The question of which animal holds the largest population on Earth leads to a contrast between familiar creatures and the microscopic majority. The sheer numerical dominance belongs almost exclusively to the invertebrates, rather than large mammals or birds. Arriving at a definitive answer is difficult due to the vastness of the planet’s ecosystems—particularly the deep ocean and the soil—where counting individuals is nearly impossible. Ultimately, the animal with the highest population is a tiny, widely distributed arthropod found in nearly every aquatic environment.
The Challenges of Quantifying Global Animal Populations
Determining the population of globally distributed animal species relies heavily on statistical inference rather than a direct census. Scientists must employ specialized sampling techniques and then extrapolate those localized counts across entire ecosystems. The choice of method depends entirely on the size, mobility, and habitat of the species being studied.
For sessile or relatively immobile organisms, such as soil mites or some marine invertebrates, researchers often use quadrat sampling. This method involves marking off a small, defined area, counting individuals within that space, and then scaling that density measurement to the total habitat area. For mobile species, the mark-recapture technique is a common approach. Here, a sample of animals is captured, marked, released, and then a second sample is taken to estimate the total population based on the ratio of marked to unmarked individuals.
These figures are always estimates because the underlying assumptions of the sampling models are rarely perfectly met in natural populations. For instance, mark-recapture assumes that marked and unmarked animals have an equal chance of being caught. The populations of short-lived or widely distributed creatures, especially those in the deep ocean, present an additional layer of difficulty, making the global figures rough approximations rather than precise counts.
The Absolute Population Champion: Microscopic Arthropods
The undisputed numerical champion within the animal kingdom is a group of microscopic crustaceans known as copepods. These tiny aquatic arthropods, typically measuring only a few millimeters in length, form an enormous portion of the zooplankton found in both marine and freshwater environments across the globe. The scale of their population is almost incomprehensible, with some estimates suggesting their global number exceeds \(1.3\) sextillion individuals (\(1.3 times 10^{21}\)).
Copepods achieve this massive population due to their small size, rapid reproductive cycles, and ubiquitous distribution, inhabiting virtually every body of water from the surface to the deep sea. On land, other microscopic arthropods also contribute significantly to the total count. Soil organisms like mites and springtails make up over 95% of the total number of soil arthropods, with their combined population estimated to be around 10 quintillion (\(1 times 10^{19}\)) individuals.
Population Count Versus Total Biomass
A distinction exists between the sheer number of individual animals and their collective mass, known as biomass. While microscopic arthropods dominate the numerical count, they do not necessarily hold the title for the greatest total weight within the animal kingdom. The entire animal kingdom’s biomass is estimated to be approximately 2 gigatons of carbon, with the largest subgroups being marine arthropods and fish.
Antarctic Krill (Euphausia superba), another small crustacean, is considered the wild animal with the largest total biomass, estimated at around 379 million tonnes. However, the total animal biomass is overwhelmingly dominated by species tied to human civilization. Livestock, specifically cattle, hold a biomass that exceeds 650 million tonnes, far surpassing the weight of all wild mammals. The combined mass of humans and domesticated animals now accounts for the vast majority of the world’s vertebrate biomass.
The Most Numerous Vertebrates
The vertebrates have populations that are dwarfed by their invertebrate counterparts. The most numerically dominant vertebrate is a group of deep-sea fish called bristlemouths, belonging to the genus Cyclothone. These small, bioluminescent fish inhabit the mesopelagic zone of the world’s oceans and are estimated to number in the quadrillions (\(10^{15}\)), making them the most abundant fish and most common vertebrate on Earth.
Among land animals, the most numerous bird species is the domesticated chicken (Gallus gallus domesticus), with a global population of around 23.7 billion individuals due to their role in agriculture. The most numerous mammal is the human species, with a population exceeding 8 billion. Even these massive numbers are several orders of magnitude smaller than the population of the microscopic copepods that fill the global oceans.