The question of how many worms exist in the world introduces a concept of scale far beyond human intuition, as most live hidden beneath the soil surface. The term “worm” is an umbrella term encompassing vast biological diversity, from familiar garden dwellers to microscopic organisms. Scientific efforts to quantify this underground biomass reveal a population so immense it redefines our understanding of life on Earth. Estimates for these global populations are derived from large-scale data collection and ecological modeling, showing a subterranean world that dramatically outweighs the visible life above ground.
Earthworms: The Quantifiable Count and Biomass
The most recognizable creatures are earthworms, classified within the phylum Annelida. While a precise global count is impossible, researchers calculate their collective mass and density across various biomes. The total biomass of earthworms in a given area can often be far greater than the biomass of all mammals in the same region, underscoring their dominance in terrestrial ecosystems.
Earthworm density varies widely based on factors like soil moisture, temperature, and organic matter content. In productive environments, such as temperate grasslands or agricultural fields, densities can exceed a million individuals per hectare. Scientists group earthworms into three main ecological types based on their behavior and habitat depth.
Anecic species are deep-burrowing, creating permanent, vertical tunnels to pull surface litter into the soil profile. Epigeic species are small, surface-dwelling organisms that specialize in the rapid decomposition of leaf litter. Endogeic species inhabit the mineral layers of the topsoil, creating horizontal networks of temporary burrows as they feed on soil and its embedded organic matter.
Researchers use these ecological distinctions to extrapolate regional and global biomass estimates. An average healthy soil often contains 100 to 500 grams of earthworm biomass per square meter. This focus on biomass provides a meaningful metric for appreciating the sheer volume of life represented by this important group.
Nematodes: The True Global Population Scale
Earthworms are dwarfed in sheer numbers by microscopic roundworms known as nematodes (phylum Nematoda). These tiny, unsegmented creatures are the most abundant multicellular animals on Earth. A single gram of soil can contain hundreds of individual nematodes, and their distribution spans virtually every terrestrial and aquatic habitat.
Global modeling estimates that approximately \(4.4 times 10^{20}\) individual nematodes inhabit the planet’s surface soils, translating to 440 quadrillion worms. This immense figure means there are roughly 60 billion nematodes for every human being alive today, establishing them as the most populous animal group.
Despite their size, the collective biomass of this population is substantial, estimated at 0.3 gigatonnes (300 million metric tons). This mass is roughly equivalent to four-fifths of the total biomass of all humans. The highest densities of nematodes are found in cold regions like sub-Arctic forests and tundra, where they correlate strongly with high levels of soil organic carbon.
How Scientists Estimate the Unseen
Arriving at these colossal figures requires specialized and often labor-intensive sampling methods for both large and microscopic worms. For earthworms, populations are estimated using a combination of physical and behavioral extraction techniques applied to defined areas called quadrats. Physical methods involve digging a soil pit and meticulously hand-sorting the soil to count and weigh the worms found within that volume.
To collect deeper-dwelling earthworms, scientists employ behavioral methods. They pour a solution of diluted irritant, such as powdered mustard or formalin, onto the soil surface. This solution mimics rain and causes the worms to rise, allowing them to be collected and counted without disturbing the entire soil profile. This approach helps researchers categorize ecological groups and translate local density measurements into broader population models.
Estimating microscopic nematode populations relies on different laboratory techniques following the collection of soil cores. Researchers collect numerous small soil samples and use extraction methods like the Baermann funnel technique, which relies on the worms’ active movement to separate them from the soil matrix. Another method, centrifugal flotation, uses specific liquids and high-speed spinning to separate the tiny worms based on density. These lab counts are combined with global environmental data and machine learning algorithms to extrapolate local densities to a global scale, accounting for differences in soil type, climate, and vegetation cover.
The Role of Worms in Ecosystems
The massive populations of earthworms and nematodes are fundamental drivers of terrestrial ecosystem function. Earthworms are recognized as ecosystem engineers, constantly modifying the physical structure of the soil through their feeding and burrowing activities. Their extensive tunneling increases soil aeration and water infiltration, which helps mitigate runoff and erosion.
Their digestive process mixes mineral soil with organic matter, speeding up decomposition and nutrient cycling. Earthworms mineralize complex organic compounds, releasing plant-available nutrients like nitrogen and phosphorus back into the soil. A healthy earthworm population is directly linked to soil fertility and overall plant productivity.
Nematodes play an equally important, microscopic role in nutrient cycling as a foundational component of the soil food web. Many nematodes are grazers, feeding on bacteria and fungi, which prevents the overgrowth of microbial populations. When these tiny worms consume microbes, they excrete excess nitrogen and other nutrients in a form plants can readily absorb, a process known as nutrient mineralization. This continuous cycle, mediated by both earthworms and nematodes, ensures the sustained fertility and health of the world’s soils.