Quantifying the number of species on Earth is an ongoing scientific endeavor with significant implications for conservation and understanding the global ecosystem. While the number of formally identified life forms is substantial, it represents only a fraction of the planet’s total biological richness. The true count is a dynamic figure, constantly refined by new discoveries and analytical models. This complexity arises because life is organized into distinct units, and quantifying those units is a monumental task.
What Defines a Species
Before counting, scientists must define the unit of measure: the species. The most widely referenced framework for sexually reproducing organisms is the Biological Species Concept (BSC). The BSC defines a species as a group of populations that can interbreed in nature and produce viable, fertile offspring, while remaining reproductively isolated from other groups.
The BSC has limitations that challenge a universal count. It cannot be applied to organisms that reproduce asexually, such as bacteria and many single-celled eukaryotes, since interbreeding is irrelevant. Furthermore, it struggles with organisms known only from fossils, where reproductive behavior cannot be observed, and with species that occasionally hybridize in the wild.
These limitations led to alternative frameworks, such as the Phylogenetic Species Concept. This concept defines a species as the smallest group of organisms that share a common ancestor and can be distinguished by a unique combination of traits or a distinct genetic lineage. The different ways scientists define a species means that the total number is not a single, fixed value, but rather an estimate dependent on the conceptual lens used for classification.
The Officially Described Count
The most concrete figure available is the number of species formally identified, named, and cataloged by taxonomists. This documented count, tracked by global databases like the Catalogue of Life and the World Register of Marine Species, is generally accepted to be just over two million unique species. This figure represents the known fraction of life on Earth, established through over 250 years of scientific exploration.
The vast majority of described species are animals, heavily dominated by insects, which account for over one million named species. Plants are the next largest kingdom, with nearly 300,000 formally named species. The number of described fungi, protozoa, and chromista is much smaller, with hundreds of thousands remaining to be documented.
This count is dynamic; taxonomists identify and name approximately 16,000 new species each year, largely insects. The number also fluctuates due to taxonomic revisions, where new evidence may “split” a single species into two or more, or “lump” previously separate species into one.
Models for Estimating Total Biodiversity
To determine the total number of species, including those yet to be discovered, scientists rely on statistical models. The most widely cited estimate for total eukaryotic species—organisms whose cells contain a nucleus—is approximately 8.7 million, with a margin of error of about 1.3 million. This estimate suggests that roughly 86% of all species on land and 91% of species in the ocean remain undescribed.
One method used to arrive at this figure is the extrapolation of taxonomic ratios. This model examines the hierarchical structure of life, from species up to phyla, noting that the higher levels of classification are much more completely known than the species level. By observing the predictable numerical pattern in the ratio of species to genera, genera to families, and so on, in well-studied groups, researchers can extrapolate this pattern to estimate the total species count for less-studied groups.
An alternative approach involves sampling methods combined with statistical species-accumulation curves. For example, in a process like fogging the canopy of a tropical rainforest, scientists collect all arthropods from a small area. They then use mathematical estimators, which look at the number of species found only once or twice in the sample, to predict how many additional species would be found with more intensive sampling. These models are useful for hyper-diverse groups like insects and fungi.
The majority of unknown species are predicted to be found in less-explored habitats, such as tropical rainforests and the deep sea, where life is most concentrated. Insects and fungi are the dominant groups in these estimates, with millions of species in each kingdom awaiting discovery.
The Separate Realm of Microbial Diversity
The estimate of 8.7 million species is limited to eukaryotes, excluding microscopic life: prokaryotes (bacteria and archaea) and viruses. Assessing the diversity of these organisms requires different methods than those applied to plants and animals. Traditional methods of culturing microbes in a lab captured only a tiny fraction of existing life, leading to significant underestimation.
The shift to genetic sequencing, specifically metagenomics, has revolutionized microbial assessment. Scientists can now extract and sequence all genetic material directly from environmental samples, such as soil or water, to identify organisms. This method sidesteps the need for culturing, revealing a far greater range of life.
In the microbial realm, the species concept is often replaced by the Operational Taxonomic Unit (OTU). An OTU is a cluster of organisms grouped by the similarity of their genetic sequence. Estimates range from the low millions of prokaryotic OTUs to predictions of up to one trillion microbial species globally, based on ecological scaling laws.
This range illustrates the difficulty in applying a traditional species concept to organisms that exchange genetic material laterally and reproduce asexually. Microbial life represents the largest reservoir of biodiversity on Earth.