life at once, notes Ron Milo. He’s a biologist at the Weizmann Institute of Science in Rehovot, Israel. Milo and his graduate student Yinon Bar-On decided to tackle that first all-inclusive tally.
It would be impossible to count each tree, mouse and bacterium. Instead, the researchers used math and statistics to make an educated guess about how much life is out there. They started with data from many individual studies in different regions of the world. Then they considered how similar other regions of the world were — places that hadn’t been surveyed in detail. Based on such data, Milo and Bar-On estimated the mass of all life on Earth.
Other researchers have looked at smaller pieces of this puzzle. They’ve also studied how humans fit in. It turns out that humans are definitely in the minority, at least in terms of weight. Yet they are hardly unimportant. Particularly in recent centuries, people have played a major role in changing the balance of other species. And they’ve done it on a planetary scale.
By the numbers
How do you tally all the living things on Earth? You could try to count individuals. Or you could count the number of species that exist. But Milo and Bar-On chose to focus on Earth’s total living biomass.
Biomass is essentially the weight of organisms. Because carbon is a key ingredient in the molecules that make up living things, biomass often is measured in terms of that carbon. When you’re talking about a lot of organisms at once, their biomass may add up to gigatons of carbon. One gigaton is equivalent to the weight of about 73.5 millionloaded school buses.
Thinking about biomass can be especially useful when comparing species of different sizes. After all, one tree might have a bigger impact on the local environment than 1,000 bugs living in its branches.
The new study estimates that the total mass of life on Earth is around 550 gigatons of carbon. Milo and Bar-On described how they came to this total last June 19 in the Proceedings of the National Academy of Sciences.
Looking at previous studies by other researchers, Milo and Bar-On identified certain patterns. There were mathematical relationships between the biomass in an ecosystem and such factors as temperature, moisture and climate. Using those patterns, the researchers could estimate biomass in places that no one had studied.
They found that mammals make up only 0.167 gigaton of the total. That’s roughly one-twelfth the mass of all animals. Livestock — mostly cows and pigs — are the majority of that. Wild mammals, from elephants and foxes to mice and deer, only make up about 4 percent of the total biomass of mammals.
Farming affected the balance of life on Earth in other ways, too. Chickens raised for meat and eggs weigh in at about 0.005 gigaton. That’s about three times the biomass of all wild birds.
As for humans, there are some 7.6 billion of us on Earth. Together, we weigh in at an estimated 0.06 gigaton of carbon. That may sound like a lot. Yet it’s roughly equal to the mass of Earth’s termites. If you add up all the arthropods — insects, spiders, crustaceans and other things with hard outer shells — they make up about 60 percent of the animal kingdom’s biomass. That’s 1.2 gigatons and a whole lot of legs!
Nearly three-quarters of the globe is covered by water. Yet 86 percent of life prefers living on land, the new research found.
For species that don’t like to live above the surface, there’s plenty of real estate below. The scientists found that there’s almost 12 times more biomass deep below ground than there is in the ocean. Most of that is microbes.
Underground aquifers — buried bodies of water — are home to many of these large subsurface communities of microbes. Together, these underground ecosystems are often called the deep biosphere (or living zone).
“The deep biosphere is a substantial part of life on Earth,” at least by mass, says Sean McMahon. He works at the University of Edinburgh in Scotland. As a geobiologist, he studies how living things and their environment interact. McMahon focuses on the deep biosphere.
“It’s a frontier of exploration we know very little about,” he says. And, he adds, it “is constantly turning up surprises.” Those include new types of microbes and new ways that microbes interact with minerals. With so little oxygen and so few nutrients down deep, life there tends to grow slowly. But that also means individuals can live a very long time.
(Actually, the very oldest microbes on Earth probably live in the deep biosphere beneath the ocean floor. Some may be thousands or even millions of years old, estimates Hans Roy of Aarhus University in Denmark.)
