Chapter 11: Organisms and Populations Application Questions | biology Class 12 CBSE

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Chapter 11: Organisms and Populations Application Questions | biology Class 12 CBSE | ByteNotes.in

Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 1

Applied Concept

A wildlife biologist is monitoring a population of Siberian cranes at Bharatpur wetlands. She records fewer than 10 individuals in a year. With reference to population attributes, explain what information she should collect to determine if this population is at risk.

The biologist should measure population density (N) over multiple years to detect trends — a consistently declining N below 10 signals serious conservation concern for this critically small population.
She should construct an age pyramid by recording the proportion of juveniles, adults, and aged individuals — if the pyramid shows a narrow base (few young), the population is declining and reproduction is insufficient for replacement.
Sex ratio data are also vital: a heavily skewed ratio (too few breeding adults of one sex) would further reduce effective breeding population size and accelerate decline, calling for urgent conservation intervention.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 2

Applied Concept

In a laboratory experiment, a flask of bacteria doubles in population every hour. The initial population is 10 cells and food supply is unlimited. With reference to exponential growth, predict the population size after 5 hours and explain why this growth pattern cannot be sustained in nature.

Under exponential growth (dN/dt = rN), with unlimited resources, the population doubles every hour — after 5 hours the population would be 10 × 2^5 = 320 cells, following the equation Nt = N0ert.
This pattern cannot be sustained because in nature, resources (food and space) are finite; as population density increases, competition for limited resources intensifies, reducing birth rates and increasing death rates.
Eventually, the population transitions to logistic growth — decelerating as density approaches the carrying capacity (K) of the environment, as described by the Verhulst-Pearl equation.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 3

Applied Concept

A national park manager wants to estimate the population density of leopards in the park without capturing or directly observing them. With reference to indirect population estimation methods described in the chapter, suggest the approach she should adopt.

The manager should use indirect estimation methods based on physical evidence — collecting and mapping pug marks (footprints) and fecal pellets in the park, which are commonly used for tiger censuses in Indian reserves and can be applied to leopards.
These indirect indicators provide relative population density estimates (relative density serves the purpose well when absolute counts are impractical), allowing comparisons across seasons and years.
The number of unique pug mark patterns or fecal pellets per unit area per survey period can be used as a proxy for population density, avoiding the need for direct capture or observation of these elusive, solitary big cats.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 4

Applied Concept

Farmers in a region are troubled by a caterpillar pest that is destroying their crops. A scientist suggests introducing a predatory wasp species that naturally parasitises the caterpillar larvae. With reference to the chapter, explain the ecological principle behind this recommendation and its advantages over chemical pesticides.

The recommendation is based on the principle of biological control — using natural predators to regulate pest populations, as illustrated by the introduction of the cactus-feeding moth to control the invasive prickly pear cactus in Australia.
Predators inherently regulate prey (pest) population densities — the parasitic wasp, by attacking caterpillar larvae, would bring the pest population down to ecologically manageable levels without eliminating it entirely (predators are 'prudent').
Unlike chemical pesticides, biological control is self-sustaining (the wasp reproduces and maintains its population), species-specific (minimal harm to non-target organisms), and eco-friendly — it aligns with natural ecological processes rather than disrupting them.

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Application Question

Easy difficulty • Concept in a practical situation

Easy

Question 5

Applied Concept

A botanist observes that Calotropis plants growing in an abandoned field are never browsed by cattle or goats, even when other plants nearby are heavily grazed. With reference to plant defences against herbivory described in the chapter, explain this observation.

Calotropis produces highly poisonous cardiac glycosides — toxic secondary metabolites that are harmful to cattle and goats, causing cardiac distress if ingested.
This is an example of chemical defence against herbivory: plants produce and store toxic chemicals that make the herbivore sick, inhibit feeding or digestion, or even kill the herbivore, deterring repeated feeding attempts.
Since cattle and goats likely learned (or instinctively avoid) the toxic effects of Calotropis, they selectively graze other less-defended plants while completely bypassing Calotropis, demonstrating the effectiveness of chemical defence in plant-herbivore interactions.

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Application Question

Easy difficulty • Concept in a practical situation

Easy

Question 6

Applied Concept

During a nature walk, you observe a cattle egret walking closely behind a herd of grazing buffalo and frequently catching insects from the grass. What type of interspecific interaction does this represent? Analyse why this is not a case of mutualism.

This interaction represents commensalism (+/0): the cattle egret benefits by catching insects that are flushed out of the vegetation by the movement of the grazing buffalo, making foraging easier and more efficient.
It is not mutualism because mutualism requires both species to benefit (+/+) — in this case, the buffalo receive no benefit from the presence of the egret; it neither helps them graze nor protects them from predators.
The egret also does not harm the buffalo in any way — the buffalo are completely unaffected (+/0), which is the defining characteristic of commensalism as opposed to mutualism or parasitism.

