#ecological diversity

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Different niches in same habitat exemplify:

“Resource partitioning” for different niches in same habitat exemplify. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. A niche is the multidimensional set of abiotic conditions, resources, and biotic relationships under which a population can persist. The fundamental niche reflects physiological and resource limits, while the realized niche is modified by competitors, consumers, mutualists, and dispersal barriers. The remaining alternatives—“Competitive exclusion”, “Complete competition”, “Character convergence”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Niche differentiation reduces overlap by separating species along resource, space, or time axes. Such partitioning can stabilize coexistence when each species performs relatively better under the conditions it uses most strongly. Field observations could test this account by measuring the proposed driver and the demographic or ecosystem response while controlling plausible confounding factors.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 13

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BIODIVERSITY & IDH

Which value of Simpson’s Index (D) shows maximum diversity?

Using Simpson’s dominance definition, D = Σpᵢ², diversity is greatest when D is smallest. The index represents the probability that two randomly sampled individuals belong to the same species. As richness and evenness increase, that probability declines because individuals are spread among many taxa. A value of zero is the theoretical lower limit and therefore represents maximum diversity among the listed values. In a real finite community containing individuals, D ordinarily remains above zero; it approaches zero as the effective number of equally abundant species becomes very large. A value of one indicates complete dominance by a single species. This direction reverses for the complementary Gini–Simpson index, 1 − D, where values near one indicate high diversity, and for the reciprocal 1/D, where larger numbers indicate more effective species. The formula must therefore accompany the name “Simpson’s index.” The keyed zero assumes the dominance convention consistently used in these rows. Interpreting the scale probabilistically prevents memorization errors: low same-species draw probability means high diversity, whereas high same-species draw probability means low diversity.

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

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BIODIVERSITY & IDH

The number of different species in a community is called:

Species richness is the count of distinct species present in a defined community, area, or sample. A community containing twenty species has greater richness than one containing ten, regardless of how individuals are distributed among them. Evenness addresses that distribution: it is high when species have similar abundances and low when one or a few dominate. Dominance likewise concerns concentration of abundance, while an index combines selected components mathematically. Shannon and Simpson measures therefore can differ between communities that have identical richness but different evenness. Richness estimates depend strongly on sampling effort, area, season, detectability, and taxonomic resolution. Rarefaction or standardized coverage is often used to compare samples of unequal size, and unseen-species estimators can account for taxa likely missed. Richness also has no information about species identity: two communities may each contain ten species yet share none, producing equal richness but high beta diversity. Despite these limitations, it is the simplest and most intuitive biodiversity component and forms the basis of species–area curves, local alpha-diversity comparisons, and regional conservation inventories.

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

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BIODIVERSITY & IDH

The diversity seen at regional level is:

Gamma diversity is the total diversity across a region or landscape that contains multiple local communities. It includes all species recorded when those communities are pooled and may be expressed as richness or as an effective diversity index. Alpha diversity describes diversity within a local habitat, while beta diversity captures differentiation or turnover among habitats. Under multiplicative partitioning, γ = α × β, so high regional diversity may result from species-rich local communities, strong compositional turnover, or both. For example, several forests with identical species lists can have high alpha but relatively low beta, limiting gamma. Conversely, individually modest sites can collectively yield high gamma if each contains different specialists. Delta diversity is sometimes used for turnover over broader biogeographic gradients, but it is not the standard term for pooled regional diversity. Defining spatial grain and extent is important because “regional” depends on study design: the same forest could be gamma scale for quadrat sampling but alpha scale in a continental analysis. Gamma remains the appropriate component whenever the aim is to summarize diversity over the full set of local habitats under consideration.

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

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BIODIVERSITY & IDH

At high disturbance frequency, diversity is low due to:

Frequent or intense disturbance repeatedly removes biomass and interrupts population growth. Species that mature slowly, reproduce infrequently, or require stable habitat cannot complete their life cycles, leaving only disturbance-tolerant taxa, rapid colonizers, or organisms with resistant stages. This strong environmental filter reduces richness and often produces communities dominated by a few stress-tolerant forms. Under the intermediate disturbance hypothesis, diversity is therefore low at the high-disturbance end. “Generalist” may describe some survivors, but broad resource use is not the universal mechanism; specialized fire-adapted or flood-adapted species can also persist. The essential point is tolerance and rapid recovery. Specialists do not generally dominate as a category, and resource richness cannot compensate if organisms are removed faster than populations establish. Disturbance frequency must be considered relative to generation time: an annual event may be severe for a long-lived tree but routine for a short-lived microbe. Spatial refuges and recolonization can preserve regional diversity even when local diversity is depressed. The hypothesis contrasts this filtering with low disturbance, where competitive exclusion rather than repeated mortality can reduce coexistence.

