The question of identifying the most common animal on Earth is deceptively simple, as the answer depends entirely on how the term “common” is defined. A person might immediately think of insects or birds, but the true scale of life is dominated by creatures that are largely invisible to the naked eye. Life’s abundance is measured not just in the number of individuals but also by the collective weight of an entire group. Understanding the planet’s animal population requires looking beyond the large, visible species to the microscopic organisms that form the foundation of global ecosystems.
Defining the Metrics of Abundance
Scientists use two distinct methods to quantify the commonality of animal life globally: population count and total biomass. Population count is a direct measure of the sheer number of individual organisms, which often favors the smallest creatures that reproduce quickly. This metric often requires scientific notation to handle the enormous scale of these populations.
Biomass, by contrast, is a measure of the collective dry weight of an entire group of organisms, often expressed in gigatons of carbon (Gt C). This metric provides a sense of the total living matter a group represents, which can be significant even if the individual members are tiny. Because these two definitions prioritize different aspects of abundance—individual quantity versus collective mass—they frequently yield vastly different results. For instance, a single large whale contributes significantly more to biomass than billions of microscopic worms.
The Most Abundant Animals By Population Count
When considering individual numbers, the most common animals are tiny invertebrates that thrive in the soil and the ocean. On land, the champions of abundance are Nematodes, or roundworms, found in virtually every terrestrial environment. These microscopic worms, typically less than a millimeter in length, exist in staggering concentrations, with a recent estimate suggesting a global population of approximately 4.4 x 10^20 individuals in the Earth’s topsoil alone.
This figure translates to roughly 60 billion nematodes for every human being on the planet, underscoring their dominance in terrestrial habitats. In a single gram of healthy soil, there can be hundreds of these tiny organisms, found everywhere from arctic tundra to tropical rainforests. Population estimates rely on sophisticated modeling based on thousands of soil samples from across the globe.
In the marine environment, the most numerous multicellular animal likely belongs to Copepods, a group of small crustaceans. These planktonic organisms, usually only a few millimeters long, inhabit both fresh and saltwater. One estimate places the total copepod population in the world’s waters at over 1.3 x 10^21 individuals, a number that surpasses the immense count of nematodes. Copepods are ubiquitous, drifting in ocean currents and occupying a wide range of aquatic habitats.
The Animal Groups with the Highest Global Biomass
Shifting the focus to collective weight reveals a different set of dominant animals, with the largest collective mass found in the oceans. The highest global animal biomass is attributed to marine arthropods, which collectively account for about 1 Gt C, or roughly half of all animal biomass on Earth. This group is primarily composed of small crustaceans like copepods and krill, demonstrating that small organisms can achieve significant collective weight through sheer number.
Krill, particularly Antarctic krill, contribute a significant portion of this biomass, with estimates reaching approximately 379 million tonnes. These tiny, shrimp-like crustaceans form dense swarms in the Southern Ocean, creating one of the largest single-species biomasses of any wild animal. Their collective weight highlights their role as a foundational food source in the marine environment.
On land, the biomass of terrestrial arthropods—which includes all insects, spiders, and mites—is estimated to be around 300 million metric tons of dry biomass. This collective weight is roughly equivalent to the combined biomass of all humans and their livestock. Even within this group, dominance is concentrated in certain types, as termites alone may account for about 40 percent of the soil arthropod biomass.
The Planetary Significance of Hyper-Abundant Species
The hyper-abundant animals, though often overlooked, perform functions foundational to the stability of global ecosystems. Nematodes, in the soil, are primary decomposers and nutrient recyclers, constantly feeding on bacteria and fungi. They are instrumental in the cycling of carbon and other nutrients, which directly impacts soil fertility and plant health worldwide.
In the oceans, copepods and krill serve as a key trophic link, transferring energy from microscopic algae and phytoplankton to larger marine consumers. They are the main food source for a vast array of animals, including commercially important fish species, seabirds, and the largest baleen whales. Without these large populations of small crustaceans, the global marine food web would collapse.
Terrestrial arthropods like ants and termites are constantly at work, turning and aerating soil, controlling other insect populations, and dispersing seeds. The health of ecosystems, from forest to farmland, relies heavily on the collective action of these organisms. Their global abundance measures their deep integration into the planet’s fundamental life support systems.