The exact number of species on Earth remains an estimate, not a precise count. Scientists face considerable challenges in cataloging life due to its sheer diversity and vast, often inaccessible environments. Understanding this estimate is important for comprehending Earth’s biodiversity and the ongoing processes of discovery and classification.
The Global Species Count
The most widely cited scientific estimate for eukaryotic species, including animals, plants, fungi, and protists, is approximately 8.7 million. Proposed by a 2011 study, this figure has a margin of error of about 1.3 million. Other studies have suggested numbers ranging from a few million to over 100 million for eukaryotes.
The number of species formally described and named by scientists is significantly lower than the estimated total. As of 2022, the IUCN Red List reported around 2.16 million identified species. A substantial portion of Earth’s species, particularly in lesser-known groups, still awaits discovery. For instance, insects represent the most diverse animal group, with over 1 million species recorded.
Prokaryotes, such as bacteria and archaea, represent the largest area of uncertainty in global species counts. Estimates for these microscopic organisms vary enormously, ranging from thousands to billions or even trillions. This highlights the challenges in cataloging the planet’s immense microbial diversity, which is far less understood than macroscopic life.
Why an Exact Count is Elusive
Achieving a precise count of all species on Earth is difficult for several reasons. Many global habitats remain largely unexplored and inaccessible, harboring countless undiscovered organisms. These include deep oceans, vast underground cave systems, tropical rainforest canopies, and even the soil beneath our feet.
Microscopic organisms, including bacteria, archaea, fungi, and protists, also contribute to the difficulty. These tiny life forms are challenging to detect, differentiate, and classify using traditional methods. Their sheer volume, often invisible to the naked eye, makes comprehensive enumeration a daunting task.
The continuous process of scientific discovery also complicates efforts to arrive at a fixed number. New species are identified and described regularly, meaning the total count is constantly changing. Additionally, the definition of a species can be complex and subject to scientific debate, particularly for organisms that reproduce asexually or exhibit subtle genetic variations.
How Scientists Estimate Species Numbers
Since counting every single organism is impossible, scientists employ various methodologies to estimate the total number of species. One common approach involves extrapolating from well-studied areas. Researchers sample a defined region, count the species present, and then use statistical models to project that diversity across larger, similar habitats. This method assumes the sampled area represents the broader environment.
Another technique is the species-area relationship (SAR), which describes the correlation between habitat size and the number of species it contains. Larger areas generally support greater species diversity. By analyzing how species accumulate with increasing area, scientists use these models to estimate total species richness within an ecosystem or biome.
Taxonomic expertise and the analysis of discovery rates also contribute to estimates. Scientists track the rate at which new species are formally described. If the rate of new discoveries within a group slows, it might suggest most species in that group have been identified. However, this method can be influenced by varying research efforts and the existence of “cryptic” species that are morphologically similar but genetically distinct.
Molecular methods, particularly DNA sequencing, have revolutionized species identification and estimation, especially for microorganisms. Techniques like DNA barcoding and 16S rRNA gene sequencing allow scientists to identify species based on unique genetic material, bypassing traditional morphological classification. This approach is effective for microbes and other organisms where physical characteristics are insufficient for precise identification. Computer models also integrate existing data on species location, geographical range, and historical discovery patterns to predict where undiscovered species are most likely to reside.