Important Questions Class 12 Biology Chapter 11 Organisms and Populations With Answers

Ecology studies how organisms interact with other organisms and with their physical environment. A population is a group of individuals of the same species living in a defined geographical area.

Ecology becomes easier when students connect individuals, populations, growth models, and species interactions. Important Questions Class 12 Biology Chapter 11 help students revise Organisms and Populations for CBSE 2026-27 board exams, school tests, and pre-board papers. NCERT Chapter 11 focuses on population attributes, age pyramids, population density, natality, mortality, immigration, emigration, exponential growth, logistic growth, life history variation, predation, competition, parasitism, commensalism, and mutualism.

Key Takeaways

• Population: A population includes individuals of the same species that live in a defined area and share resources.
• Population Attributes: Birth rate, death rate, sex ratio, age distribution, and density belong to populations, not individuals.
• Growth Models: Exponential growth forms a J-shaped curve, while logistic growth forms a sigmoid curve.
• Species Interactions: Predation, competition, parasitism, commensalism, amensalism, and mutualism shape biological communities.

Important Questions Class 12 Biology Chapter 11 Structure 2026-27

Important Questions Class 12 Biology Chapter 11 for CBSE 2026-27

Ecology questions in this chapter often ask for direct definitions with NCERT examples. Students should revise formulas, interaction signs, and examples like Pisaster, Balanus, Cuscuta, orchids, and fig-wasp mutualism.

1. What is ecology?

Ecology is the study of interactions among organisms and between organisms and their physical environment. It links living organisms with abiotic and biotic surroundings.

1. Abiotic factors include light, temperature, water, and soil.
2. Biotic factors include other organisms.
3. Ecology studies organisms, populations, communities, and biomes.

Final answer:
Ecology studies organism-environment relationships.

2. What are the four levels of biological organisation studied in ecology?

The four levels studied in ecology are organisms, populations, communities, and biomes. Chapter 11 mainly studies population-level ecology.

1. Organism: one living individual.
2. Population: individuals of one species in an area.
3. Community: populations of different species.
4. Biome: large regional ecological unit.

Final fact:
Population ecology links ecology with evolution and genetics.

3. What is a population in biology?

A population is a group of individuals of the same species living in a defined geographical area. They share or compete for similar resources.

Examples:

1. Cormorants in a wetland.
2. Rats in an abandoned dwelling.
3. Teakwood trees in a forest tract.
4. Bacteria in a culture plate.
5. Lotus plants in a pond.

Final fact:
Natural selection acts at the population level.

Organisms and Populations Class 12 Important Questions on Population Attributes

A population has measurable features that a single organism does not have. CBSE 2026-27 questions often ask birth rate, death rate, sex ratio, age pyramid, and population density.

4. What are population attributes?

Population attributes are features that describe a population, not an individual organism. They include birth rate, death rate, sex ratio, age distribution, and density.

1. An individual has birth and death.
2. A population has birth rate and death rate.
3. A population has sex ratio and age distribution.
4. Population density shows population size.

Final answer:
Population attributes help describe population status.

5. What is birth rate in a population?

Birth rate is the per capita number of births added to a population during a given period. It shows population increase through reproduction.

Example:
A pond has 20 lotus plants.
8 new plants are added in one year.

Calculation:
Birth rate = 8/20
Birth rate = 0.4 offspring per lotus per year

Final result:
Birth rate = 0.4 offspring per lotus per year

6. What is death rate in a population?

Death rate is the per capita number of deaths in a population during a given period. It shows population decrease through mortality.

Example:
A laboratory population has 40 fruitflies.
4 fruitflies die in one week.

Calculation:
Death rate = 4/40
Death rate = 0.1 individuals per fruitfly per week

Final result:
Death rate = 0.1 individuals per fruitfly per week

7. What is sex ratio in a population?

Sex ratio is the proportion of males and females in a population. An individual is male or female, but a population has sex ratio.

Example:
A population has 60 percent females and 40 percent males.

1. Sex ratio affects reproductive potential.
2. It helps study population structure.
3. It can vary across species and habitats.

Final fact:
Sex ratio is a population-level attribute.

8. What is an age pyramid?

An age pyramid is a diagram showing age distribution of individuals in a population. In humans, it usually shows male and female age groups.

1. A broad base indicates a growing population.
2. A balanced shape indicates a stable population.
3. A narrow base indicates a declining population.

Final fact:
The shape of an age pyramid reflects population growth status.

9. What is population density?

Population density is the size of a population in a given area at a given time. It is represented by N.

Population density can be measured by:

1. Total number.
2. Biomass.
3. Percent cover.
4. Relative density.

Example:
Tiger census may use pug marks and faecal pellets.

