Scientific projections estimate that approximately 8.7 million species of eukaryotes exist on Earth. Eukaryotes are life forms whose cells contain a nucleus, which includes all plants, animals, fungi, and protists. This figure is a statistical projection, not a physical count, highlighting the difficulty in determining the planet’s total species richness. The large discrepancy between known species and this estimate exists because life is distributed across inaccessible habitats and includes countless small organisms that have yet to be formally documented. Quantifying this global diversity requires sophisticated methods to extrapolate from the known to the unknown.
The Cataloged Species Count
The foundation of any estimate is the number of species that have been formally identified and described by taxonomists. This cataloged portion of life represents the established baseline for all biodiversity calculations, currently standing at around 1.2 to 1.7 million species. The process of formally naming species traces its modern roots back to the 18th century with Carl Linnaeus, who established the system of binomial nomenclature still used today. His work provided the standardized hierarchy of classification, grouping organisms into species, genus, family, and higher ranks, which allows scientists globally to track and organize known life.
However, the described species are disproportionately large and charismatic organisms, such as birds and mammals, which are relatively well-studied. Many of the known species also turn out to be synonyms, where the same organism has been described multiple times under different names, further complicating the baseline figure. This means that the vast majority of life forms, particularly those that are small or inhabit remote environments, remain outside of the formal scientific record.
Methods for Estimating Total Life
Since a direct count is logistically impossible, scientists employ statistical techniques to extrapolate the total number of species from the known data. One powerful approach, developed in 2011, uses the predictable numerical patterns observed within the hierarchical taxonomic classification system. Researchers analyzed the relationship between the number of species and the number of higher taxonomic ranks—genus, family, order, class, and phylum—for well-studied groups. By observing that the ratio of species to higher taxa tends to stabilize as a group becomes fully described, they can project the total number of species for less-studied groups where this ratio has not yet leveled off.
Another long-standing technique is the Species-Area Relationship (SAR), which is based on the ecological observation that larger geographic areas generally contain more species. This relationship is often modeled mathematically using a power law, where the number of species (\(S\)) increases with the area (\(A\)) sampled, defined by the formula \(S = cA^z\). Ecologists can use this model to estimate global species richness by sampling a small area, calculating the rate of species accumulation, and then extrapolating that rate to the total area of a continent or the entire planet. Both the taxonomic and the area-based models are statistical tools that rely on assumptions about the consistency of natural patterns.
Where the Undiscovered Species Reside
The majority of unknown life is concentrated in environments that are difficult to access or are composed of organisms too small for easy identification. Tropical arthropods, particularly insects, represent the single largest group of undescribed species, with millions thought to inhabit the canopy of tropical rainforests. The complexity and sheer volume of insect life in these regions make comprehensive sampling and identification a monumental task. This diversity is often highly specialized to specific host plants or microhabitats.
The deep ocean is another vast reservoir of undiscovered life, often referred to as the “dark biosphere.” Organisms in the deep-sea pelagic zone and beneath the seafloor sediment live under crushing pressure and in perpetual darkness, relying on chemical energy rather than sunlight. Recent expeditions continue to uncover novel life forms, including strange mollusks and carnivorous bivalves, from depths exceeding 6,000 meters. Furthermore, the world’s soil harbors an immense, hidden diversity of life, including eukaryotes such as nematodes, protists, and fungi.
Microbial life, which includes prokaryotes like bacteria and archaea, is typically counted separately from the 8.7 million eukaryote estimate due to fundamental methodological differences. The traditional species concept, which relies on sexual compatibility and stable genomes, is not applicable to prokaryotes because they frequently exchange genes across taxonomic boundaries in a process called horizontal gene transfer. Instead, prokaryotic diversity is often estimated using gene sequencing techniques and mathematical models. These methods suggest that the number of prokaryotic taxa could reach into the millions or even a trillion, far exceeding the projected total for eukaryotes.
Tracking Biodiversity Loss
Establishing a baseline for the total number of species is necessary to accurately gauge the severity of the current biodiversity loss. Extinction is a natural process, occurring at a slow, predictable rate known as the background extinction rate. This natural rate is estimated to be low, historically hovering around one to five species lost per year, or about 0.1 extinctions per million species-years.
The current rate of species disappearance, however, is estimated to be 100 to 1,000 times greater than this background rate, an acceleration driven primarily by human activities such as habitat destruction and climate change. This exponential increase in species loss is why many scientists refer to the present era as the Holocene or Sixth Mass Extinction event. The sheer magnitude of the estimated total number of species—8.7 million—provides the essential context, revealing that countless life forms may be vanishing before they are ever discovered, described, or studied.