#ecological indices

BIODIVERSITY & IDH

Which community type will have lowest Simpson’s index value (D)?

For Simpson's concentration index D, commonly written as the sum of squared species proportions, values become smallest when abundance is spread evenly across species. Squaring makes large proportions contribute disproportionately, so a dominant species raises D strongly. Equal abundance minimizes the sum for a fixed number of species and indicates low concentration and high diversity. This interpretation depends on using D itself; transformed versions such as 1-D or 1/D reverse the direction of comparison. High richness combined with low evenness can still produce substantial concentration, a

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

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BIODIVERSITY

What does a higher Simpson’s D value indicate?

A high value of Simpson’s dominance index D = Σpᵢ² indicates that abundance is concentrated in one or a few species. Since D is also the probability that two randomly chosen individuals belong to the same species, strong dominance makes this probability large and ecological diversity low. If all species are similarly abundant, each pᵢ is smaller, the squared terms shrink, and D declines. Species richness also tends to reduce D when additional species contribute appreciable abundance. Thus a high D does not imply high diversity or equal abundance, and it says nothing directly about turnover amo

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

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BIODIVERSITY

Which of the following is a community-level measure of diversity?

Gamma diversity is the total diversity of a region or landscape containing multiple local communities. It is therefore a regional-level measure, not a community-level one. Within a single community, alpha diversity can be described through species richness, evenness, or composite indices such as Shannon or Simpson diversity. Both “evenness” and “species richness” among the alternatives are legitimate community-level attributes: richness counts species, whereas evenness describes how equally individuals are distributed among them. Beta diversity then captures turnover between communities, and g

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

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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, re

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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SPECIES HYPOTHESIS

Pielou’s evenness index measures:

Pielou's evenness quantifies how uniformly individuals are distributed across the species present. It standardizes Shannon diversity by the maximum entropy possible for the observed richness, separating abundance balance from a simple count of species. Diversity has separable richness and evenness components. Shannon entropy rises when species number increases or abundances become more equal, while Pielou evenness scales Shannon entropy against the maximum possible value for that richness. Simpson measures emphasize the probability that sampled individuals belong to the same or different speci

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

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