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. Gamma diversity has therefore also been defined so that it can be measured for an existing dataset at any scale. If the area of interest is large, it is in practice impossible to carry out a complete inventory of its species. When the available sample is much smaller than the area of interest, many species that exist in the area may not be found and the gamma diversity of the entire area is underestimated by the gamma diversity of the sample. The degree of underestimation can be estimated from a species-area curve

Species-area curve

In ecology, a species-area curve is a relationship between the area of a habitat, or of part of a habitat, and the number of species found within that area. Larger areas tend to contain larger numbers of species, and empirically, the relative numbers seem to follow systematic mathematical...

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Gamma diversity consists of two components, alpha diversity

Alpha diversity

Alpha diversity is the biodiversity within a particular area, community or ecosystem, and is usually expressed as the species richness of the area. This can be measured by counting the number of taxa within the ecosystem...

 (α-diversity) and beta diversity

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....

(β-diversity):

γ=α×β

The quantification of alpha and beta diversity requires that the dataset consists of subunits at a more local scale than the entire area of interest. The subunits can be sampling units that were already used in the field or grid cells that are delimited just for the purpose of analysis. Often the subunits are thought to represent different habitats within the landscape of interest. Alpha diversity is the mean species diversity per subunit, or mean species density at the subunit scale. Beta diversity quantifies how many subunits there would be, if all subunits had the observed mean species density but shared no species.