BIODIVERSITY

BIODIVERSITY

Which of the following enhances biodiversity stability?

High evenness distributes individuals and ecological functions across many species rather than concentrating them in one dominant taxon. This can stabilize aggregate properties such as biomass production because environmental fluctuations rarely affect all species identically. When one species declines, others may maintain resource capture or compensate through increased growth—the portfolio or insurance effect. Evenness also reduces the chance that loss of a single dominant species causes a large functional collapse. By contrast, strong dominance makes community performance disproportionately

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

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BIODIVERSITY

According to ecological theory, maximum services are delivered when:

Ecosystem services are most reliably sustained when multiple species make substantial contributions rather than when function is concentrated in one dominant species. Even contribution raises functional evenness: different taxa participate in production, decomposition, pollination, nutrient retention, or trophic regulation, reducing dependence on a single vulnerable contributor. It can also strengthen the “insurance effect,” because species respond differently to drought, disease, or disturbance; decline of one contributor may be offset by another. This does not mean every species performs an

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

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BIODIVERSITY

What is the main advantage of using Simpson’s 1-D form?

The dominance form of Simpson’s index decreases as diversity rises, which often causes interpretive confusion: a numerically larger D means greater dominance and lower diversity. Transforming it to 1 − D reverses that direction. The resulting value is the probability that two individuals selected at random belong to different species, so larger values correspond directly to greater diversity. This intuitive direction is the principal advantage of the 1 − D form. It also remains bounded between 0 and 1, facilitating comparisons when methods and sampling are consistent. The transformation does n

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

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BIODIVERSITY

Which pair shows highest alpha diversity?

Alpha diversity concerns the species diversity within one local community. A ranking that assigns community A the highest alpha diversity implies that A has the largest within-community richness, the greatest evenness, or the highest value of a specified composite index relative to B, C, and D. The criterion must be kept consistent: richness counts species only, Shannon diversity weights both richness and evenness, and Simpson-based diversity places greater emphasis on common species. Consequently, a community with the longest species list need not rank highest by every index if it is overwhel

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

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BIODIVERSITY

A community with 10 species, each with 10 individuals has:

Ten species represented by ten individuals each have identical relative abundance, pᵢ = 10/100 = 0.1. This is perfect evenness because no species is numerically dominant. For a fixed richness, equal abundance maximizes both Shannon diversity and the Gini–Simpson index. Shannon diversity would be H′ = −10(0.1 ln 0.1) = ln 10, and Pielou’s evenness H′/ln S would equal 1. Simpson dominance would be 10(0.1²) = 0.1, relatively low, while 1 − D would be 0.9. The community also has a richness of ten species; whether that richness is “high” depends on the taxon, area, and comparison community, but it

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

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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

Species richness depends strongly on:

The species–area relationship shows that larger sampled or habitat areas generally contain more species. It is often represented as S = cAᶻ, where S is species richness, A is area, c depends on the taxonomic group and region, and z is the slope on logarithmic axes. Several mechanisms contribute. Larger areas contain more individuals, increasing the chance of encountering rare species; they usually encompass more habitat types and environmental gradients, creating additional niches; and larger populations have lower stochastic extinction risk. On islands or isolated habitat fragments, area also

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

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

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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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