Counting the Uncountable: The Global Biomass and Population of Land Arthropods
Featured Image Caption: Formosan termites (Coptotermes formosanus) repairing a tunnel. Termites are by far the weightiest group of soil arthropods, making up 40% of the total biomass. Image: “Termites rush to damaged portion of mound” by U.S. Department of Agriculture (taken by Scott Bauer), CC BY 2.0, via Wikimedia Commons.
Source Article: Rosenberg, Y., Bar-On, Y.M., Fromm, M., Ostikar, M., Shoshany, A., Giz, O., Milo, R. (2023). The global biomass and number of terrestrial arthropods. Sci. Adv. 9(5). https://doi.org/10.1126/sciadv.abq4049.
When we think of massive animals, our minds jump to charismatic giants like whales, elephants, and polar bears. Collectively, however, arthropods (insects, spiders, and anything else you’d be tempted to call a “bug”) put them to shame: researchers have estimated their global biomass at 1 Gigaton, exceeding all other groups of wild animals. In terms of their population numbers, arthropods are even more dominant – the global insect population alone has been estimated at 18 quintillion (that’s 1 followed by 19 zeros)!
Arthropods are an extremely diverse phylum, encompassing everything from the bees in your backyard to the mites on your scalp to the lobster on your dinner plate (if you’re into that). How do these different sub-groups contribute to the massive totals listed above? Which are the most numerous, which are the weightiest, and which are a mere rounding error?
Weighing the Unweighable, Counting the Uncountable
A study published last year estimated both population numbers and biomass for terrestrial arthropods, including both subterranean above ground species, but excluding marine species like lobsters, shrimp, and horseshoe crabs. The group behind this study includes pioneers in this field of research who have previously published global mass estimates for wild mammals, human-made structures, and the biosphere as a whole
For this study, researchers compiled a massive dataset from previous research. This included approximately 7,000 measurements of insect mass or population density conducted at hundreds of sites across the globe. The sites were classified as either in the soil or above ground. From the above-ground data, researchers were able to estimate population and biomass for arthropods as a whole. For the soil sites, which had more available data, researchers separated out the four most significant groups – termites, ants, mites, and springtails – and lumped together all remaining groups as “other.” Soil arthropod biomass and population were estimated globally and for the major terrestrial biomes, including tropical forests, deserts, and croplands.
Heavyweight Champions of the Soil
The study estimated that soil arthropods weigh a whopping 20 Megatons (that’s 200 million metric tons) globally. Termites were by far the heaviest group, comprising 40% of the total, with ants, springtails, and mites each contributing around 10% and their various relatives rounding out the picture. In terms of population, the study estimated 1 quintillion total soil arthropods (that’s 1 with 19 zeros, as you may recall), of which the infinitesimal mites and springtails together comprised 95% of the total.
Comparing different biomes, researchers found that tropical rainforests contain the most soil arthropods, both in terms of population and biomass. This may come as a surprise, given that tropical soils are famously thin and nutrient poor; they compensate, however, by offering arthropods a rich layer of topsoil full of decomposing animal and plant matter. In terms of biomass, pastures and croplands are the next richest habitats. Even the frigid and inhospitable tundra holds a surprising number of arthropods in its soils, though both ants and termites are absent from these regions. Mediterranean habitats and deserts brought up the rear, ranking at the bottom in terms of both biomass and population.
The study estimated an above-ground biomass of 50 Megatons for the entire Arthropod lineage. They add, however, that this may be an undercount, given the difficulties of collecting data, especially in important habitats like tropical rainforests. They therefore calculated an “upper estimate” of 200 Megatons, which would make the above-ground total similar to that underground. The authors highlight that more research is needed in this area to derive a more accurate estimate.
Altogether, the researchers come to an estimated 300 Megatons for terrestrial arthropods both above and below soil (though they add that it could be as low as 100 or as high as 500). How does this compare to other animal groups? While this is more than 10x greater than the biomass of wild mammals, it is about equal to that of humans and our extremely heavy livestock. And this number is completely overshadowed by the weight of crustaceans, the undersea cousins of terrestrial arthropods, which are an order of magnitude heavier.
A Tool for Tracking Species Decline
For the bug-obsessed among us, these numbers are inherently fascinating. But what is the practical value of such estimates? We are, of course, in the midst of a human-driven mass extinction crisis, and arthropod population declines are a key part of that crisis. Most studies focus on trends in population or biomass, which are of course useful. Calculating these baseline numbers, however, gives scientists and conservationists a benchmark against which their declines can be compared. It also gives us a more holistic sense of what of our planet’s ecosystems are, how they function, and how they can be protected.
