The question of which animal is the most numerous on Earth is counter-intuitive, as the answer rarely involves the large, familiar creatures that capture human attention. The true numerical champions are small, widespread invertebrates that thrive globally, particularly in the oceans. The sheer scale of life in the invertebrate world dwarfs the combined populations of all mammals and birds. Abundance is determined by a capacity for rapid reproduction and global dispersal, not size or intelligence.
Earth’s Numerical Champions Arthropods and Marine Invertebrates
The definitive numerical champions are invertebrates, specifically within the phylum Arthropoda, which includes insects and crustaceans. The undisputed most populous animal species is the copepod, a tiny crustacean found in nearly every freshwater and saltwater environment. With an estimated global population reaching approximately 1.3 nonillion individuals, these creatures may outnumber every other animal on the planet. Their small size, typically only a few millimeters long, coupled with their role as primary consumers of algae and phytoplankton, allows for their widespread success.
On land, the insect kingdom establishes its dominance, with ants being the most numerous terrestrial animals. Global estimates suggest the world is home to roughly 20 quadrillion individual ants. This enormous population is attributed to their colonial social structure, high reproductive rates, and ability to colonize almost every landmass on Earth. Antarctic krill, another small crustacean, also exists in vast numbers, with the Antarctic species alone estimated to number over 500 trillion individuals. Krill form the foundation of the marine food web, and their high fecundity ensures their numerical supremacy.
The Most Abundant Vertebrates
The population totals for vertebrates are significantly smaller than their invertebrate counterparts. The most numerous vertebrate is believed to be the bristlemouth, a tiny, bioluminescent deep-sea fish belonging to the family Gonostomatidae. Trillions, possibly even quadrillions, of these fish swim in the world’s oceans, inhabiting deep, dark layers. Their sheer numbers make them the most populous fish and the single most abundant vertebrate species.
Among terrestrial vertebrates, birds and mammals are far less numerous globally. The Red-billed Quelea (Quelea quelea) is widely considered the most numerous wild bird species, with estimates ranging from one to ten billion individuals across two-thirds of Africa. Their success is driven by their ability to form immense flocks and consume grass seeds. The most populous mammal species is Homo sapiens, with the global human population currently exceeding eight billion individuals.
The scale of human-managed populations significantly alters the vertebrate landscape. The domesticated chicken has an estimated population exceeding 23 billion, making it the most numerous bird overall, far outnumbering any wild avian species. Furthermore, the biomass of domesticated cattle, raised for meat and dairy, vastly exceeds the total biomass of all wild land mammals combined.
Population Density and Distribution
Total population count offers only one perspective, as it does not account for how individuals are concentrated. Population density, the number of individuals per unit area or volume, highlights localized concentrations of life. For instance, while bristlemouths are globally ubiquitous, Antarctic krill can form massive swarms with densities reaching 10,000 to 30,000 animals per cubic meter.
In the deep ocean, hydrothermal vent communities exemplify extreme localized density in an otherwise sparsely populated environment. These vents, fueled by chemosynthetic bacteria, support animal populations that are 10,000 to 100,000 times denser than the surrounding deep-sea floor. Species such as giant tube worms, clams, and shrimp cluster tightly around the vent openings, creating isolated oases of high biomass sustained by chemical energy. On land, the Argentine ant provides a terrestrial example, forming massive supercolonies that can stretch for hundreds of miles.
Counting Difficulties and Biomass Estimates
Calculating the populations of the most numerous animals is a profound scientific challenge due to their small size, rapid turnover, and inaccessible habitats. Scientists rely on sampling methods like trawl nets and acoustic surveys to estimate densities, which are then extrapolated across the entire ocean volume. These extrapolations are subject to sampling errors and the inherent difficulty of surveying vast, deep-sea, and remote environments.
The alternative metric, biomass, measures the total mass or weight of a species, shifting the focus from individual numbers to collective physical presence. While copepods are the numerical champions, large species can accumulate more total mass despite having fewer individuals. The wild animal with the largest estimated single-species biomass is the Antarctic krill, with a cumulative wet mass of approximately 379 million metric tons. This mass rivals that of the entire human population. The largest animal biomass overall belongs to domesticated cattle, which exceed 400 million tonnes. The total biomass of ants, despite their minute size, is also staggering, outweighing the combined mass of all wild birds and non-human mammals.