Life comes in all sizes and shapes – but certain size ranges are more common than others. For the first time, a study has cataloged all known organisms on earth according to their size and biomass. The result: the largest and smallest living beings, i.e. huge trees and tiny microorganisms, have the largest proportions of the biomass on our planet. We humans also belong to the largest size category. On the other hand, creatures of the order of insects make the least difference in terms of their biomass.
In the early 1970s, US researchers published a sensational study. According to this, the size distribution of living beings in the sea follows a mathematical law: If the body mass of a class increases tenfold, the number of organisms decreases by a factor of ten. As a result, all size classes of organisms should have approximately the same amount of biomass. This so-called Sheldon size spectrum theory has since been confirmed in further studies on marine organisms, with more recent studies showing that human influences shift the distribution.
Surprising size distribution
But what about on land? In order to clarify this question, a team led by Eden Tekwa from the University of British Columbia in Canada recorded the body sizes of all organisms living on earth – and came up with an astonishing distribution: “The smallest and largest organisms clearly outweigh all other organisms ‘ says Tekwa. “This appears to be a new pattern that needs to be explained, and we don’t yet have theories on how to explain it.”
Accordingly, large organisms with an individual weight of around ten tons make up the largest proportion of the biomass. These are mainly large forest trees. The organisms in this size class weigh a total of 65 gigatonnes. Other organisms at the upper end of the size scale – weighing between 100 grams and 1,000 tons – also make up a large proportion of the total biomass on earth. In contrast, all weight classes of living things between one picogram and 10 milligrams – including insects, fungi and small plants – make up a total of only about one gigatonne of biomass. In comparison, the organisms at the lowest end of the size scale weigh astonishingly large: Microorganisms such as bacteria and archaea, which individually weigh only about 0.01 picograms, collectively contribute around 15 gigatonnes to the total biomass on Earth.
Microbes and trees predominate
“The discovery that life on Earth is predominantly packaged in the largest and smallest sizes surprised us,” says Tekwa’s colleague Malin Pinsky. “Sometimes it seems like mosquitoes, flies or ants rule the world, but when we did the math, we found that our world is dominated by microbes and trees. They are the silent partners that recycle the nutrients and freshen the air around us.”
Regarding the distribution of biomass in the oceans, Tekwa and her team point out that previous studies have left out important organisms: “The common phrase ‘from bacteria to whales’, which is intended to represent the entire size range of the sea, omits macroalgae, seagrass, hard corals and mangroves. According to our results, these organisms make up 45 percent of all marine biomass.” Nevertheless, the team concludes, in broad agreement with Sheldon’s size spectrum theory, that the biomass in the sea is actually distributed more evenly across the different size classes than on land.
Baseline for future studies
For both aquatic and terrestrial organisms, the research team found that maximum body size was in a similar range for different species in different environments. “The largest body sizes are found in several groups of species, and their maximum body sizes are all within a relatively narrow range,” Tekwa said. “Trees, grasses, underground fungi, mangroves, corals, fish, and marine mammals all have similar maximum body sizes. This could indicate that there is a universal upper size limit due to ecological, evolutionary, or biophysical constraints.”
The author team points out that the current mass estimates are subject to a lot of uncertainty. “Our results provide an initial rough roadmap for possible patterns, which are likely to change drastically as size-mass spectra become the target of research programs,” the team said. Nevertheless, the study offers an important basis for future surveys. “The survey establishes a baseline of the current state that already includes man-made impacts. For example, fish biomass is probably half what it was before humans arrived, but deducing these patterns becomes increasingly difficult the further we go back in geological time.” Further studies are important to capture such changes. “Global environmental assessments should be an integral part of sustainability initiatives,” the team said.
Source: Eden Tekwa (University of British Columbia, Vancouver, Canada) et al., Plos One, doi: 10.1371/journal.pone.0283020