Nature favours creatures in largest and smallest sizes

Life comes in all shapes and sizes, but some sizes are more popular than others
General Sherman is a giant sequoia tree located in Sequoia National Park. / Le « General Sherman » est un séquoia géant situé dans le Sequoia National Park.
The extreme sizes dominate life across the biosphere. Illustration by Sylvia Heredia. / Les tailles extrêmes occupent une place prépondérante dans la biosphère. Illustration de Sylvia Heredia
Published: 10 May 2023

Surveying the body sizes of Earth’s living organisms, researchers from McGill University and University of British Columbia found that the planet’s biomass – the material that makes up all living organisms – is concentrated in organisms at either end of the size spectrum.

The researchers spent five years compiling and analyzing data about the size and biomass of every type of living organism on the planet—from tiny one-celled organisms like soil archaea and bacteria to large organisms like blue whales and sequoia trees.

They found that the pattern favouring large and small organisms held across all types of species and was more pronounced in land-based organisms than in marine environments. Interestingly, maximum body size seemed to reach the same upper limits across multiple species and environments.

“Trees, grasses, underground fungi, mangroves, corals, fish, and marine mammals all have similar maximum body sizes. This might suggest that there is a universal upper size limit due to ecological, evolutionary, or biophysical limitations,” says lead author Eden Tekwa, a former postdoctoral fellow at University of British Columbia and now a research associate with McGill University’s department of biology.

"Life constantly amazes us, including the incredible range of sizes that it comes in,” says co-author Malin Pinsky, an associate professor in the department of ecology, evolution, and natural resources at Rutgers University. “If the tiniest microbe was the size of the period at the end of this sentence, the largest living organism, a sequoia tree, would be the size of the Panama Canal."

“As for humans, we already know we comprise a relatively small biomass, but our size among all living things reveals our place in the global biome. We belong to the size range that comprises the highest biomass, which is a relatively large body size,” says Tekwa.

Predicting the effects of climate change

Cataloguing which body sizes are most common is a key step towards understanding the world around us, say the authors. These results also have important implications for predicting the impacts of climate change and human activity on the planet’s biomass.

“For example, fish biomass is probably half of what it was before humans arrived, but it gets harder and harder to infer those patterns as we go farther back in time,” says Tekwa. “We need to think about how the distribution of body size biomass will change under environmental pressures.”

About the study

"The sizes of life" by Eden Tekwa, Katrina Catalano, Anna Bazzicalupo, Mary O’Connor, and Malin Pinsky was published in PLOS ONE.

About McGill University

Founded in 1821, McGill University is home to exceptional students, faculty, and staff from across Canada and around the world. It is consistently ranked as one of the top universities, both nationally and internationally. It is a world-renowned institution of higher learning with research activities spanning three campuses, 12 faculties, 14 professional schools, 300 programs of study and over 39,000 students, including more than 10,400 graduate students.

McGill’s commitment to sustainability reaches back several decades and spans scales from local to global. The sustainability declarations that we have signed affirm our role in helping to shape a future where people and the planet can flourish.

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