It seems like an impossible question to answer: if you put all the forms of life into groups (sea creatures, bacteria, animals, etc.) and plopped them on a scale, which group would weigh the most? I was shocked to find that scientists have figured out a pretty reliable way to give us an answer - and the results are not at all what I expected!
Let's start with how scientists tackle finding the mass of every living thing on Earth. First, they settled on gigatons of carbon (GtC) as a measurement unit. Carbon can be found floating in the air, mixed with ocean water, or locked away in rocks. It's also stored in anything that's alive!
Scientists will survey a specific type of biome, like a deciduous forest, and calculate the carbon stored in each population. They get a decent count of the living things found in that area and apply that number to every similar biome on the planet! 🌎 It's a rough estimate, for sure, but it reveals some jaw-dropping patterns.
So is the planet straining under the weight of people? Ocean creatures? Bacteria? Nope.
The heavyweight that contributes the most mass in gigatons of carbon is land plants - by a LOT. 🌳 The human population has a biomass of 0.06 Gt C. Plants have a total biomass of 450 Gt C. That's 7,500 times MORE than the mass of every person alive! 😱
Now, things got weird for me when I dug into the biomass data. There are some strange rankings in the smallest-to-largest biomass lineup!
First, let's look deeper at the big picture. At the kingdom level, the world's bacteria biomass is around 70 Gt C. Can you imagine the size of the bacteria population it would take to make up that much biomass??
Viruses, though, have the tiniest biomass - just 0.2 Gt C. There are an estimated 10 nonillion (10 with 30 zeroes) viruses on Earth! According to the National Human Genome Research Institute, if you could place all those viruses end to end, you could make a line to the next galaxy--and back!--40 times. 😱 Can you imagine how lightweight those viruses must be to have so little biomass?
Diving into the 2 Gt C of the animal kingdom, leggy arthropods (terrestrial + marine) weigh in at 1.2 Gt C - by far the heaviest group.
That makes sense to me, but do you know I didn't expect? Humans are outweighed by cnidarians! Those are creatures like jellyfish, sea anemones, and corals. I had no idea they would outweigh all of humanity.
I feel like students could really get engrossed finding relationships in this data. If I were going to present these visuals to my class, here are a couple questions I might ask students to really get them thinking.
💡 Terrestrial (land) plants contribute much more to the world's biomass than marine plants. What resources are more easily available on land compared to water? This one gets learners thinking about the needs of living things that use photosynthesis. Gas dissolved in the atmosphere, soil, and access to sun (the ocean gets dark pretty quickly as you go down) would all be good answers.
💡 Check out the biomass of wild birds. What do we know about their biology that might contribute to their ranking? This one gets learners thinking about the adaptations and skeletal systems, like how the pneumatic bones of birds have adapted to have so much space for air.
💡 Scientists predict that the biomass of the animal kingdom looked a lot different when the human population was smaller. What do you think those differences are? This helps students look at relationships between humans and other members of the biosphere. For example, it's likely that the livestock biomass would be smaller but wild mammal and plant biomass would have been greater.
- Chris
Link to National Human Genome Research Institute: https://www.genome.gov/about-genomics/fact-sheets/Genomics-and-Virology
Link to graph data source: https://www.pnas.org/doi/10.1073/pnas.1711842115
