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#Simpson index

3 public questions tagged with this topic.

BIODIVERSITY

Which index represents dominance?

Simpson’s D, in its dominance form, is calculated as the sum of squared relative abundances, Σpᵢ². Squaring gives disproportionate weight to common species, so D becomes large when one or a few taxa account for most individuals. It can also be viewed as the probability that two randomly sampled individuals are conspecific. Both interpretations make D a dominance measure: high D indicates concentrated abundance and therefore low diversity. Shannon’s index incorporates richness and evenness through −Σpᵢ ln pᵢ but is not conventionally labelled a dominance index. The Gini–Simpson index, 1 − D, reverses the scale and measures the probability that two individuals belong to different species. Alpha diversity is a spatial category—diversity within a local community—rather than a particular mathematical index. For example, two communities may each contain ten species, yet the one in which a single species forms 90% of all individuals will have much higher D. This sensitivity to abundant taxa makes Simpson’s D relatively robust to rare species missed by sampling, but also means it describes dominance more strongly than simple richness.

Ref: NCERT Biology Class 12, Ch. 15 Biodiversity and Conservation

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BIODIVERSITY

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, respectively, while D − 1 would be non-positive over the usual range and is not the standard diversity transformation. For a finite sample, an unbiased form may use counts as 1 − Σnᵢ(nᵢ − 1)/[N(N − 1)], but its probabilistic interpretation remains the same. Naming the formula matters because “Simpson’s index” is used inconsistently across texts. Stating 1 − D removes that ambiguity and makes larger numerical values correspond intuitively to greater diversity.

Ref: NCERT Biology Class 12, Ch. 15 Biodiversity and Conservation

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BIODIVERSITY

Which one is used to assess similarity between two communities?

Sørensen’s coefficient compares the composition of two communities using shared species. For presence–absence data it is commonly calculated as Cₛ = 2c/(a + b), where a and b are the numbers of species in the two communities and c is the number occurring in both. The coefficient ranges from 0, indicating no shared species, to 1, indicating identical species lists. Because shared species are weighted twice, the numerator corresponds to their contribution to both lists. It is useful for comparing sites, seasons, successional stages, or treatment plots and is closely related to beta diversity: lower similarity generally implies greater species turnover. Shannon and Simpson indices, by contrast, summarize diversity within a community from relative abundances; they do not directly state how many species are shared between two sites. Alpha diversity likewise names the within-site scale rather than a pairwise comparison statistic. Sørensen’s measure can be sensitive to sampling completeness because undetected rare species appear as absences, so comparable effort is important. Abundance-based extensions exist, but the classic coefficient specifically answers the compositional similarity question.

Ref: NCERT Biology Class 12, Ch. 15 Biodiversity and Conservation

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