The magnitude of biodiversity

Biodiversity is most frequently quantified as the number of species. Estimates of the number of species currently living on Earth range widely, largely because most living species are microorganisms and tiny invertebrates, but most estimates fall between 5 million and 30 million species. Roughly 1.75 million species have been formally described and given official names. Insects comprise over half of the described species, and ~3/4 of known animal species. The number of undescribed species is undoubtedly much higher, however. Particularly in inaccessible environments, and for inconspicuous groups of organisms, collecting expeditions routinely discover many undescribed species. Estimates of the total numbers of species on Earth have been derived variously by extrapolating from the ratios of described to previously unknown species in quantitative samples, from the judgment of experts in particular taxonomic groups, and from patterns in the description of new species through time. For most groups of organisms other than vertebrates, such estimates are little more than educated guesses, explaining the wide range in estimates of global species diversity. Since insects are essentially absent from the sea, the species diversity of the oceans is generally considerably lower than terrestrial ones.

Species can be grouped on the basis of shared characteristics into hierarchical groups, or taxa, reflecting their shared evolutionary history. At the highest level of classification (or deepest branches in the evolutionary tree of life) organisms are divided into three Domains: 1) the Bacteria, which are microorganisms lacking a cellular nucleus or other membrane-bound organelles; 2) the relatively recently discovered Archaea, microorganisms of primarily extreme environments such as hot springs, which are superficially similar to Bacteria but fundamentally different at biochemical and genetic levels; and 3) the Eukarya, which include all other organisms based on nucleated cells. The Eukarya includes the four "kingdoms", the protists, animals, plants, and fungi. Each of the eukaryotic kingdoms in turn is divided into a number of phyla. At this higher taxonomic level, the oceans are far more diverse than those on land, likely reflecting the marine origins of life on Earth. Nearly half the phyla of animals occur only in the sea (e.g., the sea stars and other echinoderms), whereas only one (the obscure Onychophora, or velvet worms) is restricted to land.