Nature favours all creatures great and small over medium-sized, study suggests

Life may come in all shapes and sizes, but nature favours the biggest and smallest creatures over the medium-sized, research suggests.

A survey of body sizes of Earth organisms suggests 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 analysing data about the size and biomass of every type of living organism on the planet.

This ranged from tiny one-celled organisms such as soil archaea and bacteria to large organisms such as blue whales and sequoia trees.

According to the study, 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.

Malin Pinsky, an associate professor at the Rutgers School of Environmental and Biological Sciences, America, and one of the study authors, said: “This conclusion – that life on Earth comes packaged predominantly in the largest and smallest sizes – was a discovery that surprised us.

“Sometimes it seems like mosquitoes or flies or ants must run the world, and yet, when we did the numbers, we found that our world is dominated by the microbes and the trees.

“These are the silent partners that recycle the nutrients and replenish the air all around us.”

The researchers said humans belong to the size range that comprises the highest biomass, which is a relatively large body size.

Eden Tekwa, lead author of the study who conducted the research first as a postdoctoral fellow at Rutgers, then at the University of British Columbia, said: “The largest body sizes appear across multiple species groups, and their maximum body sizes are all within a relatively narrow range.”

Now a research associate with McGill University’s Department of Biology in Canada, they added: “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.”

The scientists suggest the work, published in the science journal Plos One, opens the door to a better understanding of elemental life processes.

Dr Pinsky added: “Body size is a fundamental feature of life, governing everything from metabolic rates to birth rates and generation times.

“Cataloguing which body sizes are most common is a key step towards understanding the world around us.”