Species Composition and Diversity

Two forests can both have 10 species and still feel completely different. In one, each species has roughly 10% of the individuals — walk through it and you bump into something new constantly. In the other, one species makes up 91% of all individuals and the other nine are barely there — walk through it and you mostly see the same tree over and over. Counting species, called richness, misses this difference entirely. Diversity needs a second ingredient: evenness, how equally individuals are distributed across those species.

Simpson's index folds richness and evenness into one number. For each species, find its proportion of the total community (its count divided by the total count), square that proportion, and add up the squared proportions across all species. That sum tells you the probability that two randomly picked individuals belong to the same species — high when one species dominates. Subtract that sum from 1, and D = 1 - sum(p_i²) now rises toward 1 as diversity increases: more species, more evenly spread, means any two individuals you grab are less likely to match.

This sits inside the bigger picture of community structure, which also includes which species are present and how they interact, but diversity gives a quick, comparable snapshot of a community's makeup. It matters because diversity often tracks ecosystem stability and resilience — a community that depends heavily on one dominant species is more vulnerable to a crash than one where several species share the load. That's why a richer-looking community can actually score as less diverse than a simpler, more balanced one.