Simpson's diversity index is one of the two most widely used biodiversity metrics in ecology, alongside the Shannon index. It measures how likely two randomly chosen individuals from a community are to belong to different species — a single number that captures both how many species are present and how evenly individuals are distributed among them.
How the calculation works
Simpson's index starts with the dominance score D = Σp_i², where p_i is each species' share of the total population. Because you square each proportion before summing, abundant species contribute disproportionately more to D than rare ones — a single species at 90% abundance contributes 0.81 to D on its own, while ten species at 10% each only contribute 0.10 combined. This weighting is the key mathematical difference from the Shannon index, which uses a logarithmic weighting that treats rare species somewhat more generously.
The calculator reports 1−D as the headline result because it reads more intuitively as a "diversity" number: it climbs toward 1 as diversity increases, whereas D climbs toward 1 as dominance (low diversity) increases. The reciprocal 1/D is a third useful lens — it rescales the score into an "effective number of species," so a score of 4 means the community behaves, in diversity terms, like four perfectly even species.
Reading your species counts
Enter each species you identified along with a raw count of individuals — you don't need to convert to percentages first, the calculator does that automatically. Species names are optional and only used to label the Dominance tab's abundance table; the math only depends on the counts. You need at least two species with a positive count, since a single species always has D = 1 and 1−D = 0, which isn't a meaningful diversity comparison.
The Dominance tab shows exactly how much each species contributes to D via its p_i² term, which makes it easy to spot which single species (if any) is driving your overall score down. The Reciprocal tab converts the same result into an effective-species count that's often easier to communicate to a non-technical audience than a decimal dominance index.
Simpson's index vs. the Shannon index
Both indices summarize richness (how many species) and evenness (how uniformly distributed) into a single number, but they weight rare species differently. Simpson's index (Σp_i²) is dominated by the most abundant species, since squaring proportions shrinks the contribution of rare species much faster than the logarithmic weighting used by Shannon's index (−Σp_i·ln p_i). In practice this means Simpson's index is more sensitive to changes in the dominant species, while Shannon's index is more sensitive to the presence of additional rare species. Neither is strictly "more correct" — ecologists often report both, since they can occasionally rank two communities in different orders when abundance patterns diverge sharply between them.