Alpha, beta and gamma diversity. Robert Whittaker (1920-1980), American ecologist and researcher of succession and vegetation gradients, proposed three measures of ecosystem diversity: alpha (α), beta (β), and gamma (γ). Alpha is the diversity within an ecosystem which is generally described as the number of species. Beta diversity involves the comparison of different ecosystems in environmental gradients, for example, in a mountainous area within a coastal area. Beta diversity shows us the size of the change of species from one ecosystem to another. Gamma diversity refers to the total diversity of a region, i.e. the geographic diversity. It is the sum of the alpha diversity of various ecosystems.