'Biodiversity' can be used as a synonym for living nature, with an emphasis on its complexity, at genetic, species and ecosystem levels. This complexity is vital in the maintenance of healthy ecosystem functioning, and it is important to recognise this when developing policies to protect the natural environment.
The genetic diversity inherent in most species provides the raw material to respond rapidly to changed circumstances. Change is, of course, the normal state of affairs in the living world. What makes our present situation unique is the rapidity and scale of the change. Our fragmentation and destruction of habitats constitutes a massive uncontrolled experiment in ecology and genetics. Knowledge of population structures, i.e. the distribution and amount of genetic variation, of a wide range of organisms is necessary, as is a much deeper understanding of the biological significance of different sorts of variation.
Although much debate on biodiversity focuses on 'species', it is important to note that this is not a standard unit. The way species are defined differs between groups and between taxonomists. However, despite these ambiguities, measures of species richness and distribution are important tools in assessing the state of the environment and the direction and speed of change. The number of described species is now around 1.7 million. The estimated total number of species in existence ranges in order of magnitude from around 10 million to 100 million.
Species are unevenly distributed over the Earth's surface. Most vertebrate species have small ranges, and these narrowly-distributed species tend to co-occur in "centres of endemism", which are concentrated in the tropical regions. This means there are some areas of the world, and some countries, with much higher levels of species richness than others.
Although patterns of terrestrial diversity are better understood than those in the aquatic realm, it is clear that there are also areas of high marine biodiversity such as coral reef ecosystems.
Numbers of described species by taxonomic group (IUCN Red List version 2010.3)
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