Table of Contents
What Is Simpson's Diversity Index?
Simpson's Diversity Index measures the probability that two randomly selected individuals from a community belong to different species. It accounts for both species richness (number of species) and evenness (relative abundance). The index was proposed by Edward H. Simpson in 1949 and remains one of the most widely used diversity metrics in ecology.
There are three common forms: Simpson's D (dominance), 1-D (diversity), and 1/D (reciprocal). The Gini-Simpson index (1-D) ranges from 0 to 1, where higher values indicate greater diversity. The reciprocal index (1/D) represents the effective number of equally common species and ranges from 1 to the total number of species.
Formulas
Interpretation
| 1-D Value | Diversity Level |
|---|---|
| 0.0 - 0.3 | Low diversity (dominated by few species) |
| 0.3 - 0.6 | Moderate diversity |
| 0.6 - 0.8 | High diversity |
| 0.8 - 1.0 | Very high diversity (even distribution) |
Frequently Asked Questions
How is Simpson's Index different from Shannon entropy?
Simpson's index gives more weight to dominant species, while Shannon entropy gives more weight to rare species. Simpson's is less sensitive to species richness and more sensitive to evenness. In practice, both are used together to provide a complete picture of community diversity.
Why are there different versions of Simpson's Index?
Simpson originally defined D as a dominance measure (higher D = lower diversity). Because this is counterintuitive, ecologists created the complement (1-D) where higher values mean more diversity, and the reciprocal (1/D) which represents the effective number of species.