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.
Biodiversity depends on two key components: species richness (the number of different species present) and species evenness (how evenly individuals are distributed among those species). Communities with similar richness can have very different diversity if one species dominates the population.
A forest contains ten tree species, but 90% of all trees belong to a single species. Although species richness is high, diversity is reduced because one species dominates the community.
A grassland contains four plant species, each contributing roughly 25% of the individuals. Although there are fewer species, the community is more evenly balanced and often has a higher diversity index than expected.
Simpson's Diversity Index reflects both richness and evenness, making it useful for comparing ecological communities rather than simply counting how many species are present.
There is no single measure that completely describes biodiversity. Different diversity indices highlight different characteristics of a biological community, so ecologists often report several metrics together.
Reporting several indices provides a more complete picture of community structure than any single metric alone.
| Feature | Simpson's Index | Shannon Index |
|---|---|---|
| Most sensitive to | Dominant species | Both common and rare species |
| Best for | Comparing community dominance | Overall biodiversity assessment |
| Effect of rare species | Relatively small | Greater influence |
| Typical use | Vegetation surveys, dominance studies | General ecological diversity comparisons |
Compare plant diversity across forests, grasslands, wetlands, or restoration sites.
Measure bacterial diversity in environmental, soil, marine, or human microbiome samples.
Assess river, lake, and stream health using aquatic macroinvertebrate or fish communities.
Monitor biodiversity changes following habitat restoration, land management, or species recovery programs.
Compare biodiversity before and after construction, mining, agriculture, or other environmental disturbances to evaluate ecological impacts.
Higher diversity values generally indicate healthier and more balanced ecological communities, while lower values suggest that one or a few species dominate the ecosystem.
Example: A monoculture plantation where nearly every individual belongs to a single species.
Example: A managed grassland with several species present but varying abundances.
Example: A mature rainforest where many species occur with relatively balanced abundances.
Interpretation should always consider the habitat, sampling method, and ecological context, since expected diversity naturally varies between ecosystems.
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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