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Organisms and Populations

NCERT Class 12 · Biology Based on NCERT Class 12 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter introduces ecology at the population level, explaining how organisms interact with each other and their environment. It covers population attributes, growth models, life history variations, and interspecific interactions. The one big idea is that populations, not individuals, evolve and are shaped by interactions like predation, competition, parasitism, commensalism, and mutualism, all of which affect population size and dynamics.
Key Concepts (25)
Ecology
The study of interactions among organisms and between the organism and its physical (abiotic) environment.
e.g. Studying how lotus plants in a pond interact with water, sunlight, and fish.
Population
A group of individuals of a given species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding.
e.g. All cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract.
Population density (N)
The size of a population, technically called population density, measured in numbers, biomass, or percent cover depending on the species.
e.g. 200 carrot grass plants vs. one huge banyan tree — biomass is a more meaningful measure for the banyan.
Birth rate (Natality)
The number of births during a given period in the population that are added to the initial density, expressed per capita.
e.g. If 8 new lotus plants are added to 20 existing ones, birth rate = 8/20 = 0.4 offspring per lotus per year.
Death rate (Mortality)
The number of deaths in the population during a given period, expressed per capita.
e.g. If 4 fruitflies die out of 40 in a week, death rate = 4/40 = 0.1 individuals per fruitfly per week.
Sex ratio
The proportion of males and females in a population.
e.g. 60% females and 40% males in a population.
Age pyramid
A graphical representation of the age distribution (percentage of individuals of a given age or age group) of males and females in a population, reflecting its growth status (growing, stable, or declining).
e.g. A pyramid with a broad base indicates a growing population.
Immigration
The number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
e.g. Birds migrating to a wetland for breeding.
Emigration
The number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.
e.g. Fish moving out of a pond due to overcrowding.
Intrinsic rate of natural increase (r)
The per capita rate of increase in a population, calculated as birth rate minus death rate (b – d), representing the inherent potential of a population to grow.
e.g. For Norway rat r = 0.015; for flour beetle r = 0.12; for human population in India in 1981 r = 0.0205.
Carrying capacity (K)
The maximum population size of a species that a given habitat can support indefinitely without degradation.
e.g. A forest can support a maximum of 50 deer; beyond that, resources become limiting.
Exponential growth
Population growth under unlimited resource conditions, resulting in a J-shaped curve when population density (N) is plotted against time.
e.g. A Paramecium population doubling every day under ideal conditions.
Logistic growth
Population growth in a habitat with limited resources, showing a lag phase, acceleration, deceleration, and finally an asymptote at carrying capacity, resulting in a sigmoid (S-shaped) curve.
e.g. A bacterial population in a petri dish with limited nutrients.
Competitive Exclusion Principle (Gause's Principle)
Two closely related species competing for the same resources cannot co-exist indefinitely; the competitively inferior one will be eliminated eventually.
e.g. Balanus (larger barnacle) excludes Chathamalus (smaller barnacle) from the intertidal zone on Scottish coasts.
Resource partitioning
A mechanism by which competing species avoid competition by using different times for feeding or different foraging patterns, promoting co-existence.
e.g. Five closely related species of warblers living on the same tree avoid competition due to behavioural differences in foraging activities (MacArthur's study).
Predation
An interaction where one organism (predator) kills and eats another organism (prey); includes herbivory.
e.g. Tiger eating deer; sparrow eating seeds.
Parasitism
An interaction where one organism (parasite) benefits and the other (host) is harmed; the parasite lives on or inside the host.
e.g. Lice on humans (ectoparasite); liver fluke in humans (endoparasite).
Commensalism
An interaction where one species benefits and the other is neither harmed nor benefited.
e.g. Orchid growing as an epiphyte on a mango branch; barnacles on a whale; cattle egret and grazing cattle.
Mutualism
An interaction where both interacting species benefit.
e.g. Lichens (fungus and algae); mycorrhizae (fungi and plant roots); fig tree and wasp pollination.
Amensalism
An interaction where one species is harmed and the other is unaffected.
e.g. A large tree shading out smaller plants beneath it.
Brood parasitism
A form of parasitism in birds where the parasitic bird lays its eggs in the nest of its host, letting the host incubate them.
e.g. Cuckoo (koel) laying eggs in crow's nest.
Co-evolution
The reciprocal evolutionary process where two species (e.g., flower and pollinator) evolve in response to each other.
e.g. Mediterranean orchid Ophrys mimics female bee to attract male bee for pollination.
Phytophagous
Feeding on plant sap and other parts of plants; nearly 25% of all insects are phytophagous.
e.g. Caterpillars feeding on leaves.
Cryptic colouration (Camouflage)
A defence mechanism where an organism is coloured to blend with its environment, avoiding detection by predators.
e.g. Some species of insects and frogs are cryptically-coloured.
Darwinian fitness
The reproductive fitness of an organism, measured by its intrinsic rate of natural increase (r value).
e.g. Populations evolve to maximise their r value in their habitat.
Formulas & Equations (5)
Nt+1 = Nt + [(B + I) – (D + E)]
Reads as: Population density at time t+1 equals population density at time t plus births plus immigration minus deaths minus emigration.
Variables: Nt = population density at time t; B = number of births; I = number of immigrants; D = number of deaths; E = number of emigrants.
Use when: Calculating change in population density over a given time period.
⚠ Forgetting to include immigration and emigration; students often only consider births and deaths.
dN/dt = (b – d) × N
Reads as: The rate of change of population size with respect to time equals the difference between per capita birth rate and per capita death rate multiplied by the population size.