Final fact:
Population density need not always mean total number.

Population Growth Class 12 Biology Questions With Answers

Population size changes due to births, deaths, immigration, and emigration. Board questions often ask the population density equation and the meaning of each factor.

10. What are the four basic processes that change population density?

The four processes are natality, mortality, immigration, and emigration. Natality and immigration increase population density.

1. Natality adds new individuals through birth.
2. Immigration adds individuals from outside.
3. Mortality reduces individuals through death.
4. Emigration removes individuals from the habitat.

Final answer:
Population density changes through B, I, D, and E.

11. Define natality, mortality, immigration, and emigration.

Natality and immigration increase population size, while mortality and emigration decrease it. These four factors decide population change.

Definitions:

1. Natality: number of births during a given period.
2. Mortality: number of deaths during a given period.
3. Immigration: individuals entering a habitat.
4. Emigration: individuals leaving a habitat.

Final fact:
Births and deaths usually influence density most under normal conditions.

12. Write the equation for population density at time t + 1.

The equation is Nt+1 = Nt + [(B + I) - (D + E)]. It shows change in population density over time.

Formula:
Nt+1 = Nt + [(B + I) - (D + E)]

Where:

1. Nt = population density at time t.
2. B = births.
3. I = immigrants.
4. D = deaths.
5. E = emigrants.

Final fact:
Population increases when B + I is greater than D + E.

13. When does immigration become important in population growth?

Immigration becomes important when a new habitat is being colonised. Incoming individuals may increase population faster than births.

1. A species enters a new habitat.
2. Individuals arrive from other areas.
3. Population density rises due to incoming members.

Final fact:
Immigration has special importance during colonisation.

Exponential Growth Class 12 Biology Questions

Exponential growth occurs when resources such as food and space remain unlimited. The curve becomes J-shaped because the population grows at its full potential.

14. What is exponential growth?

Exponential growth is population growth under unlimited resources. The population grows in a geometric pattern.

1. Food and space remain abundant.
2. Death rate stays lower than birth rate.
3. The species reaches its innate growth potential.
4. The growth curve becomes J-shaped.

Final answer:
Exponential growth occurs under ideal unlimited conditions.

15. Write the equation for exponential growth.

The exponential growth equation is dN/dt = rN. Here, r is the intrinsic rate of natural increase.

Formula:
dN/dt = rN

Where:

1. N = population density.
2. t = time.
3. r = intrinsic rate of natural increase.

Also:
r = b - d

Final fact:
The value of r measures population growth potential.

16. Write the integral form of the exponential growth equation.

The integral form is Nt = N0ert. It calculates population density after time t.

Formula:
Nt = N0ert

Where:

1. Nt = population density after time t.
2. N0 = population density at time zero.
3. r = intrinsic rate of natural increase.
4. e = base of natural logarithms.
5. t = time.

Final fact:
The value of e is approximately 2.71828.

17. What is intrinsic rate of natural increase?

Intrinsic rate of natural increase is the inherent potential of a population to grow. It is represented by r.

1. r depends on per capita birth and death rates.
2. r = b - d.
3. It helps assess effects of abiotic and biotic factors.

Examples from NCERT:
Norway rat has r = 0.015.
Flour beetle has r = 0.12.
Human population in India had r = 0.0205 in 1981.

Final fact:
A higher r value shows faster potential growth.

Logistic Growth Class 12 Biology Questions

Logistic growth is more realistic because natural resources are limited. The curve shows lag, acceleration, deceleration, and a stable phase near carrying capacity.

18. What is logistic growth?

Logistic growth is population growth under limited resources. It forms a sigmoid or S-shaped curve.

1. The population first grows slowly.
2. Then it enters acceleration.
3. Growth slows during deceleration.
4. The population finally reaches carrying capacity.

Final answer:
Logistic growth is more realistic for natural populations.

19. What is carrying capacity?

Carrying capacity is the maximum population size a habitat can support with available resources. It is represented by K.

1. Food and space limit growth.
2. Competition increases near K.
3. No further growth occurs beyond K.

Final fact:
Carrying capacity sets the upper limit for population growth.

20. Write the Verhulst-Pearl logistic growth equation.

The Verhulst-Pearl logistic growth equation is dN/dt = rN[(K - N)/K]. It describes population growth under limited resources.

Formula:
dN/dt = rN[(K - N)/K]

Where:

1. N = population density at time t.
2. r = intrinsic rate of natural increase.
3. K = carrying capacity.

Final fact:
The logistic growth curve becomes sigmoid.

21. Why is logistic growth more realistic than exponential growth?

Logistic growth is more realistic because natural habitats have limited resources. No population can grow exponentially forever.

