Simpson Diversity Index

Simpson Diversity Index measures how likely it is that two individuals chosen at random from the same community belong to different species. The term \(\sum p_i^2\) adds up the probabilities that both picks come from the same species, so subtracting from 1 leaves the probability of a mismatch. It is especially sensitive to dominant species, which is why communities with strong dominance by one or a few species get lower diversity scores.

Use this when comparing biodiversity across habitats, sample plots, or time points in ecology and conservation studies. It is useful when you want a single number that reflects both species richness and how evenly individuals are distributed among species.

• Using the raw species counts instead of converting them to proportions first
• Forgetting to square each proportion before summing
• Confusing Simpson diversity index with Simpson dominance or the reciprocal form of the index