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Application Question

Hard difficulty • Concept in a practical situation

Hard

Question 7

Applied Concept

A coral reef ecologist notes that on a reef where the top predatory fish were removed by overfishing, the populations of certain invertebrate prey species exploded while others went extinct. With reference to the chapter, explain this observation using the concept of predator-regulated species diversity.

This parallels the Pisaster starfish removal experiment on the American Pacific Coast — when a keystone predator is removed, prey species populations are no longer regulated, leading to competitive dominance by certain species.
Without predators controlling the dominant prey species, interspecific competition among prey becomes unchecked — the competitively superior prey species monopolises resources, driving less competitive prey species to local extinction.
The chapter states that predators maintain species diversity by reducing competition intensity among prey; their removal disrupts this balance, creating a simplified, less diverse community dominated by a few competitively superior species — mirroring the scenario described.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 8

Applied Concept

A birdwatcher in spring observes a female cuckoo secretly laying an egg in a crow's nest while the crow is briefly away. Upon returning, the crow incubates all the eggs without apparent awareness of the foreign egg. With reference to parasitism, explain the evolutionary adaptations behind this behaviour.

This is brood parasitism — the cuckoo (koel) parasitises the crow by laying its eggs in the crow's nest, letting the host incubate and raise the parasitic chick, saving the parasite's own parental investment.
The cuckoo's egg has evolved through co-evolution to mimic the crow's eggs in size and colour, reducing the probability that the crow will detect and eject the foreign egg from the nest.
This is a classic example of host-parasite co-evolution — the parasite evolves to resemble the host's eggs, while the host evolves improved recognition ability to detect and reject foreign eggs, driving a continuous evolutionary arms race between cuckoo and crow.

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Application Question

Medium difficulty • Concept in a practical situation

Medium

Question 9

Applied Concept

Two species of birds are introduced to a remote island with limited insect prey. Initially both species coexist, but after five years only one species remains. With reference to Gause's principle and competitive exclusion, explain the likely outcome and the condition required for coexistence.

With limited insect prey and presumably identical resource use, Gause's Competitive Exclusion Principle predicts that the competitively superior species will eventually eliminate the inferior one — which is the likely explanation for one species disappearing after five years.
For coexistence to have occurred, the two species would have needed to partition resources — using the same prey at different times, foraging in different habitat layers, or targeting prey of different sizes, thus reducing direct competitive overlap.
This case parallels the Galapagos tortoise-goat competition: when two species compete for the same limiting resource, the one with the higher intrinsic efficiency (or r value) survives, while the other is competitively excluded.

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Application Question

Easy difficulty • Concept in a practical situation

Easy

Question 10

Applied Concept

An orchid grows on the branch of a mango tree in a garden. The orchid absorbs moisture and nutrients from the air and photosynthesises independently. What type of interspecific interaction does this represent? Justify your answer.

This represents commensalism (+/0): the orchid benefits by using the mango tree as a physical support (phoresy), gaining access to better light conditions high in the canopy without being rooted in competition for soil nutrients.
The mango tree is neither benefited nor harmed — the orchid does not extract water or nutrients from the mango's vascular system (it is an epiphyte, not a parasite), and the added weight is negligible.
If the orchid were extracting nutrients from the mango (as Cuscuta does from its host), it would be parasitism (–) rather than commensalism (0) — the distinction lies in whether the host suffers any measurable negative effect.

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Chapter sections

Chapter 11: Organisms and Populations | Application Questions

A wildlife biologist is monitoring a population of Siberian cranes at Bharatpur wetlands. She records fewer than 10 individuals in a year. With reference to population attributes, explain what information she should collect to determine if this population is at risk.

In a laboratory experiment, a flask of bacteria doubles in population every hour. The initial population is 10 cells and food supply is unlimited. With reference to exponential growth, predict the population size after 5 hours and explain why this growth pattern cannot be sustained in nature.

A national park manager wants to estimate the population density of leopards in the park without capturing or directly observing them. With reference to indirect population estimation methods described in the chapter, suggest the approach she should adopt.

A botanist observes that Calotropis plants growing in an abandoned field are never browsed by cattle or goats, even when other plants nearby are heavily grazed. With reference to plant defences against herbivory described in the chapter, explain this observation.

During a nature walk, you observe a cattle egret walking closely behind a herd of grazing buffalo and frequently catching insects from the grass. What type of interspecific interaction does this represent? Analyse why this is not a case of mutualism.

Two species of birds are introduced to a remote island with limited insect prey. Initially both species coexist, but after five years only one species remains. With reference to Gause's principle and competitive exclusion, explain the likely outcome and the condition required for coexistence.

An orchid grows on the branch of a mango tree in a garden. The orchid absorbs moisture and nutrients from the air and photosynthesises independently. What type of interspecific interaction does this represent? Justify your answer.

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