#Shannon index

BIODIVERSITY

Shannon index value is lowest when:

Shannon diversity declines when abundance becomes concentrated in a single species. In H′ = −Σpᵢ ln pᵢ, species with moderate and similar proportions collectively create high uncertainty about the identity of a randomly selected individual. If one species dominates, its pᵢ approaches 1 and the remaining proportions approach 0; the identity of a sampled individual becomes highly predictable, so H′ becomes small. In the limiting case of only one species, p = 1 and H′ = −1 ln 1 = 0. Equal abundance instead maximizes H′ for a given richness, with H′max = ln S. High richness also tends to increase

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

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BIODIVERSITY

The value of Shannon’s index (H) increases with:

Shannon diversity is H′ = −Σpᵢ ln pᵢ, where pᵢ is the proportional abundance of each species. The index increases when more species are added and when individuals are distributed more evenly among the species already present. A rare species contributes relatively little because its pᵢ is small, while a strongly dominant species lowers overall uncertainty: a randomly chosen individual becomes easier to predict. For a fixed richness S, H′ reaches its maximum when every species has abundance 1/S, giving H′max = ln S. Evenness can therefore be expressed as J′ = H′/ln S. Dominance and unevenness mo

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: lo

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

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BIODIVERSITY

The Shannon diversity index (H) values for communities C1 and C2 are:

Shannon diversity is H′ = −Σpᵢ ln(pᵢ), so both richness and abundance evenness affect the result. Applying each community's relative frequencies to that expression yields approximately 1.4 for C1 and 0.69 for C2, indicating greater effective diversity in C1. Community indices compress abundance data, so their assumptions and mathematical conventions must remain explicit. Shannon entropy gives weight to both common and uncommon species, Simpson measures are more strongly influenced by dominant species, and Sørensen similarity usually uses presence–absence overlap. Equal abundance provides usefu

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

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