Simpson D (dominância)
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
About this calculator
The Simpson Dominance Index (D) measures the probability that two randomly selected individuals from a community belong to the same species. It is an indicator of dominance, where high values signal low diversity, with few species dominating. The formula is D = Σ(pi²), where pi is the proportion of individuals of the i-th species. It is calculated by summing the squares of each species' proportions.
This index is widely used in ecology to compare community structures, such as forests or insect assemblages. When D is close to 1, the community is dominated by a few species; close to 0, high diversity with equitable distribution. It is common to use the complement 1-D (Simpson Index) or 1/D (inverse) to express diversity.
Calculation requires relative abundance data. It is sensitive to sample size: small samples tend to underestimate D. It also does not distinguish between richness and evenness, being more influenced by abundant species. It should be interpreted together with other indices, such as Shannon-Wiener.
Applications include environmental monitoring, conservation studies, and agriculture. For example, assessing the effect of a disturbance on the dominance of invasive species. Cautions: ensure representative sampling and avoid comparisons between communities with very different sample sizes.
Frequently asked questions
What does a high D value mean?
It indicates that the community is dominated by few species, meaning low diversity.
What is the difference between Simpson's Index (D) and Shannon?
Simpson gives more weight to abundant species, while Shannon considers richness and evenness. Simpson is more sensitive to changes in dominant species.
How do I interpret the complement 1-D?
1-D is the probability that two individuals belong to different species, a measure of diversity. Higher values indicate more diverse communities.
Can I use D to compare communities with different sample sizes?
It is risky because D is influenced by the number of individuals sampled. Prefer to standardize sampling effort.
Can the index exceed 1?
No, D ranges from 0 to 1, since the sum of pi² is at most 1 (when one species completely dominates).