Measure biodiversity and species dominance in ecological communities. Calculate Simpson's D, 1-D, reciprocal index, Shannon diversity, and evenness.
Last updated: March 2026
Simpson's Diversity Index is a measure of biodiversity that quantifies the diversity of species in a community or ecosystem. Originally developed by Edward Hugh Simpson in 1949, it takes into account both the number of species present (richness) and the relative abundance of each species (evenness).
The index exists in several forms. Simpson's D measures dominance (the probability that two randomly selected individuals belong to the same species). Simpson's Index of Diversity (1-D) inverts this to measure diversity directly. The reciprocal form (1/D) represents the "effective number of species" — the number of equally common species needed to produce the observed diversity.
Values range from 0 to 1 for 1-D (higher = more diverse), while the reciprocal ranges from 1 to the number of species. A community dominated by one species has low diversity (1-D close to 0), while a community with many equally abundant species has high diversity (1-D close to 1).
Forest Tree Survey Example
This forest has moderately high diversity (0.74) with an effective species count of ~3.88. The community is not dominated by a single species.
D measures dominance (probability two individuals are the same species). 1-D is the diversity index (0-1 scale, higher = more diverse). 1/D is the reciprocal index representing effective number of equally common species.
Use 1-D for a simple 0-1 diversity score that's intuitive. Use 1/D when you want results in units of 'effective species'. Use D when specifically interested in dominance. All are valid and commonly used.
Simpson's index is less sensitive to rare species and emphasizes dominant species. Shannon's index weights all species more equally. Simpson's is simpler to interpret; Shannon's is more sensitive to richness changes.
Evenness (Pielou's E) measures how equal species abundances are. E = 1 means all species have identical counts. E near 0 means one species dominates. It separates the effects of richness from abundance distribution.
Yes! Simpson's index is standardized (0-1 or effective species count), making it ideal for comparing different communities, habitats, or time periods. Just ensure sampling methods are consistent.
There's no universal 'good' value — it depends on the ecosystem. Tropical rainforests typically have 1-D > 0.9, while agricultural monocultures approach 0. Compare to similar ecosystems or track changes over time.
Minimum 2 species are required. For reliable results, aim for at least 50-100 total individuals across multiple species. Larger samples give more accurate estimates of true diversity.
Simpson's D uses n(n-1)/N(N-1) which corrects for finite sample size, making it relatively unbiased. However, larger samples still provide more accurate estimates of the true population diversity.
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