A new study measuring the size of life on planet Earth has found an overwhelming pattern — most creatures grow to be either very big or very small.
The first-of-its-kind study, led by researcher Eden Tekwa at the University of British Columbia and published in the journal PLOS ONE, offers a window into how evolutionary forces have driven the living world to look the way it is.
“I imagined myself as an alien interstellar explorer. I get to Earth, and the first thing I would do is scan how much life is of different sizes,” said Tekwa.
“You would expect that to be a relatively easy thing to do, given the kind of technological advances we have. But actually, we haven't done that for our own planet.”
Without a hi-tech space ship, the team of researchers from UBC and Rutgers University turned to what’s considered the most comprehensive database of life’s biomass on Earth, published five years ago by Israeli researcher Yinon Moise Bar-On after drawing on hundreds of other research papers. They supplemented that with data from citizen naturalists and a vast range of research papers.
The study found organisms in the the small and large range outweigh intermediate-sized life by a factor of at least 10.
Micro-life dominates at sea, mega-flora on land
Terrestrial life accounted for the largest share of living mass, with trees and grass outweighing everything else. The total mass of Homo sapiens — part of the large category of life — was found to be equivalent to roughly half the world’s livestock.
Just the planet’s soil fungi was found to weigh 214 times more than the planet’s eight billion humans, whereas humans outweighed wild land mammals by a factor of 18.
“Things like reptiles and mammals comprise very little after total biomass,” said Tekwa.
In the ocean, the species with the most biomass were mangrove forests, an aquatic tree that provides refuge and habitat for countless other lifeforms.
Past studies focusing on predator-prey relationship — where big things eat smaller things — have often failed to count such species.
“Things like coral reefs, mangroves, the grass and kelps are almost always excluded from these empirical and theoretical analyses,” said the researcher, now at McGill University. “They just don't fit into that ‘big things-small things’ paradigm.”
“It turns out, they make up almost half of the oceans' biomass.”
Microscopic organisms were found to dominate the biomass in marine ecosystems. But on land, big, woody plants reigned supreme.
Tekwa said there are no theories explaining how life evolved to dominate larger and smaller size extremes. What is certain: medium-sized life — that occupies masses ranging from an amoeba to a frog — is much less common.
An evolutionary story of climate and hunting
Tekwa said it’s not totally clear how climate affects the size of species as they evolve. When dinosaurs dominated the planet more than 200 million years ago, temperatures were higher and there was more oxygen in the atmosphere. On the other hand, the fossil record has been largely built on finding creatures with skeletons or shells, which could warp our understanding of what life was life, said the researcher.
For the past 100,000 years, the planet's climate has shifted as it has slipped in and out of a number of ice ages. Meanwhile, explosions of human populations has led to the downfall of large mega-fauna in some places, past studies have found.
Today, after more than a century burning fossil fuels and carrying out industrial-scale fishing, humanity is once again running an unintended natural experiment that could impact the survival of some of the biggest and smallest species on the planet.
In the face of that risk, Tekwa says their study could help humanity target conservation species that are important sources of food and certain parts of the living world whose health is critical in adapting to a changing climate.
“How much there is of life at different sizes determines how fast and how much the biosphere provides what we need to survive,” said Tekwa. “Size determines how long carbon gets stored and released into the atmosphere.”
“And we know when carbon is released into the atmosphere, it increases global warming.”
Big species — from Sequoia trees and blue whales to red mangroves and stony coral — store more carbon for longer.
The researchers also found a surprising symmetry in how life exists on the planet.
In one example, they found all the world's coral has roughly the same biomass as all of the world's fish.
“It illustrates in some way the balance of life,” Tekwa said.