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

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BIODIVERSITY & IDH

The IDH predicts highest diversity at:

The intermediate disturbance hypothesis predicts a hump-shaped relationship between disturbance and species diversity. At very low disturbance, strong competitors can monopolize limiting resources and exclude inferior competitors, reducing coexistence. At very high disturbance, mortality or biomass destruction occurs so frequently or intensely that only rapid colonizers and highly tolerant species persist. Intermediate disturbance can prevent complete competitive exclusion while allowing enough time for slower-growing species to establish, so early and late successional taxa coexist. Disturbance may be fire, grazing, floods, storms, treefall, or physical disruption, and both frequency and intensity matter. The hypothesis is not universal: empirical studies often find monotonic, weak, or context-dependent patterns because productivity, spatial scale, dispersal, disturbance type, and evolutionary history alter outcomes. “No disturbance” is effectively the low-disturbance extreme rather than a separate route to maximum diversity. The mechanism is a balance between exclusion and environmental filtering, not a claim that moderate damage always benefits ecosystems. When applying IDH, investigators must define disturbance quantitatively and distinguish richness from evenness, since different diversity components may peak at different disturbance levels.

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

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BIODIVERSITY & IDH

Shannon-Weaver index (H’) accounts for:

Shannon–Weaver diversity is calculated as H′ = −Σpᵢ ln pᵢ. The number of terms reflects species richness, while the relative abundances pᵢ determine evenness. Adding species generally raises H′, and distributing individuals more equally raises it further. For a fixed richness S, the maximum is ln S when every species has pᵢ = 1/S. This permits Pielou evenness to be calculated as J′ = H′/ln S. The index can be interpreted as uncertainty in predicting the species identity of a randomly selected individual: many equally likely species create high uncertainty, whereas dominance makes identity predictable and H′ low. Richness-only metrics ignore abundance, and dominance-only descriptions emphasize concentration without jointly representing both components in this form. Turnover is beta diversity and requires comparison among communities rather than calculation within one abundance distribution. Shannon values depend on logarithm base and sampling completeness, particularly detection of rare species, so valid comparisons need consistent methods. The index’s sensitivity to both how many species occur and how their individuals are apportioned explains its widespread use in community ecology.

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

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BIODIVERSITY & IDH

What does Simpson’s index (D) value close to 1 indicate?

In the dominance convention, Simpson’s D = Σpᵢ² is the probability that two randomly selected individuals belong to the same species. A value close to 1 occurs when one species contains nearly all individuals. The community is then highly dominated, the identity of a sampled organism is predictable, and diversity is low. Maximum evenness would spread abundance among species, reduce each squared proportion, and drive D downward. Infinite or maximum diversity is therefore incompatible with D near 1. The notation requires care because some texts label 1 − D as “Simpson’s index”; that complementary form has the opposite direction, approaching 1 as diversity increases. Others use 1/D, whose effective-number interpretation also rises with diversity. Here the answer set and surrounding items clearly employ D as dominance. For illustration, proportions 0.95, 0.03, and 0.02 give D = 0.904, whereas three equal species give D ≈ 0.333. Richness alone cannot determine D because the distribution of individuals matters strongly, and squaring makes abundant species much more influential than rare ones.

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

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

Which of the following about diversity components is incorrect?

Alpha, beta, and gamma diversity refer to different spatial scales. Alpha diversity is measured within a relatively homogeneous local habitat or community and may be represented by species richness or an index combining richness and evenness. Beta diversity quantifies differentiation among habitats, often as species turnover or as a multiplicative relation such as β = γ/α. Gamma diversity is the total diversity of the larger landscape or region containing those local communities. Consequently, describing alpha diversity as regional diversity reverses the established scale terminology. A forest quadrat’s species diversity is alpha diversity; the change from forest to grassland is beta diversity; and the pooled diversity of the full landscape is gamma diversity. These levels are linked but not interchangeable. High gamma diversity can arise from high local richness, high turnover among sites, or both. Likewise, a landscape whose sites are individually rich may still have low beta diversity if their species lists are nearly identical. The scale-based definitions make “Alpha = regional diversity” the incorrect statement, while within-community alpha, between-community beta, and regional gamma are mutually consistent.

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

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