#community ecology

Niche CD RP CEP

Which is NOT a mechanism for species coexistence?

Stable coexistence requires mechanisms that reduce average interspecific competition or create stabilizing advantages when a species becomes rare. Niche differentiation and complementarity achieve this by allowing species to use different resources, places, or times, thereby limiting direct overlap. If the number of independently limiting resources or niche dimensions is sufficiently large relative to competitors, classical resource-competition theory can also permit coexistence, though the wording given is simplified. Niche overlap, by itself, is not a coexistence mechanism; it indicates shared use and can intensify competition when the shared resource is limiting. Partial overlap is compatible with coexistence when other stabilizing processes operate, but increasing overlap does not explain that stability. Competitive exclusion is most likely when species have nearly identical requirements and no fitness or stabilizing trade-off. Modern coexistence theory separates niche differences, which generate negative frequency dependence, from average fitness differences, which favor one species. Coexistence occurs when stabilizing niche differences are strong enough to overcome those fitness inequalities. Thus overlap describes potential competitive similarity rather than a process that maintains diversity.

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

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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 & IDH

A community with high evenness and richness will have:

Species richness supplies the number of different taxa, while evenness describes how uniformly individuals are distributed among them. A community high in both components has many possible species identities and little numerical dominance, producing high values of composite diversity measures. Shannon H′ rises with richness and reaches its maximum for a fixed richness when all species are equally abundant. Simpson dominance D becomes low under even abundance, while its complement 1 − D becomes high. Thus “low Simpson index” could be misleading: it is true only for the dominance form D, not for the Gini–Simpson diversity form. “High biodiversity” is the convention-independent description. High dominance directly contradicts high evenness, and low diversity contradicts the combination of many species and equitable abundance. Ecologically, such communities may use resources complementarily and can show insurance against environmental fluctuation, although diversity does not guarantee stability in every food web. Sampling effort and taxonomic resolution must be comparable when communities are contrasted. This example shows why richness alone is incomplete: a species-rich assemblage dominated by one taxon can be less diverse by abundance-sensitive indices than a somewhat poorer but highly even assemblage.

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

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BIODIVERSITY

In species-area curves, which relation is NOT valid?

A species–area relationship specifically links the number of species S to sampled area A, commonly expressed as S = cAᶻ or, after logarithmic transformation, log S = log c + z log A. The curve rises because larger areas include more individuals and habitats, but its slope typically diminishes on arithmetic axes. “Richness versus area” directly represents this relationship. Prey killed versus prey density instead describes a predator functional response, while photosynthesis versus light describes physiological resource saturation. Tree richness versus evapotranspiration is an environment–diversity relationship, often used to examine how water–energy availability correlates with plant diversity; it is not a species–area curve because its independent variable is not area. Therefore the item’s wording makes more than one alternative fall outside species–area theory: A, B, and D all represent different relationships, while C is the only literal species–area relation. The keyed choice D can be understood as one non-species–area example, but it is not uniquely defensible. This is an item-construction mismatch rather than a sound single-answer comparison; the key should remain unchanged in the source, but the stem requires revision or a missing graph/context.

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

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ECOLOGICAL SUCCESSION

What increases the chance of dominant species being present?

Greater species richness increases the sampling probability that a highly productive or otherwise dominant species is included in a community. This selection effect can raise measured ecosystem function even without every species contributing a unique mechanism, because diverse assemblages draw more species from the regional pool. Species richness counts taxa but does not show their abundance, traits, or interaction strengths. Dominant species may control bulk process rates, rare species may provide specialized functions or future insurance, and predators can restructure whole food webs through indirect effects. Functional groups summarize role overlap, while keystone effects identify unusually strong influence. Separating these dimensions explains why equal losses of richness can have very different ecological consequences and why conservation cannot rely solely on the number of species remaining. In this context, the keyed term, High richness, identifies the relevant mechanism or quantitative relationship and links the observed pattern to its underlying ecological cause.

Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 20

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