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 dependent on one population and can suppress subordinate species through competitive exclusion. Primary productivity is an ecosystem rate, not inherently a cause of biodiversity stability; very high productivity may even intensify dominance in some systems. Keystone predators can promote coexistence by limiting superior competitors, but their effect is context-specific, and reliance on one keystone can itself create vulnerability. High evenness is therefore the most general property listed that enhances diversity stability. Stability has several components—resistance, resilience, and temporal invariability—and evenness may affect each differently. Richness, response diversity, food-web structure, and spatial connectivity also matter, so evenness is supportive rather than sufficient on its own.

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 identical role or contributes exactly the same amount. Complementarity among distinct functional traits may be more important than numerical equality, and some keystone species have disproportionate effects. Functional redundancy can buffer loss, but redundancy alone does not guarantee maximum service if redundant species all respond similarly to stress. Communities composed entirely of predators or mutualists are trophically unrealistic and omit processes needed for complete ecosystem functioning. Among the alternatives, even contribution best captures broad participation and low dominance. The general principle is that service delivery depends on abundance distribution, functional identity, and complementarity together, so evenness supports stability without implying that species are ecologically interchangeable.

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 not reduce the data requirement; relative abundances are still needed. Nor is it mathematically more informative than D, because each can be obtained exactly from the other. “Simplicity” is too vague to distinguish the forms, whereas direct interpretation identifies the specific benefit. Users must nevertheless name the index because some publications call D itself “Simpson’s index” and others use that phrase for 1 − D. In a highly dominated community, D approaches 1 but 1 − D approaches 0, aligning the latter’s scale with the everyday expectation that low numbers mean low diversity.

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 overwhelmingly dominated by one species. Alpha diversity should also not be confused with beta diversity, which compares turnover between communities, or gamma diversity, which describes the pooled regional species set. Ecologically, high alpha diversity can reflect resource heterogeneity, moderate disturbance, productive conditions within limits, or reduced competitive exclusion, but the observed ranking depends on the supplied abundance data. Since the table, graph, or “pair” information referenced by the item is absent from the extracted text, community A cannot be independently recalculated. The keyed result is interpretable only as A having the strongest local diversity statistic in the omitted source material.

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 is not low merely because each species has ten individuals. High dominance is excluded by the equal counts, and low diversity is inconsistent with the simultaneous richness and maximum evenness. This illustrates why abundance data improve on a species count alone. Two communities may each contain ten species, but one with 91 individuals of one species and one individual of each remaining species would have markedly lower evenness and composite diversity.

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 the index by adding possible species identities, provided the added species are represented. The phrase “all species are abundant” is imprecise, but if it implies comparable abundances it would support high, not low, diversity. This example separates richness from evenness: a community can contain many species yet have a low Shannon value when almost all individuals belong to one taxon. Dominance therefore supplies the clearest condition for the lowest index.

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 affects extinction rates and interacts with distance-dependent colonization. Competition, predation, and mortality can strongly alter local composition, but their effects on richness vary with intensity, trophic structure, and environmental context. Area has a broad, repeatedly observed scaling relationship with richness across taxa and regions, making it the strongest general predictor among these alternatives. Sampling design still matters: simply surveying more area detects more species even without ecological change, whereas genuine habitat-area effects operate through population persistence and heterogeneity. Log-transforming the relation yields log S = log c + z log A, allowing comparisons of slopes.

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 among sites, which is beta diversity. Numerical interpretation must follow the stated convention: some sources call 1 − D “Simpson’s diversity index,” in which case high values indicate high diversity, while 1/D is a reciprocal diversity measure with the same direction. Here the symbol D is being used consistently as the dominance form. For instance, a community with proportions 0.9, 0.05, and 0.05 has D = 0.815, reflecting far lower effective diversity than three equally abundant species, for which D is about 0.333.

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 gamma diversity pools diversity across the broader region. The keyed choice D conflicts with this accepted spatial hierarchy and also fails the stem because gamma is explicitly larger-scale. This is a demonstrable key mismatch, compounded by the presence of more than one plausible community-level alternative. The scientifically sound correction would require rewriting the stem—for example, asking for a regional measure, which would make gamma unambiguous—or replacing the choices with alpha, beta, gamma, and another scale. The workbook key should remain untouched as instructed, but learners should not internalize gamma diversity as a within-community metric.

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, 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

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 move H′ downward, not upward, because most individuals become concentrated in a few taxa. “Gamma” describes regional diversity and is a spatial scale, not a direct driver within the formula. Shannon’s index is often interpreted as the uncertainty in predicting the species identity of a randomly sampled individual. High richness supplies more possible identities, and high evenness keeps those possibilities similarly likely. Its joint sensitivity to both components explains why two communities with equal species counts can have different H′ values.

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

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