Variables: dN/dt = rate of change in population size; b = per capita birth rate; d = per capita death rate; N = population size.
Use when: Describing exponential growth when resources are unlimited.
⚠ Confusing per capita rates (b and d) with total numbers (B and D).
dN/dt = rN
Reads as: The rate of change of population size equals the intrinsic rate of natural increase multiplied by the population size.
Variables: r = intrinsic rate of natural increase (b – d); N = population size.
Use when: Exponential growth equation; r is a key parameter for comparing population growth potential.
⚠ Forgetting that r is per capita and unitless (per time).
Nt = N0 e^(rt)
Reads as: Population density after time t equals initial population density times e raised to the power of r times t.
Variables: Nt = population density after time t; N0 = population density at time zero; r = intrinsic rate of natural increase; e = base of natural logarithms (2.71828); t = time.
Use when: Calculating population size after a given time under exponential growth.
⚠ Using log base 10 instead of natural log (e); misplacing the exponent.
dN/dt = rN ((K – N)/K)
Reads as: The rate of change of population size equals the intrinsic rate of natural increase times population size times the fraction of carrying capacity still available.
Variables: N = population density at time t; r = intrinsic rate of natural increase; K = carrying capacity.
Use when: Describing logistic growth when resources are limited.
⚠ Writing the equation as dN/dt = rN (K – N) without dividing by K; forgetting the (K – N)/K term.
How It Works
Population Growth (Exponential)
  1. Step 1: Resources are unlimited in the habitat.
  2. Step 2: Per capita birth rate (b) exceeds per capita death rate (d), giving a positive intrinsic rate of natural increase (r).
  3. Step 3: Population size (N) increases at a rate proportional to its current size: dN/dt = rN.
  4. Step 4: The population grows in a J-shaped curve when N is plotted against time.
  5. Step 5: In the absence of checks, population can reach enormous densities in a short time (e.g., chessboard wheat grain story).
💡 Exponential growth is theoretical and rarely occurs in nature for long because resources are always limited.
Population Growth (Logistic)
  1. Step 1: Resources in the habitat are finite, leading to competition.
  2. Step 2: The habitat has a carrying capacity (K) — the maximum population size it can support.
  3. Step 3: Initially, growth is slow (lag phase), then accelerates as resources are abundant.
  4. Step 4: As population approaches K, growth decelerates due to resource limitation.
  5. Step 5: Population density stabilises at K (asymptote), producing an S-shaped (sigmoid) curve.
  6. Step 6: The equation is dN/dt = rN ((K – N)/K).
💡 The logistic model is more realistic than exponential; the term (K – N)/K represents the unused carrying capacity.
Population Density Change (Four Basic Processes)
  1. Step 1: Population density changes due to four processes: natality (B), mortality (D), immigration (I), and emigration (E).
  2. Step 2: Natality and immigration increase population density.
  3. Step 3: Mortality and emigration decrease population density.
  4. Step 4: The net change is given by Nt+1 = Nt + [(B + I) – (D + E)].
💡 Births and deaths are the most important factors under normal conditions; immigration and emigration become significant during colonisation of new habitats.
Predator-Prey Dynamics and Defence Mechanisms
  1. Step 1: Predators keep prey populations under control, preventing ecosystem instability.
  2. Step 2: Predators help maintain species diversity by reducing competition among prey (e.g., Pisaster starfish experiment).
  3. Step 3: Prey species evolve defences: cryptic colouration (camouflage), poisonous chemicals (e.g., Monarch butterfly), and morphological structures (thorns in Acacia, Cactus).
  4. Step 4: Plants produce chemical defences like cardiac glycosides (Calotropis), nicotine, caffeine, quinine, etc.
💡 Predators are 'prudent' — overexploitation would lead to extinction of both predator and prey.
Competitive Exclusion and Resource Partitioning
  1. Step 1: Two species compete for the same limiting resource.
  2. Step 2: According to Gause's Competitive Exclusion Principle, the superior competitor eliminates the inferior one.
  3. Step 3: Evidence includes the Abingdon tortoise extinction after goat introduction, and Connell's barnacle experiment (Balanus excludes Chathamalus).
  4. Step 4: To avoid exclusion, species may evolve resource partitioning — using different times or foraging patterns (e.g., MacArthur's warblers).
💡 Competition can occur even when resources are not limiting (interference competition).
Parasitism and Life Cycle Complexity
  1. Step 1: Parasites evolve host-specificity and co-evolve with hosts.
  2. Step 2: Adaptations include loss of sense organs, adhesive organs/suckers, loss of digestive system, high reproductive capacity.
  3. Step 3: Life cycles often involve intermediate hosts or vectors (e.g., liver fluke needs snail and fish; malarial parasite needs mosquito).
  4. Step 4: Ectoparasites (e.g., lice, ticks) live on host surface; endoparasites (e.g., liver fluke) live inside host body.
  5. Step 5: Brood parasitism (e.g., cuckoo) involves laying eggs in host's nest.
💡 An ideal parasite should not kill its host quickly, but natural selection has not led to totally harmless parasites because hosts evolve defences.
Mutualism in Plant-Animal Interactions
  1. Step 1: Plants need animals for pollination and seed dispersal.
  2. Step 2: Plants offer rewards: pollen, nectar, juicy fruits.
  3. Step 3: Co-evolution ensures tight relationships (e.g., fig tree and wasp — one-to-one pollination).
  4. Step 4: Some orchids use sexual deceit (e.g., Ophrys mimics female bee to attract male bee for pollination).
💡 Mutualistic systems must safeguard against 'cheaters' — animals that take rewards without pollinating.
Exam Traps — Don't Make These Mistakes
⚠ Confusing per capita rates with total numbers
✗ Birth rate = total number of births in a population.
→ Birth rate in ecology is per capita (per individual), not total. NCERT defines it as 'change in numbers with respect to members of the population'.
✓ Birth rate is the number of births per individual per unit time, e.g., 0.4 offspring per lotus per year.
⚠ Thinking exponential growth is always J-shaped and logistic is always S-shaped
