Gini-Simpson index is represented by:
Simpson’s dominance index D = Σpᵢ² measures the probability that two randomly selected individuals belong to the same species. Taking its complement gives 1 − D, the probability that the two individuals belong to different species. This complementary measure is called the Gini–Simpson index and increases with both richness and evenness. A community dominated by one species has D near 1 and 1 − D near 0; an increasingly even, species-rich community has smaller D and a Gini–Simpson value approaching 1. The alternatives 1/D and D represent the reciprocal Simpson index and dominance index, respect
Ref: NCERT Biology Class 12, Ch. 15 Biodiversity and Conservation