Simpson's Diversity Index Calculator

Simpson's Diversity Index (1 − D) is a measure of biodiversity that accounts for both the number of species present and their relative abundance. It ranges from 0 to 1, where values closer to 1 indicate higher diversity and values closer to 0 indicate lower diversity or dominance by a single species. It was introduced by Edward H. Simpson in 1949.

First, calculate Simpson's Index D = Σ(nᵢ / N)², where nᵢ is the count of each species and N is the total number of individuals across all species. Then compute the Diversity Index as 1 − D. A value of 1 − D close to 1 means high diversity; close to 0 means low diversity. This calculator does all the arithmetic for you automatically.

Simpson's Index D (also called the Dominance Index) represents the probability that two individuals picked at random from the same community belong to the same species. A high D means low diversity. Simpson's Diversity Index (1 − D) inverts this so that higher values always indicate higher biodiversity, making it more intuitive to interpret.

A high value (close to 1) means the community is highly diverse — there are many species and/or their individuals are spread relatively evenly among species. This suggests a healthy, stable ecosystem with no single species dominating the population.

A low value (close to 0) indicates low diversity, meaning the community is dominated by one or a few species. This could suggest ecological stress, habitat degradation, or an invasive species outcompeting others. A value of 0 would mean all individuals belong to a single species.

The Reciprocal Index is calculated as 1 divided by D. Its minimum value is 1, representing a community with only one species, and its maximum value equals the number of species (S) when all species are equally abundant. It expresses diversity in a more intuitive unit — the 'effective number of species'.

This calculator supports up to 10 species directly in the input fields. For most ecological fieldwork and classroom exercises, 10 species is sufficient. If you need more, you can calculate Simpson's Index manually by summing (n/N)² for each additional species and entering the combined result.

The formula is D = Σ(nᵢ / N)², where nᵢ is the number of individuals of species i and N is the total number of all individuals. Equivalently, an unbiased version uses D = Σ[nᵢ(nᵢ − 1)] / [N(N − 1)]. This calculator uses the proportional (Σpᵢ²) form, which is the most widely cited version introduced by Simpson in 1949.