1. Food becomes limiting.
2. Space becomes limiting.
3. Competition increases.
4. Growth slows near carrying capacity.

Final answer:
Most animal populations follow logistic growth over time.

Age Pyramid Class 12 Biology and Life History Variation Questions

Population structure and reproductive strategies reveal how species survive in their habitats. NCERT connects life history variation with Darwinian fitness and selection pressure.

22. How does an age pyramid show population growth status?

An age pyramid shows whether a population is growing, stable, or declining. Its shape reflects age distribution.

1. Growing population: more young individuals.
2. Stable population: balanced age groups.
3. Declining population: fewer young individuals.

Final fact:
Age pyramids show future population trends.

23. What is life history variation?

Life history variation refers to differences in reproductive strategies among organisms. These traits evolve under habitat conditions.

Examples:

1. Pacific salmon breeds once in its lifetime.
2. Bamboo also breeds once in its lifetime.
3. Most birds and mammals breed many times.
4. Oysters produce many small offspring.
5. Birds and mammals produce fewer large offspring.

Final fact:
Life history traits evolve under biotic and abiotic constraints.

24. What is Darwinian fitness in population ecology?

Darwinian fitness is reproductive fitness that helps a population maximise its r value. It reflects successful survival and reproduction.

1. Organisms evolve efficient reproductive strategies.
2. Selection pressures shape life history traits.
3. Different habitats favour different reproductive patterns.

Final fact:
Darwinian fitness connects ecology with evolution.

Population Interactions Class 12 Important Questions

No species lives alone in nature. Population interactions explain how species benefit, suffer, or remain unaffected in a biological community.

25. What are population interactions?

Population interactions are relationships between populations of two different species. They may be beneficial, harmful, or neutral.

Types:

1. Mutualism: + +
2. Competition: - -
3. Predation: + -
4. Parasitism: + -
5. Commensalism: + 0
6. Amensalism: - 0

Final fact:
Interaction signs show the effect on both species.

26. Distinguish between mutualism, competition, predation, parasitism, commensalism, and amensalism.

These interactions differ by benefit, harm, or neutrality for the two species involved.

Final fact:
The symbols +, -, and 0 show ecological effect.

Predation Class 12 Biology Questions With Answers

Predation transfers energy across trophic levels and controls prey population size. NCERT also treats herbivory as a broad form of predation.

27. What is predation?

Predation is an interaction where one species benefits and the other species is harmed. The predator kills or consumes the prey.

Examples:

1. Tiger eating deer.
2. Sparrow eating seeds.
3. Herbivores feeding on plants.

Final fact:
Predation helps transfer energy to higher trophic levels.

28. What are the important roles of predators?

Predators transfer energy, control prey populations, and maintain species diversity. They support ecosystem stability.

Roles:

1. They act as conduits for energy transfer.
2. They keep prey population under control.
3. They reduce competition among prey species.
4. They help maintain species diversity.

Example:
Pisaster starfish maintains diversity in rocky intertidal communities.

29. How did prickly pear cactus spread in Australia and get controlled?

Prickly pear cactus spread rapidly in Australia because its natural predators were absent. It was controlled by introducing a cactus-feeding moth.

1. The cactus entered Australia in the early 1920s.
2. It spread over millions of hectares.
3. The invaded habitat lacked natural predators.
4. A cactus-feeding moth controlled the cactus.

Final fact:
This shows biological control through predator introduction.

30. What defence mechanisms do prey species use against predators?

Prey species use camouflage, poison, distasteful chemicals, and other defences against predators.

Examples:

1. Some insects and frogs use camouflage.
2. Some species are poisonous.
3. Monarch butterfly is distasteful to birds.
4. It gets its chemical defence during the caterpillar stage.

Final fact:
Prey defences reduce predation risk.

31. Name important defence mechanisms in plants against herbivory.

Plants use morphological and chemical defences against herbivores. These defences reduce feeding damage.

Morphological defence:

1. Thorns in Acacia.
2. Thorns in cactus.

Chemical defence:

1. Calotropis produces poisonous cardiac glycosides.
2. Plants produce nicotine, caffeine, quinine, strychnine, and opium.

Final fact:
Many plant chemicals evolved as anti-herbivore defences.

Competition Class 12 Biology Questions With Answers

Competition reduces fitness when two species use similar resources or interfere with each other. It can occur between related or unrelated species.

32. What is competition in ecology?

Competition is an interaction where the fitness of one species decreases in the presence of another species. Both species may suffer.

1. Competition can occur for food.
2. It can occur for space.
3. It can occur even when species are unrelated.

Example:
Flamingoes and fishes compete for zooplankton in some South American lakes.

