Diversity of Life

Abstract Diversity of life (also called biological diversity or biodiversity) is the variety of living systems. It may refer to extant organisms, but also to their diversity in the past. It is usually meant to encompass multiple levels of biological systems, from the gene level, through the level of populations and species, up to the communities of organisms and the ecosystems to which they belong. Habitat fragmentation and host specialisation are two of the major causes explaining the origin of the multiplication of living species in the course of evolutionary history. About 2 million species have been described to date and some 17 000–19 000 new entries are added every year to this list. The number of living species existing on Earth is unknown, current estimates favouring figures in the range of 8–12 millions. Key Concepts: There are three major ranks of biological diversity on Earth, at the gene, species and ecosystem level respectively. Species diversity is fostered mainly by geographical isolation and by strict interspecific relationships such as parasitism, food specialisation and plant pollination by insects. All animal phyla are represented in the sea and several phyla including echinoderms, ctenophores and brachiopods are exclusively marine, but only 15% of all living species described to date inhabit the sea. Insects represent more than one half of the total species‐level biological diversity on Earth. Key events in the history of life were the Cambrian ‘explosion’ and the invasion of land by plants (Middle Silurian), arthropods (Upper Silurian) and vertebrates (Upper Devonian). The history of biological diversity was punctuated by major critical events (mass extinctions) of which the most severe marks the end of the Palaeozoic era. Recent descriptions of previously unknown species include a few cetaceans and other large animals as well as small animals representing completely new classes or phyla, such as Loricifera, Cicliophora and Micrognathozoa. A sensible approach to estimating the number of existing species is to compare the number of described and undescribed species collected by prolonged sampling efforts in biologically rich and hitherto less investigated areas. Global estimates of existing biodiversity range from 5 to 130 million species, a figure of 8–12 millions being currently favoured.