—
—
—
100
—
—
—
100
The Simpson Diversity Index Calculator computes the Simpson index and its complement and reciprocal forms from species abundance data. The Simpson index represents the probability that two randomly chosen individuals belong to the same species. Its complement (1-D) represents the probability that they belong to different species, making it an intuitive measure of diversity.
Enter the abundance for up to three species. The calculator returns D (concentration), 1-D (diversity), and 1/D (reciprocal index), along with the total number of individuals. This simplified version works for up to three species and serves as a learning and quick-estimation tool.
The Simpson index (D) is calculated as:
D = SUM(pi²)
Where pi = ni/N is the proportion of species i. D represents dominance or concentration and ranges from 1/S (perfect evenness) to 1 (one species dominates completely).
Two common transformations make D a diversity measure:
For finite samples, the unbiased form uses n(n-1)/(N(N-1)) instead of pi², but the pi² form is commonly used and is computed here.
Inputs
Results
With fairly even abundances (40, 35, 25), D = 0.345 and 1-D = 0.655. The reciprocal 2.90 is close to the maximum of 3 (for 3 species), indicating good diversity.
Inputs
Results
When one species dominates at 90%, D = 0.815 (high dominance), 1-D = 0.185 (low diversity), and 1/D = 1.23 (effectively just over one species).
The Simpson index gives more weight to common (abundant) species and is less sensitive to rare species, while the Shannon index weights all species by their proportion. Simpson is easier to interpret probabilistically (probability of same-species encounter). Shannon is more sensitive to changes in rare species. Both are widely used and often reported together to provide complementary information.
D (Simpson concentration) measures dominance and decreases with diversity, which can be counterintuitive. 1-D (Gini-Simpson index) increases with diversity and ranges from 0 to 1, making it easy to interpret. 1/D (reciprocal) also increases with diversity and can be interpreted as the effective number of equally common species. Always clearly state which form you are using in publications.
The effective number of species (true diversity) is the number of equally abundant species that would produce the same diversity index value as the observed community. For Simpson, this is 1/D. A community with 10 species but heavy dominance might have an effective number of species of only 3, meaning its diversity is equivalent to 3 equally common species.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
How helpful was this calculator?
Be the first to rate!
