Measure how diverse a community is using Simpson’s diversity index, from the counts of up to five species.
How it works
The formula is D = 1 – Σ(nᵢ(nᵢ-1)) ÷ (N(N-1)), reported in its complement form so a higher number means higher diversity. Four species with 10 individuals each gives a diversity of 0.769; one dominant species (50) alongside three rare ones (2 each) gives only 0.203.
What this does not include
This is distinct from this site’s biodiversity index calculator, which uses Shannon’s index — a different formula that weights rare species more heavily. Simpson’s index weights dominance more heavily instead, so the two can rank the same community differently.
How to use this calculator
- Enter the count for each species present (up to five).
A worked example
Four species with equal counts of 10 each: Simpson’s diversity index = 0.769231 — high diversity, reflecting perfect evenness.
Four species with uneven counts (50, 2, 2, 2): diversity index = 0.202597 — much lower, since one species dominates the community.
What the variables mean
| Variable | Meaning |
|---|---|
| Species counts | Population count for each species present |
Edge cases worth knowing
Simpson’s index ranges from 0 (no diversity) to just under 1 (maximum diversity) — it’s typically interpreted as the probability that two randomly selected individuals belong to different species.
A single species present makes the index meaningless for measuring diversity, so the calculator declines to show a result for that degenerate case.
Why does one dominant species lower the diversity score so much?
Simpson’s index is more sensitive to the probability that two randomly picked individuals belong to the same species — when one species dominates, that probability is high, pulling the diversity score down.
What does a diversity score of exactly 0 mean?
Every individual belongs to the same single species — no diversity at all.
How is this different from Shannon’s diversity index?
Both measure diversity but weight it differently — Shannon’s index is more sensitive to rare species, while Simpson’s index is more sensitive to how dominant the most common species is.
