Biological diversity, or biodiversity, is the number and variety of species and other taxa in any locality, ecosystem, region or the biosphere. The commonest measure of biodiversity is the number of species present in the area of interest. Species number, species richness and species diversity are used almost interchangeably in the ecological literature, although diversity may be employed when considering both the species number and their abundances in a sample (see below) (Calow 1998, 1999).

Learning how many species there are in a particular locality has formed a major basis of enquiry in ecology. Once descriptive data are available another basic question becomes why numbers of species differ in different localities, on different plant species, or in different lakes? With increasing knowledge we can begin to ask questions about the total number of species on this earth, and on the patterns of their distribution in space and time. Thus, the subject of biological diversity reaches the local, to regional, to global scale, encompassing species richness in component communities to the biogeography of species and interactions among species and the physical environment.