The total number of animal species, known as zoological biodiversity, is not a fixed figure but a dynamic and constantly changing measurement. Scientists must differentiate between the species that have been formally identified and named and the much larger number that are theorized to exist but remain undiscovered. Understanding this difference is fundamental to grasping the immense scale of life across global ecosystems.
The Current Count of Documented Species
The count of animal species currently cataloged, named, and officially documented by the scientific community rests conservatively between 1.5 and 2.16 million species. This figure represents the known biological inventory accumulated over centuries of taxonomic work. This documented number is heavily skewed toward a few dominant groups, illustrating a significant imbalance in the known animal kingdom.
The phylum Arthropoda, which includes insects, spiders, and crustaceans, accounts for the overwhelming majority of known species. More than 1.05 million species, primarily insects, have been described, making up well over half of all officially documented animal life. In stark contrast, all vertebrates—mammals, birds, reptiles, amphibians, and fish—combined account for only about 65,000 known species. This disparity shows that the animals most familiar to people represent a tiny fraction of the world’s documented biodiversity.
Scientific Methods for Estimating Total Diversity
The known count is vastly different from the total number of species thought to exist, which is estimated to be many millions higher. Ecologists and taxonomists use specialized mathematical models to project the true scope of global animal diversity. One widely cited scientific estimate places the total number of animal species at approximately 7.77 to 8.7 million.
One key method involves extrapolating from the ratio of known to unknown species in well-studied groups. For example, intensive sampling of insects, such as beetles, within a limited area determines the proportion of species new to science. This ratio is then scaled up to estimate the total number of species across similar, less-studied environments globally, leading to some projections as high as 30 million.
Another estimation technique uses a statistical approach based on the hierarchical structure of taxonomy. This method analyzes the relationship between the number of taxa at each classification level (phyla, classes, orders, and families). By analyzing how these higher-level groups relate to the number of described species within them, scientists can statistically model and predict the total number of species that must exist. This approach provides a rigorous, non-field-based estimate of the total species count.
Organizing Life: The Major Animal Phyla
Scientists organize this immense collection of life using a hierarchical system called taxonomy. A phylum represents one of the largest and most fundamental divisions of the animal kingdom, categorizing “types” of animals based on fundamental body plans and evolutionary relationships. While the animal kingdom contains around 35 recognized phyla, a small number of these groups contain the vast majority of all animal species.
The phylum Arthropoda is the most species-rich, encompassing insects, arachnids, and crustaceans. The second-largest phylum, Mollusca, includes snails, clams, and octopuses, contributing over 100,000 described species. Other major phyla include Chordata (all vertebrates) and Nematoda (roundworms). Although Nematoda has only about 20,000 described species, they are estimated to number in the millions due to their prevalence in soil and water.
Habitats Driving New Species Discovery
The total estimated number remains much higher than the documented count because immense regions of the globe remain poorly studied, acting as reservoirs for undiscovered life. Tropical rainforests, especially their towering canopies, are massive biodiversity hotspots that are difficult to access and survey systematically. These environments still yield thousands of new species annually as researchers employ new climbing and sampling techniques.
The deep sea and abyssal zones are significant frontiers of discovery, as vast portions of the ocean floor have never been mapped or explored. New deep-sea expeditions consistently reveal novel marine species adapted to extreme pressure and darkness. Soil and sediment ecosystems, populated by microfauna like nematodes and tiny arthropods, also represent an immense, largely invisible domain of life. The ongoing process of discovery is increasingly aided by molecular techniques, which distinguish between morphologically similar species and reveal hidden genetic diversity.