Earthworms, classified as invertebrates in the phylum Annelida, are segmented worms that play a profound role in terrestrial ecosystems. While the question of how many earthworms exist globally is common, obtaining a single, precise number is scientifically impossible. Scientists cannot take a literal census of all soil-dwelling creatures. Instead, they rely on complex methods to estimate their massive population, examining local densities and collective weight to understand the health of our planet.
The Challenge of Counting
Quantifying the worldwide earthworm population is hindered by fundamental obstacles inherent to their existence. Earthworms are cryptic organisms, spending most of their lives hidden beneath the surface, making direct observation across large territories impractical. Their vast global distribution encompasses nearly every biome on every continent except Antarctica, further complicating any attempt at a comprehensive census.
The sheer diversity of these creatures also presents a challenge to standardized counting and identification. While roughly 5,679 species have been formally described, modeling suggests the true global diversity could be as high as 30,000 species. This enormous species pool means counting methods must account for organisms of vastly different sizes, behaviors, and habitat preferences.
Estimating Population Density
Because a global count is unattainable, scientists focus on determining local population density, which is the number of individuals found within a specific area. This local data is then used to extrapolate estimates for larger regions and the globe. Density is typically measured in individuals per square meter, with figures varying widely based on soil type and land use.
The most common method for measuring density is the quadrat method. This involves digging a defined soil pit, such as \(20 \text{cm} \times 20 \text{cm}\), and manually sorting the soil to count the worms. This labor-intensive technique is often supplemented with extraction methods to draw out deeply burrowing species. Chemical irritants, like a dilute solution of formalin, or electrical stunning can be applied to force the earthworms to the surface for collection.
The resulting densities illustrate a dramatic scale difference between environments. Poor quality soils might host fewer than 50 earthworms per square meter, while rich agricultural land can support populations exceeding 500 individuals per square meter. In highly concentrated environments, such as vermicomposting systems, densities can temporarily rise above 2,000 individuals per square meter.
Global Biomass and Scale
To grasp the magnitude of the earthworm population on a planetary scale, scientists often shift the focus from individual numbers to collective physical mass, known as biomass. Biomass is typically calculated by wet weight or by the total mass of carbon contained within the organisms. Studies have shown that earthworms represent a massive proportion of the total living matter in terrestrial ecosystems.
In many forests, the estimated live weight of earthworms below ground is considerably greater than the weight of all animals living above the soil surface. Earthworms are often the most abundant animal biomass within most soil ecosystems. The total estimated carbon biomass for all Annelids (a phylum that includes earthworms, leeches, and ragworms) is approximately 0.2 gigatons of carbon.
This single group of primarily soil-dwelling animals outweighs the collective biomass of all humans, estimated at 0.06 gigatons of carbon. Furthermore, the Annelid biomass is greater than that of all livestock—cows, pigs, and chickens—which total about 0.1 gigatons of carbon.
Ecological Importance of Their Numbers
The enormous number and collective biomass of earthworms are directly tied to their significant role as “ecosystem engineers.” Their constant movement through the soil physically alters the environment, providing numerous benefits for plant life and soil fertility. This large population drives the process of bioturbation, which involves the mixing of soil layers and the transport of organic matter deep underground.
Their burrowing activity creates macropores and intricate tunnel networks throughout the soil, which greatly improves water infiltration and aeration. This network acts like a natural drainage system, reducing runoff and helping plant roots access oxygen. Earthworms consume organic matter and excrete it as nutrient-rich fecal pellets known as casts.
These casts are richer in nitrogen, phosphorus, and other minerals than the surrounding soil, acting as a natural, slow-release fertilizer. The high magnitude of the earthworm population ensures that these processes occur continuously across vast tracts of land, underpinning the health of global ecosystems, particularly in agricultural regions.