A bacterial cell is only about a tenth the size of most animal cells. Yet even though microbes are too small to see, they still have mass. And given the Earth’s enormous populations of microbes, that mass adds up. The biomass of bacteria is greater than all birds and mammals combined. That’s also true of each of the other microscopic kingdoms of life: archaea (Ar-KEE-uh) and protists. It’s even true of viruses.
“Dinosaurs never ruled the Earth — it was bacteria all along,” says McMahon.
Naturally, such small organisms are hard to count. Most surveys only estimate their numbers. To calculate their biomass, researchers multiply the number of cells by the likely weight of each cell (based on its size).
But scientists’ ideas of an average cell size have been changing over the years, notes Barny Whitman. That makes things complicated. Whitman is a microbiologist at the University of Georgia, in Athens.
“In the late ‘90s, most estimates of cell size were about four or five times larger than they are now,” he says. “Since then, people have discovered large numbers of very small cells.” So biologists have had to adjust their biomass tallies to account for smaller bacteria.
This kind of uncertainty is an important part of any study. Science is rarely certain. Instead, researchers try to express how well they know something, or how much bigger or smaller a number might be. This uncertainty often reflects how hard it is to measure something. For example, scientists don’t know exactly how big microbes are, and many of them are hidden underground. Plants are easier to find and measure, so their biomass has less uncertainty.
Ultimately, Whitman argues, the overall results about Earth’s biomass are more important than the details. Even if scientists can’t be completely certain how many single-celled organisms there are, they still know bacteria vastly outweigh any animal.
Graphic: L. Steenblik Hwang; Data: Y.M. Bar-On et al/PNAS 2018
Modern humans have only been around for some 300,000 years. That’s just a flash in geological history. Still, we’ve left our mark on the planet. We’ve changed Earth’s terrain and its chemistry. We’ve also affected the diversity, abundance and biomass of many other species.
For example, between 15,000 and 10,000 years ago, half of Earth’s large mammal species died out. (“Large” in this case means mammals weighing more than 45 kilograms, or 100 pounds.) Human activities, such as hunting or destroying habitats, seem to have largely driven that extinction. That’s what a 2014 study concluded, at least. The study was done by Christopher Sandom and his colleagues at Aarhus University in Denmark.
Milo and Bar-On now estimate that over humanity’s history, 90 percent of animal biomass has disappeared.
The plant kingdom has taken a similar hit. Its mass has declined by half since humans began roaming the planet. And the crops people are farming only make up 2 percent of greenery today. In other words, we haven’t fully replaced the plants that we cleared away to make room for farms and expanding cities.
With climate change, the total amount of plant life on Earth will continue to decrease, according to research by Tom Crowther. He’s an ecologist at ETH Zurich, a university in Switzerland. He has been studying how our planet’s plant life will change in the future.
Today, 43 percent of Earth’s trees grow in the tropics. That’s the warm region close to the equator. Its growing season lasts all year and largely explains why plants can grow more densely there than at higher latitudes. (Latitude measures how far a place is from the equator. Sites with high latitudes are nearer to the poles, like Siberia or northern Alaska.)
But human-caused climate change has been warming the high latitudes faster than the rest of the planet. That’s especially true in the Arctic. As a result, more plants can move in and grow there, Crowther says. So the world’s total amount of forested land will increase.
However, climate change and the cutting down of greenery at tropical sites have made some of those sites drier. “The trees don’t like that,” Crowther says. So even if more land is warm enough to support trees, he predicts that the total biomass of trees will fall. How much? Stay tuned. Crowther is working on some calculations right now. In any case, plants will likely still dominate life on Earth. But the living planet could look very different.
By 2050, Earth may host 9.7 billion people. Our human biomass by then still won’t outweigh bacteria. But the balance of animals and plants may continue to shift so that we’ll have even more livestock and crops and even less wildlife. This means Earth’s biodiversity — the variety of life — will shrink. And that can be bad for the health of habitats. If trends continue, there will be fewer life forms for you to write your pen pal about.
