Simpson's Diversity Index Calculator

Simpson's Diversity Index (1-D) is a measure of biodiversity that accounts for both the number of species (richness) and the relative abundance of each species (evenness). It was introduced by E.H. Simpson in 1949. Values range from 0 (no diversity) to 1 (infinite diversity), where higher values indicate a more diverse community.

First, calculate Simpson's Index D = Σ(pᵢ²), where pᵢ is the proportion of individuals belonging to species i (i.e., nᵢ/N). Then the Simpson's Diversity Index is 1-D. For example, if D = 0.35, then the diversity index is 1 - 0.35 = 0.65.

Simpson's Index D represents the probability that two randomly selected individuals belong to the same species — so a higher D means less diversity. The Diversity Index (1-D) inverts this so that higher values mean more diversity. The Reciprocal Index (1/D) gives the effective number of species, where a value of 4 means the community behaves like 4 equally abundant species.

A high value of 1-D (close to 1) indicates a highly diverse community where many species are present in relatively equal abundances. This suggests a healthy, well-balanced ecosystem with no single species dominating the population.

A low value of 1-D (close to 0) indicates low biodiversity, typically meaning one or a few species dominate the community while others are rare. This can signal ecological stress, habitat degradation, or a naturally specialized environment.

You need at least two species to get a meaningful result, though technically the formula works with one species (giving D = 1 and 1-D = 0). The more species you include with accurate population counts, the more reliable your diversity estimate will be.

Yes. While originally developed for ecology, Simpson's Index can be applied to any domain where you want to measure diversity or concentration — such as linguistics (word variety in text), economics (market diversity), or demographics (ethnic or cultural diversity in populations).

The Reciprocal Index ranges from 1 (minimum diversity — only one species present) upward with no theoretical upper limit, though in practice it is bounded by the number of species. A value of 1/D = S (where S is the number of species) would indicate perfect evenness — all species equally abundant.