✗ Exponential growth always gives a J-shaped curve, and logistic growth always gives an S-shaped curve.
→ While that is generally true, students often forget that exponential growth can also be geometric and that the J-shape is only when resources are unlimited. Logistic growth always gives a sigmoid curve only when resources become limiting.
✓ Exponential growth results in a J-shaped curve when N is plotted against time under unlimited resources. Logistic growth results in a sigmoid (S-shaped) curve when resources are limiting, with an asymptote at carrying capacity K.
⚠ Confusing commensalism with mutualism
✗ Cattle egret and cattle is an example of mutualism because both benefit.
→ In commensalism, one species benefits and the other is unaffected. The cattle are neither harmed nor benefited by the egret; the egret benefits by catching insects stirred up by cattle.
✓ Cattle egret and grazing cattle is an example of commensalism: the egret benefits (gets food), the cattle are unaffected.
⚠ Thinking the female mosquito is a parasite
✗ The female mosquito is a parasite because it sucks blood.
→ The female mosquito is not considered a parasite because it does not live on or in the host; it only takes a blood meal for reproduction and then leaves. Parasitism involves a prolonged association.
✓ The female mosquito is not a parasite; it is a temporary feeder. Parasitism requires a sustained relationship where the parasite lives on or inside the host.
⚠ Misapplying Competitive Exclusion Principle
✗ Two species competing for the same resource can never co-exist.
→ Gause's principle states they cannot co-exist indefinitely if resources are limiting. However, species can co-exist through resource partitioning or if resources are not limiting.
✓ According to Gause's Competitive Exclusion Principle, two closely related species competing for the same limiting resources cannot co-exist indefinitely; the inferior competitor is eliminated. However, co-existence is possible through resource partitioning.
⚠ Confusing age pyramid shapes with population status
✗ A pyramid with a narrow base indicates a growing population.
→ A broad base (many young individuals) indicates a growing population. A narrow base indicates a declining population.
✓ A broad-based age pyramid indicates a growing population; a bell-shaped pyramid indicates a stable population; a narrow-based pyramid indicates a declining population.
Compare & Contrast
Resource availability
Exponential growth
Logistic growth
Exponential growth occurs under unlimited resources (J-shaped curve), while logistic growth occurs under limited resources (S-shaped curve) with an asymptote at carrying capacity K.
Effect on the other organism
Predation
Parasitism
In predation, the predator kills and consumes the prey (immediate death), while in parasitism, the parasite lives on or inside the host and usually does not kill the host immediately (though it may weaken it).
Benefit to the second species
Commensalism
Mutualism
In commensalism, one species benefits and the other is unaffected (e.g., orchid on mango tree). In mutualism, both species benefit (e.g., lichens — fungus and algae).
Location on/in host
Ectoparasite
Endoparasite
Ectoparasites feed on the external surface of the host (e.g., lice on humans), while endoparasites live inside the host body (e.g., liver fluke in liver).
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA (2-3 marks), 1 LA (5 marks) — typically 8-10 marks total
Most asked: Population attributes (birth rate, death rate, sex ratio, age pyramid) · Exponential vs. logistic growth (including formulas and graphs) · Interspecific interactions (predation, competition, parasitism, commensalism, mutualism) with examples · Competitive Exclusion Principle and resource partitioning · Defence mechanisms in plants against herbivory · Age pyramids and their interpretation
Don't skip: Derive and explain exponential and logistic growth equations with graphs.; Explain Gause's Competitive Exclusion Principle with an example.; Differentiate between predation, parasitism, commensalism, and mutualism with examples.; Describe the four processes affecting population density and the equation Nt+1 = Nt + [(B+I) – (D+E)].
One-Line Revision (read aloud before the exam)
  1. Ecology is the study of interactions among organisms and between organisms and their abiotic environment.
  2. A population is a group of individuals of the same species living in a defined geographical area, sharing resources and potentially interbreeding.
  3. Population density (N) can be measured in numbers, biomass, or percent cover depending on the species.
  4. Birth rate and death rate are per capita rates, not total numbers.
  5. Age pyramids show the age distribution of males and females and indicate whether a population is growing, stable, or declining.
  6. Population density changes due to natality (B), mortality (D), immigration (I), and emigration (E): Nt+1 = Nt + [(B+I) – (D+E)].
  7. Exponential growth (dN/dt = rN) occurs under unlimited resources, producing a J-shaped curve.
  8. Logistic growth (dN/dt = rN ((K–N)/K)) occurs under limited resources, producing an S-shaped curve with carrying capacity K.
  9. The intrinsic rate of natural increase (r) is the per capita difference between birth and death rates.
  10. Predators keep prey populations in check and help maintain species diversity.
  11. Plants defend against herbivory using thorns (Acacia, Cactus) and chemicals (cardiac glycosides in Calotropis, nicotine, caffeine).
  12. Gause's Competitive Exclusion Principle states that two species competing for the same limiting resources cannot co-exist indefinitely.
  13. Resource partitioning allows competing species to co-exist by using different times or foraging patterns.
  14. Parasitism benefits the parasite and harms the host; ectoparasites live on the host surface, endoparasites live inside.
  15. Brood parasitism (e.g., cuckoo laying eggs in crow's nest) is a form of parasitism in birds.
  16. Commensalism benefits one species while the other is unaffected (e.g., orchid on mango tree).
  17. Mutualism benefits both species (e.g., lichens, mycorrhizae, fig tree and wasp).
  18. Co-evolution is seen in tight plant-pollinator relationships, like the Mediterranean orchid Ophrys mimicking female bees.
  19. The female mosquito is not a parasite because it does not live on the host; it only takes a blood meal temporarily.
  20. Predators are 'prudent' — overexploitation would lead to extinction of both predator and prey.

