How many different living things inhabit our planet? Providing a precise answer is a complex challenge. Earth’s ecosystems, from the deepest oceans to the highest mountains, harbor an astonishing diversity of organisms, many of which are yet to be discovered.
The Scale of Life’s Diversity
Scientists estimate the total number of species on Earth to be in the millions, with a significant gap existing between what is known and what is yet to be discovered. Approximately 1.7 to 2.2 million species have been formally described. Projections for the total number vary widely, from a few million to potentially trillions, especially for microorganisms. One widely cited estimate suggests around 8.7 million total species of eukaryotes, including animals, plants, fungi, and protists.
Within these broad categories:
Animals represent the largest group of described species, with over 1.5 million known, including about 1.05 million insects. Estimates suggest there could be as many as 7.77 million animal species in total.
Plants account for approximately 391,000 known species, with about 369,000 being flowering plants.
Fungi have around 120,000 to 155,000 described species, though estimates for their total number range from 2.2 to 3.8 million, and some suggest up to 12 million.
Protists, a diverse group of single-celled eukaryotes, have over 60,000 described species, but estimates for their total diversity can be as high as 1.2 to 10 million.
The greatest uncertainty lies with bacteria and archaea, where estimates range from millions to billions, with some studies suggesting up to a trillion microbial species.
Viruses, while not typically classified as living organisms, also represent immense diversity, with thousands of known types and potentially millions more undiscovered.
Why an Exact Count Remains Elusive
Providing a definitive number of species presents a challenge due to several complexities. Vast areas of the planet remain largely unexplored, such as deep oceans, subterranean ecosystems, and dense tropical rainforest canopies. Many organisms in these environments are microscopic or have cryptic lifestyles, making them difficult to detect. For instance, a significant portion of fungal species spend much of their life cycle hidden underground as networks of fine threads, making them challenging to find and identify.
Defining what constitutes a “species” can also be ambiguous, especially for microorganisms like bacteria and archaea. These microbes often exhibit genetic exchange and reproduction patterns that do not fit the traditional species concept for larger, sexually reproducing organisms. The logistical scale of biodiversity surveys required to catalog all life forms is immense, necessitating extensive resources, advanced technologies, and global effort from trained taxonomists.
Scientific Approaches to Estimation
Given the challenges of a direct count, scientists employ various methodologies to estimate the total number of species on Earth. One approach involves extrapolation from well-studied areas, using the ratio of known to unknown species to project diversity in less explored environments. For example, early estimates for insect diversity in tropical rainforests were based on new beetle species found on a single tree, then extrapolated to millions of trees.
Statistical modeling plays a significant role, identifying mathematical relationships between different taxonomic levels, such as the number of genera and families within a kingdom, to predict species numbers. The 2011 estimate of 8.7 million eukaryotic species, for instance, used patterns in the taxonomic classification system. Genetic sequencing techniques, particularly metagenomics, are revolutionizing the estimation of microbial diversity. By analyzing DNA directly from environmental samples, researchers identify unique genetic sequences from organisms never cultured or observed, providing insights into the vast unseen world of bacteria, archaea, and viruses.
Undiscovered Life and Biodiversity Hotspots
The majority of undiscovered species reside in specific regions that are either highly diverse or largely inaccessible. Tropical rainforests and coral reefs are recognized as biodiversity hotspots, characterized by a high concentration of unique species. These regions are complex and varied, providing numerous ecological niches that support diverse life forms. Much of their microscopic and invertebrate life remains undocumented.
Beyond these well-known hotspots, less-explored frontiers hold potential for new discoveries. Deep-sea environments, including hydrothermal vents and abyssal plains, harbor unique ecosystems adapted to extreme conditions. Polar regions, subterranean caves, and soil are also thought to contain millions of undescribed species. These hidden realms represent significant gaps in our current understanding of global biodiversity, contributing to the ongoing uncertainty about the exact number of living things.
The Ever-Changing Inventory of Life
The inventory of life on Earth is not static; it is a dynamic system continuously shaped by species discovery and extinction. Each year, thousands of new species are identified and formally described by scientists, adding to the known catalog of life. For instance, approximately 2,000 new plant species are discovered annually. This ongoing process of exploration and classification means that our understanding of life’s diversity is always expanding.
Despite these discoveries, the planet faces a concerning rate of species extinction. Many species are being lost due to human activities, often before they can even be discovered or studied. The total count of living things is thus increasing with new discoveries but also decreasing as species disappear, sometimes unnoticed. This highlights the constant flux in Earth’s biological richness.