The variety of life on Earth, known as biodiversity, is a fundamental concept in biology, yet its true scale remains one of science’s greatest puzzles. While we share the planet with millions of different organisms, only a fraction of them has been formally recognized and cataloged by researchers. Understanding this disparity is a first step toward grasping the true richness of Earth’s ecosystems and the full scope of life’s complexity.
The Known Baseline
The baseline for measuring undiscovered life is the count of species that have been officially described, named, and recorded in scientific literature. Current estimates place the total number of formally described species globally at approximately 1.9 to 2.16 million. The cataloging process is meticulous, requiring a published description that follows strict international rules of nomenclature (e.g., the International Code of Zoological Nomenclature for animals). Each new species must be differentiated from all others, often with a designated type specimen and a unique two-part Latin name.
This count forms the foundation of our biological knowledge, maintained in central databases like the Catalogue of Life. The number of truly unique species is likely slightly lower, however, as the same species can sometimes be described and named multiple times, resulting in “synonyms.” Despite efforts to reconcile these duplicates, the described number represents only a small percentage of the total species thought to exist.
Methods for Estimating Global Diversity
Since a full, direct count is impossible, scientists rely on statistical methods to estimate the total number of species.
The first powerful approach is the taxonomic ratio method, which identifies predictable numerical relationships within the Linnaean classification system. Researchers analyze groups where the description of higher taxonomic levels, such as genera and families, is considered relatively complete. By observing the consistent ratio of higher ranks to the species level in these well-known groups (like mammals or birds), scientists can extrapolate that ratio to less-studied groups to predict their total species count.
Another technique is the species-area relationship (SAR), which is based on the ecological observation that larger habitats generally contain more species. This relationship is often expressed mathematically as a curve relating the number of species (S) to the area (A). By sampling species richness across smaller, defined areas and observing how the number of species increases with the size of the sampled area, researchers can extrapolate this curve to estimate the total species count across much larger, unsampled regions or globally.
A third method involves extrapolation from sampling curves, also known as species accumulation curves. This technique involves repeatedly sampling a habitat and plotting the cumulative number of unique species found against the sampling effort or number of individuals collected. When the curve begins to flatten out, it suggests that most species in that local area have been discovered, allowing statistical models to estimate the asymptotic total, which is the maximum expected number of species in that community. These estimates for local communities can then be scaled up to generate regional or global projections.
The Scale of the Unknown
Applying these methodologies produces a wide range of estimates for the total number of species on Earth. For eukaryotic life—organisms whose cells contain a nucleus, including animals, plants, and fungi—the most frequently cited estimate is 8.7 million species. This figure, derived from the taxonomic ratio method, suggests that roughly 86% of terrestrial species and 91% of marine species remain undiscovered and undescribed.
The total global estimate becomes far larger when all forms of life are considered, especially microorganisms. While the 8.7 million figure typically excludes prokaryotes (bacteria and archaea) and viruses, other studies incorporating molecular data have drastically expanded the scale of the unknown. Estimates that include this microbial diversity can range from tens of millions to over a trillion species, reflecting the staggering number of unique genetic lineages found in environmental samples. This disparity highlights the challenge of applying a traditional “species” concept to the microbial world, where genetic sequencing is revealing complexity at an unprecedented scale.
Where the Undiscovered Species Reside
The vast majority of undiscovered species are concentrated in specific, difficult-to-access habitats and within certain hyper-diverse taxonomic groups.
Insects are the single largest contributor to the unknown, with estimates suggesting millions of species, particularly in the canopies of tropical rainforests. These elevated “forests within forests” are incredibly rich but have historically been challenging to survey, necessitating specialized techniques like canopy walkways and cranes. The discovery of new arthropods, even in small, localized surveys, confirms the enormous diversity hidden in these aerial habitats.
The deep ocean represents another significant frontier, characterized by extreme pressure and darkness, where exploration is still limited. A large percentage of marine life, including species living in deep-sea trenches and hydrothermal vents, remains uncataloged. Discoveries in the deep sea often reveal entirely new species, such as carnivorous bivalves and unique parasitic isopods, underscoring the novelty of life in these remote environments.
Finally, the kingdoms of Fungi and Protists are thought to contain millions of undescribed species, many of which are microscopic and exist in the soil, water, and as symbionts. While only about 150,000 fungal species have been formally described, current estimates suggest the total number is between 2.2 and 3.8 million, with some molecular analyses pointing even higher. The use of environmental DNA (eDNA) sampling, which extracts genetic material directly from the environment, now allows scientists to detect these previously invisible “dark taxa,” revealing a massive hidden world of life.