Life on Earth exhibits an astonishing array of forms, from the smallest microorganisms to the largest whales. Each distinct type of organism, capable of interbreeding and producing fertile offspring, is generally recognized as a species. Cataloging and studying these unique life forms helps scientists grasp the complexity of ecosystems and the intricate web of life that sustains our planet.
The Current Count
Scientists have formally described and named approximately 2.16 million species globally. This figure is continuously updated, though some described species may later be found to be synonyms, meaning they were already known but given a different name. After accounting for such duplicates, the number of truly unique described species might be closer to 1.5 to 1.9 million.
Insects represent the largest portion of this known biodiversity, with over 1 million described species, making them the most diverse group of animals. Plants also contribute significantly to the count, with around 374,000 to 391,000 described species, including vascular plants. The fungal kingdom, encompassing yeasts, molds, and mushrooms, has about 120,000 to 155,000 formally described species. Vertebrates, which include mammals, birds, reptiles, amphibians, and fish, make up a smaller but well-studied fraction, with roughly 62,000 to 75,923 described species.
How Species Are Identified
Identifying and describing a new species is a rigorous scientific endeavor. When a scientist encounters an organism suspected to be new, they undertake a detailed study of its physical characteristics, known as morphological analysis. This involves examining features like size, shape, and anatomical structures, comparing them to known species to determine if there are sufficient differences.
Alongside morphological examination, molecular evidence has become a standard tool in modern taxonomy. Scientists often sequence the organism’s DNA to compare its genetic makeup with that of related known species. If both morphological and genetic analyses indicate that the organism is distinct, a formal scientific paper is prepared, proposing it as a new species and assigning it a unique scientific name. This paper then undergoes peer review by other experts in the field, and if accepted, the new species is officially recognized and its description published in scientific literature. Museum collections play a crucial role by housing physical specimens, serving as reference material for comparative analysis by taxonomists worldwide.
Challenges in Discovery
Discovering and cataloging Earth’s species presents numerous challenges. Many habitats remain largely unexplored, such as the deep ocean, dense rainforest canopies, and subterranean environments. These remote and often extreme locations likely harbor countless undiscovered species, but their inaccessibility and cost limit discovery.
Many organisms are also inherently difficult to find due to their small size, elusive behavior, or cryptic nature, blending seamlessly with their surroundings. Microscopic life forms, like bacteria, archaea, and many fungi, are particularly challenging to identify and differentiate using traditional methods alone.
A global shortage of trained taxonomists, who specialize in species identification and classification, also slows the documentation of Earth’s biodiversity. This limited human resource, coupled with the immense volume of potential new species, makes the task a slow and demanding process.
Estimating Earth’s Biodiversity
Millions of species have been described, but scientists agree this is only a fraction of Earth’s total life forms. Estimates for the actual number of species range broadly, from a conservative 5 million to as many as 11 million eukaryotic species. Some projections, particularly those including microbial life, suggest even billions of species exist. This difference highlights the vast “unseen majority” of life.
Scientists employ various methods to arrive at these larger estimates. One common approach involves extrapolating from the rate at which new species are discovered in well-studied groups or areas. By analyzing species accumulation curves, which plot new species found over time, researchers project when discovery rates might plateau, indicating a more complete inventory.
Another method involves comparing the ratio of known to unknown species in thoroughly surveyed groups and applying that ratio to less-explored groups. Advances in molecular techniques, such as environmental DNA sequencing, are also revealing immense diversity, especially among microorganisms, that traditional methods often miss. Estimating total biodiversity is crucial for conservation and understanding Earth’s ecosystems.