33. What is Gause’s Competitive Exclusion Principle?

Gause’s Competitive Exclusion Principle states that two closely related species competing for the same limiting resources cannot coexist indefinitely. The inferior species gets eliminated.

1. The principle works when resources are limited.
2. The superior competitor survives better.
3. The inferior competitor may disappear locally.

Final fact:
Resource limitation is important for competitive exclusion.

34. Explain competition between Balanus and Chathamalus.

Balanus excludes Chathamalus from the intertidal zone because it is the larger and superior competitor. Connell showed this through field experiments.

1. Balanus dominates the rocky sea coast of Scotland.
2. Chathamalus is smaller and competitively inferior.
3. Balanus restricts Chathamalus from the preferred zone.

Final fact:
This is strong evidence for competition in nature.

35. What is resource partitioning?

Resource partitioning is a method by which competing species avoid competition by using resources differently. It helps species coexist.

1. Species may feed at different times.
2. Species may use different foraging patterns.
3. Species may use different parts of the same habitat.

Example:
MacArthur showed that five warbler species coexisted by different foraging activities.

Final fact:
Resource partitioning reduces direct competition.

Parasitism Class 12 Biology Questions With Answers

Parasitism benefits the parasite and harms the host. Parasites often show special adaptations for attachment, reproduction, and host dependence.

36. What is parasitism?

Parasitism is an interaction where the parasite benefits and the host is harmed. The parasite depends on the host for food or shelter.

1. Parasites may reduce host survival.
2. They may reduce host growth.
3. They may reduce host reproduction.
4. They may make the host vulnerable to predators.

Final fact:
Parasitism is a + - interaction.

37. What adaptations are seen in parasites?

Parasites show adaptations such as adhesive organs, high reproductive capacity, and loss of unnecessary organs.

Adaptations:

1. Loss of unnecessary sense organs.
2. Adhesive organs or suckers.
3. Loss of digestive system in some forms.
4. High reproductive capacity.
5. Complex life cycles with intermediate hosts or vectors.

Final fact:
Parasite adaptations support survival inside or on hosts.

38. Distinguish between ectoparasites and endoparasites.

Ectoparasites live on the external surface of the host, while endoparasites live inside the host body.

Final fact:
Both types depend on hosts for survival.

39. What is brood parasitism?

Brood parasitism occurs when a bird lays eggs in another bird’s nest and lets the host incubate them.

1. The parasitic bird avoids parental care.
2. The host bird incubates the eggs.
3. Parasitic eggs resemble host eggs in size and colour.
4. This reduces rejection by the host.

Example:
Koel and crow show brood parasitism.

Commensalism Class 12 Biology and Mutualism Questions

Commensalism benefits one species without affecting the other. Mutualism benefits both species and often shows close evolutionary adjustment.

40. What is commensalism?

Commensalism is an interaction where one species benefits and the other remains unaffected. It is represented as + 0.

Examples:

1. Orchid growing on mango branch.
2. Barnacles growing on whale.
3. Cattle egret feeding near grazing cattle.
4. Clown fish living among sea anemone tentacles.

Final fact:
The host species gets no apparent benefit or harm.

41. What is mutualism?

Mutualism is an interaction where both species benefit. It is represented as + +.

Examples:

1. Lichen: fungus and algae or cyanobacteria.
2. Mycorrhiza: fungi and roots of higher plants.
3. Fig tree and pollinator wasp.
4. Orchid and pollinator insect.

Final fact:
Mutualism often involves co-evolution.

42. Explain mutualism in lichens.

Lichens show mutualism between a fungus and a photosynthetic alga or cyanobacterium. Both partners benefit.

1. The alga or cyanobacterium performs photosynthesis.
2. It provides food to the fungus.
3. The fungus offers protection and support.

Final fact:
Lichen is a classic example of mutualism.

43. Explain mutualism between fig tree and wasp.

Fig tree and wasp show a one-to-one mutualistic relationship. A given fig species is pollinated only by its partner wasp species.

1. The female wasp uses fig fruit for egg-laying.
2. Wasp larvae feed on some developing seeds.
3. The wasp pollinates the fig inflorescence.
4. The fig provides food and oviposition site.

Final fact:
Fig-wasp mutualism shows co-evolution.

44. How does Ophrys orchid use sexual deceit?

Ophrys orchid uses sexual deceit by making one petal resemble a female bee. The male bee attempts pseudocopulation and transfers pollen.

1. One petal resembles the female bee in size, colour, and markings.
2. Male bee mistakes it for a female.
3. Pollen sticks to the bee.
4. The bee transfers pollen to another flower.

Final fact:
Pollination success depends on flower-pollinator co-evolution.

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