Understanding Biodiversity Indices

In ecology, a diversity index is a quantitative measure that reflects how many different types (such as species) there are in a dataset, and simultaneously takes into account how evenly the basic entities (such as individuals) are distributed among those types.

Richness vs. Evenness

Species Richness is simply the number of different species present in a community. A forest with 50 species is 'richer' than one with 10.

Species Evenness compares the similarity of the population size of each species present. If one species dominates 90% of the forest, the evenness is low. If all species have roughly equal populations, the evenness is high.

How Simpson's Index Works

Simpson's Index takes both richness and evenness into account. The formula calculates the probability that two individuals randomly selected from a sample will belong to the same species. A highly diverse ecosystem will have a low probability (a low D value), which is why we often use 1 − D to represent the final score, where a higher score means higher diversity.