- biological diversity
- the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are a part; this includes diversity within species, between species, and of ecosystems."
- biodiversity properly refers to the variety of living organisms.
- Living organisms play central roles in the cycles of major elements (carbon, nitrogen, and so on) and water in the environment, and diversity specifically is important in that these cycles require numerous interacting species.
- 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.
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 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…