3.1.8—Simpson's reciprocal index

Simpson’s reciprocal index D = N(N−1) ÷ Σn(n−1); higher D generally indicates greater richness and/or evenness.

N is total individuals and n is each species count. Compute every n(n−1), sum, then divide; compare samples with similar effort and habitat.

For counts 5 and 5, N=10 and D=90/40=2.25; equal abundances raise evenness.

This reciprocal form increases as dominance falls and diversity rises.

Do not compare indices from unequal sampling effort or silently switch to a different Simpson convention.