Biomass studies such as these are one way to project what the future may look like, and how quickly major changes in species or ecosystems may occur. They also help us understand the role people are playing in these changes. That could help us learn what organisms (big or small) most need protecting.
alien (in astronomy) Life on or from a distant world.
aquifer Rock that can contain or transmit groundwater.
archaea (singular: archaeon) One of the three domains of life on Earth. This group consists of single-celled prokaryotes — organisms without a cell nucleus. Archaea are best known for living in extremely harsh environments, such as very salty water or highly acidic or hot places.
Arctic A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.
arthropod Any of numerous invertebrate animals of the phylum Arthropoda, including the insects, crustaceans, arachnids and myriapods, that are characterized by an exoskeleton made of a hard material called chitin and a segmented body to which jointed appendages are attached in pairs.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.
bacterial Having to do with bacteria, single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.
biodiversity (short for biological diversity) The number and variety of species found within a localized geographic region.
biology The study of living things. The scientists who study them are known as biologists.
biomass Matter that contains carbon and can be used as a fuel, especially in a power station for the generation of electricity. Plants are a kind of biomass.
biosphere The sites on Earth's surface, in its atmosphere and in its waters that host living organisms. Or it can be equivalent sites on other planets.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
census An official count or survey of a population.
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
colleague Someone who works with another; a co-worker or team member.
crop (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers.
crustaceans Hard-shelled water-dwelling animals including lobsters, crabs and shrimp.
diversity A broad spectrum of similar items, ideas or people. In a social context, it may refer to a diversity of experiences and cultural backgrounds. (in biology) A range of different life forms.
ecology (adj. ecological) A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
exoskeleton A hard, protective outer body covering of many animals that lack a true skeleton, such as an insect, crustacean or mollusk. The exoskeletons of insects and crustaceans are largely made of chitin.
extinction The permanent loss of a species, family or larger group of organisms.
factor Something that plays a role in a particular condition or event; a contributor.
federal Of or related to a country’s national government (not to any state or local government within that nation). For instance, the National Science Foundation and National Institutes of Health are both agencies of the U.S. federal government.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.
geologic or geological An adjective that refers to things that are related to Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.
habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.
host (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
latitude The distance from the equator measured in degrees (up to 90). Low latitudes are closer to the equator; high latitudes are closer to the poles.
livestock Animals raised for meat or dairy products, including cattle, sheep, goats, pigs, chickens and geese.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
microbiology The study of microorganisms, principally bacteria, fungi and viruses. Scientists who study microbes and the infections they can cause or ways that they can interact with their environment are known as microbiologists.
microscopic An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.
moisture Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
oxygen A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
poles (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator; the upper and lower ends of the virtual axis around which a celestial object rotates.
population (in biology) A group of individuals from the same species that lives in the same area.
Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.
protist A broad group of mostly single-celled organisms that are neither plants nor animals. Some, like algae, may appear plant-like. Those known as protozoans may appear animal-like. And still others appear fungi-like.
resident Some member of a community of organisms that lives in a particular place. (Antonym: visitor)
Siberia A region in northern Asia, almost all of which falls within Russia. This land takes its name from the language of the Tatar people, where Siber means sleeping land. This region is vast. It has become famous for its long, harsh winters, where temperatures can fall to −68° Celsius (−90° Fahrenheit).
species A group of similar organisms capable of producing offspring that can survive and reproduce.
statistics The practice or science of collecting and analyzing numerical data in large quantities and interpreting their meaning. Much of this work involves reducing errors that might be attributable to random variation. A professional who works in this field is called a statistician.
survey To view, examine, measure or evaluate something, often land or broad aspects of a landscape. (with people) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
termite An ant-like insect that lives in colonies, building nests underground, in trees or in human structures (like houses and apartment buildings). Most feed on wood.
terrain The land in a particular area and whatever covers it. The term might refer to anything from a smooth, flat and dry landscape to a mountainous region covered with boulders, bogs and forest cover.
tropics The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.