alpha beta gamma & diversity index

The term alpha diversity (α-diversity) was introduced by R. H. Whittaker^[1][2]

• together with the terms beta diversity (β-diversity) and gamma diversity (γ-diversity).
•  Whittaker's idea was that the total species diversity in a landscape (gamma diversity) is determined by two different things, the mean species diversity in sites or habitats at a more local scale (alpha diversity) and the differentiation among those habitats (beta diversity).

A diversity index is a quantitative measure

•  that increases when the number of types into which a set of entities has been classified increases, and obtains its maximum value for a given number of types when all types are represented by the same number of entities.
• When diversity indices are used in ecology, the entities of interest are usually individual plants or animals, and the types of interest are species or other taxa.
•  In demography, the entities of interest can be people, and the types of interest various demographic groups, and in information science, the entities can be characters and the types the different letters of the alphabet.
• The most commonly used diversity indices are simple transformations of the effective number of types (also known as 'true diversity'), but each diversity index can also be interpreted in its own right as a measure corresponding to some real phenomenon (but a different one for each diversity index).^[