is the biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
within a particular area, community
The term community has two distinct meanings:*a group of interacting people, possibly living in close proximity, and often refers to a group that shares some common values, and is attributed with social cohesion within a shared geographical location, generally in social units larger than a household...
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
, and is usually expressed as the species richness
Species richness is the number of different species in a given area. It is represented in equation form as S.Species richness is the fundamental unit in which to assess the homogeneity of an environment. Typically, species richness is used in conservation studies to determine the sensitivity of...
of the area. This can be measured by counting the number of taxa
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
(distinct groups of organisms) within the ecosystem (eg. families, genera
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
). However, such estimates of species richness are strongly influenced by sample size, so a number of statistical techniques can be used to correct for sample size to get comparable values.
Simpson's Diversity Index
- S is the number of species
- N is the total number of organisms
- ni is the number of organisms of species i
- S is the number of species. Also called species richness
- pi is the relative abundance of each species, calculated as the proportion of individuals of a given species to the total number of individuals in the community:
- ni is the number of individuals in each species; the abundance of each species
- N is the total number of all individuals
This is a parametric index of diversity that assumes that the abundance of species follows the log series distribution.
where each item gives the number of species predicted to have 1,2,3,....n individuals in the sample. The index is the α parameter. This is a useful index which has been widely used.
Rarefaction takes hypothetical subsamples of n organisms from the more-sampled region, and calculates the average number of species in such subsamples. This average can be compared to the number of species actually found in the less-sampled region
Alpha diversity has been measured both in extant ecosystems and in extinct communities. It is an especially useful measure to track Earth's past biodiversity because if it is used correctly, diversity can be measured independently of the fossil record which is not complete and therefore subject to errors. Examples include:
- A study of the alpha diversity of Triassic tetrapod families found that recovery after the Permo-Triassic extinction event took 30 million years
- A study on trilobites revealed that following the end-Ordovician extinction, trilobite alpha diversity was comparable to those of the Late Cambrian communities (Adrain et al. 2000)
Other measures of diversity
Alternative ways to measure biodiversity include:
- Beta diversity
Beta diversity is a measure of biodiversity which works by comparing the species diversity between ecosystems or along environmental gradients. This involves comparing the number of taxa that are unique to each of the ecosystems....
– species diversity between ecosystems; this involves comparing the number of taxa that are unique to each of the ecosystems
- Gamma diversity
Gamma diversity is used in ecology to refer to the total species diversity in an area or region of interest. The area of interest may be of very different sizes in different situations, so it has been difficult to reach consensus on what spatial scales are appropriate to quantify gamma diversity...
– taxonomic diversity of a region with several ecosystems
- Phylogenetic diversity
Phylogenetic diversity is a measure of biodiversity which incorporates taxonomic difference between species. It is defined and calculated as "the sum of the lengths of the all the branches that are members of the corresponding minimum spanning path", in which 'branch' is a segment of a cladogram,...
– or 'Omega diversity'. The differences or diversity between taxa
- Global diversity
The biodiversity of planet Earth is the total variability of life forms forms. Currently about 1.9 million species are known, but this is thought to be a significant underestimate of the total number of species....
– overall biodiversity of Earth
Diversity may not be congruent at all taxonomic levels and diversity patterns may vary depending on the type of diversity measured, as seen in the example to the left.