Animal Species and Biodiversity

A species is the fundamental unit by which biologists name and compare living things. The most widely taught definition, the biological species concept advanced by the evolutionary biologist Ernst Mayr, describes a species as a group of organisms that can interbreed in nature and produce fertile offspring, and that is reproductively isolated from other such groups. Modern practice uses several complementary species concepts, including morphological, ecological, and phylogenetic definitions, because the biological concept is difficult to apply to fossils, asexual organisms, and microbes.

Biodiversity is typically described at three interlocking levels. Genetic diversity refers to variation within a species, the raw material of evolution and the reservoir that allows populations to adapt to disease, drought, and other pressures. Species diversity refers to the number and abundance of species in a given area, the level most familiar to the public. Ecosystem diversity refers to the range of habitats and ecological processes across a landscape or the planet as a whole, from coral reefs and mangroves to grasslands, boreal forests, and abyssal plains.

The Catalogue of Life, a consortium maintained in partnership with the International Union for Conservation of Nature and Species 2000, compiles validated species checklists from hundreds of taxonomic databases. Its 2024 release included roughly 2.16 million described species across all kingdoms of life, of which approximately 1.5 million are animals. A highly cited statistical analysis by Camilo Mora and colleagues, published by the Royal Society in 2011, extrapolated from the slowing pace of new higher-rank discoveries and estimated that roughly 8.7 million eukaryotic species exist on Earth, of which about 7.8 million are animals. Uncertainty around that figure is large, and revised estimates published since continue to debate both the total and how quickly it is converging.

Whichever estimate one prefers, the implication is the same: the majority of animal species alive today have not yet been formally described by science. Most of them are small invertebrates in tropical forests, in soil, and in the deep ocean. Understanding who they are, where they live, and how they interact is the central scientific task of modern biodiversity research, and it is increasingly urgent as habitats are transformed by land use and climate change.