The question of how many different forms of life inhabit the Earth is one of the most fundamental, yet least understood, in science. Biologists have attempted to catalog the planet’s inhabitants for centuries, but the total number remains an estimate and a subject of ongoing debate. The sheer scale of global biodiversity makes a precise count impossible with current resources. Scientists must rely on sophisticated mathematical models and statistical extrapolations, resulting in a number that is constantly being refined as new habitats are explored and new organisms are genetically sequenced.
The Vast Difference Between Known and Estimated Species
The species that have been formally identified, named, and cataloged represent only a small fraction of the total life on Earth. Since the Linnaean system of classification was introduced, scientists have described approximately 1.2 to 1.5 million species, primarily consisting of larger, more accessible organisms like plants and vertebrates. This inventory is the cumulative result of more than 250 years of taxonomic effort.
This known figure contrasts sharply with the most widely accepted scientific projection for the total number of species. Based on a landmark 2011 study, the estimated total for all eukaryotic life—organisms whose cells have a nucleus, including animals, plants, fungi, and protists—is approximately 8.7 million, with an uncertainty range of about 1.3 million. This estimate suggests that about 86% of terrestrial species and 91% of marine species are still waiting to be discovered, named, and documented.
The Hidden Kingdoms: Why Counting is So Difficult
The immense gap between described species and the total estimate exists because certain kingdoms of life and specific environments present significant barriers to traditional cataloging. The sheer size and inaccessibility of many habitats mean that physically collecting and examining specimens is often impossible. This is particularly true for the deep ocean, where an estimated 2.2 million species reside, and the canopies of tropical rainforests, which shelter millions of invertebrates.
Another major challenge lies in the nature of the organisms themselves, especially microorganisms. The 8.7 million estimate largely excludes prokaryotes, such as bacteria and archaea, and viruses, whose total numbers are thought to be millions or even trillions. For these microbes, the concept of a distinct species is often fluid, and identification relies heavily on genetic sequencing rather than physical traits. Furthermore, the two largest and most diverse groups of eukaryotes—insects and fungi—are undersampled, with the vast majority of their species concentrated in remote tropical regions that lack sufficient taxonomic infrastructure.
How Scientists Calculate the Total Estimate
To arrive at a figure like 8.7 million, scientists employ rigorous mathematical and statistical techniques that move beyond simple physical counting. The most influential method, used in the 2011 study, involves analyzing the hierarchical structure of the Linnaean classification system. This approach relies on the observation that the number of known species within higher taxonomic ranks, such as phylum, class, and order, follows a consistent and predictable pattern.
Researchers examined the relationship between the number of known species and the number of higher taxa they belong to. By using this mathematical relationship, they could extrapolate the expected total number of species in groups that are still undersampled. For instance, they analyzed the ratio of known species to known genera and families, projecting this ratio downward to estimate the final total of species for an entire kingdom.
Other methods also contribute to refining the overall estimate, particularly for groups that do not fit the taxonomic hierarchy model well. Species accumulation curves track the rate at which new species are discovered over time and use predictive modeling to estimate when the discovery rate will level off, or “asymptote.” Furthermore, modern molecular approaches like metagenomics allow scientists to sample environmental DNA, identifying unique genetic sequences that represent distinct organisms without needing to see the physical specimen. These genetic surveys increase estimates of diversity for groups like fungi and protists.
Where Biodiversity is Most Concentrated
The distribution of Earth’s species is heavily skewed, both taxonomically and geographically, with a few groups and locations accounting for the majority of life. Taxonomically, the animal kingdom dominates the eukaryotic total, with an estimated 7.8 million species. Within this group, invertebrates, particularly insects, represent the majority of undescribed life, far eclipsing the approximately 66,000 described species of vertebrates.
Fungi, which include molds, yeasts, and mushrooms, also constitute a large but poorly understood kingdom, with estimates suggesting there are hundreds of thousands of species yet to be described. Geographically, the greatest concentration of unknown species is found in tropical environments. Tropical rainforests and coral reefs are recognized as major biodiversity reservoirs, harboring immense numbers of endemic species found nowhere else on the planet.
These areas, along with deep-sea hydrothermal vents, represent the primary frontiers of discovery. Terrestrial “hotspots” contain nearly 60% of the world’s plant, bird, mammal, reptile, and amphibian species on less than 3% of the Earth’s land surface. These hotspots are defined by having high numbers of endemic plant species and having lost a significant portion of their original habitat.