#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, and merely having few individuals does not determine D unless their allocation among species is known. Ecological categories are simplified models, yet they remain valuable when their assumptions are stated. The selected description captures the dominant net effect, while real systems may vary with density, habitat, life stage, and environmental conditions. This reasoning connects organismal behavior with broader ecological consequences. A mechanism that changes encounter rates, resource use, or reproductive success can scale up to alter population trajectories and the structure of species interactions.

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

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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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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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 species, depending on convention. Every calculation therefore requires explicit relative abundances and a stated formula; comparing values produced under different conventions can reverse interpretation. In this context, the keyed term, Distribution of individuals among species, identifies the relevant mechanism or quantitative relationship and links the observed pattern to its underlying ecological cause. The distinction is testable by measuring changes in organisms, resources, or process rates through time rather than relying on the label alone.

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

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