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

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

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Question 1

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What is a population in ecological terms? Give two examples from the chapter.

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources.
Example 1: All cormorants in a wetland constitute a population.
Example 2: All lotus plants in a pond constitute a population.

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Question 2

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How is birth rate calculated in a population? Give the example of lotus plants from the chapter.

Birth rate is calculated as the number of new individuals added through reproduction divided by the initial population size, expressed as offspring per individual per year.
In the chapter example, if a pond had 20 lotus plants and 8 new plants were added, birth rate = 8/20 = 0.4 offspring per lotus per year.
Birth rate is a per capita measure and is an attribute of the population, not of an individual organism.

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Question 3

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What is an age pyramid and what does its shape indicate?

An age pyramid is a graphical representation of the age distribution (proportion of individuals in different age groups) of males and females in a population.
The shape of the pyramid reflects the growth status of the population — a broad base indicates a growing population, an even distribution indicates a stable population, and a narrow base indicates a declining population.
Age pyramids are used especially for human populations to analyse demographic trends and plan resources.

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Question 4

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Why is biomass sometimes a more meaningful measure of population density than total number?

Total number can underestimate the ecological role of a species when individual sizes vary greatly — for example, one large banyan tree contributes far more to the community than 200 small carrot grass plants.
In such cases, per cent cover or biomass gives a more realistic measure of a species' contribution and dominance in the habitat.
Similarly, when populations are enormous (like bacteria in a petri dish), counting individuals is impractical, and biomass or density estimates are more appropriate.

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Question 5

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Name the four processes that bring about changes in population density and state how each affects population size.

Natality (births) and immigration increase population density by adding new individuals to the existing population.
Mortality (deaths) and emigration decrease population density by removing individuals from the population.
The equation Nt+1 = Nt + [(B + I) – (D + E)] summarises how these four processes together determine population density at any given time.

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Question 6

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What is the intrinsic rate of natural increase (r)? Give examples of r values for two species mentioned in the chapter.

The intrinsic rate of natural increase (r) is the difference between per capita birth rate (b) and per capita death rate (d); it represents the inherent potential of a population to grow.
For the Norway rat, r = 0.015; for the flour beetle, r = 0.12; and for the human population of India in 1981, r = 0.0205.
The value of r is used to assess the impact of any biotic or abiotic factor on population growth.

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Question 7

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Distinguish between exponential and logistic population growth in terms of resource availability and curve shape.

Exponential growth occurs when resources are unlimited; the population grows at its maximum potential rate, producing a J-shaped curve.
Logistic growth occurs when resources are limited; growth initially accelerates, then decelerates as the population approaches carrying capacity (K), producing a sigmoid (S-shaped) curve.
Exponential growth is described by dN/dt = rN, while logistic growth is described by the Verhulst-Pearl equation dN/dt = rN[(K–N)/K].

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Question 8

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What is carrying capacity (K)? Why is logistic growth considered a more realistic model for natural populations?

Carrying capacity (K) is the maximum number of individuals of a species that a habitat can sustainably support, given its available resources.
Logistic growth is considered more realistic because, in nature, resources (food and space) are finite and become limiting sooner or later, preventing unlimited exponential growth.
In the logistic model, population growth slows as density approaches K, reflecting the competition for limited resources that actually occurs in natural habitats.

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Question 9

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What is life history variation? Distinguish between organisms that breed once and those that breed many times in their lifetime.

Life history variation refers to the differences in reproductive strategies — including frequency of breeding and number/size of offspring — that species have evolved to maximise their Darwinian fitness in their habitats.
Organisms like Pacific salmon and bamboo breed only once in their lifetime (semelparous), whereas birds and mammals breed multiple times (iteroparous).
Some organisms (oysters, pelagic fishes) produce many small offspring, while others (birds, mammals) produce fewer but larger offspring — these strategies reflect different trade-offs in energy allocation.

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Question 10

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Using a table of signs (+, –, 0), list any four types of interspecific interactions with the effect on each species.

Mutualism: Species A (+), Species B (+) — both species benefit from the interaction.
Predation/Parasitism: Species A (+), Species B (–) — one species benefits while the other is harmed.
Commensalism: Species A (+), Species B (0) — one benefits while the other is unaffected; Amensalism: Species A (–), Species B (0) — one is harmed while the other is unaffected.

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Question 11

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Explain the role of predators in maintaining species diversity in a community with the example of Pisaster.

Predators help maintain species diversity by controlling the population of dominant prey species, thereby reducing interspecific competition among prey.
In the rocky intertidal communities of the American Pacific Coast, the starfish Pisaster is an important predator. When all Pisaster were removed, more than 10 species of invertebrates became extinct within a year due to unchecked interspecific competition.
This demonstrates that the presence of a keystone predator prevents any single prey species from monopolising resources and dominating the community.

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Question 12

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What morphological and chemical defences have plants evolved against herbivory? Give two examples each.

Morphological defences include thorns — for example, thorns in Acacia and Cactus deter herbivores from feeding on the plant.
Chemical defences include the production of toxic or deterrent substances — Calotropis produces cardiac glycosides that are toxic to cattle and goats; nicotine, caffeine, quinine, strychnine, and opium are alkaloids produced by plants as chemical defences against grazers.
These defences evolved because plants, unlike animals, cannot move away from herbivores and must rely on passive strategies for protection.

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Question 13

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How does the Monarch butterfly protect itself from predation? What is the source of its chemical defence?

The Monarch butterfly contains a special chemical that makes it highly distasteful to its predator (birds), which then avoid feeding on it.
The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.
This is an example of chemical defence in prey animals, where a toxin originally from a plant is incorporated into the animal's body to deter predators.

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Chapter sections

Chapter 11: Organisms and Populations | Short Questions

What is a population in ecological terms? Give two examples from the chapter.

How is birth rate calculated in a population? Give the example of lotus plants from the chapter.

What is an age pyramid and what does its shape indicate?

Why is biomass sometimes a more meaningful measure of population density than total number?

Name the four processes that bring about changes in population density and state how each affects population size.

What is the intrinsic rate of natural increase (r)? Give examples of r values for two species mentioned in the chapter.

Distinguish between exponential and logistic population growth in terms of resource availability and curve shape.

What is carrying capacity (K)? Why is logistic growth considered a more realistic model for natural populations?

What is life history variation? Distinguish between organisms that breed once and those that breed many times in their lifetime.

Using a table of signs (+, –, 0), list any four types of interspecific interactions with the effect on each species.

Explain the role of predators in maintaining species diversity in a community with the example of Pisaster.

What morphological and chemical defences have plants evolved against herbivory? Give two examples each.

How does the Monarch butterfly protect itself from predation? What is the source of its chemical defence?

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