MCQs — 10 Questions with Answers

Q1. Which of the following is an attribute unique to populations and not found in individual organisms?

  • A. Birth and death
  • B. Birth rate and death rate ✓
  • C. Feeding behaviour
  • D. Reproductive potential

Answer: B — Individuals have absolute births and deaths; populations have per capita birth and death rates as group-level statistical attributes.

Q2. If a bacterial culture of 10,000 cells experiences 500 new cells produced per hour, what is the birth rate?

  • A. 50 cells/hour
  • B. 0.05 cells per bacterium per hour ✓
  • C. 500 cells per hour
  • D. 0.5 cells per bacterium per hour

Answer: B — Birth rate = (number of births) / (population size) = 500 / 10,000 = 0.05 cells per bacterium per hour.

Q3. An age pyramid showing a wide base, narrow middle, and pointed top indicates a population that is:

  • A. Stable and not changing
  • B. Declining in size
  • C. Rapidly growing ✓
  • D. At carrying capacity

Answer: C — A pyramidal (wide base, pointed top) shape reflects high proportion of young individuals, indicating rapid population growth potential.

Q4. Why is biomass a more appropriate measure of population density for a forest containing one large banyan tree and 200 small carrot grass plants?

  • A. Because the banyan tree occupies more space
  • B. Because biomass reflects actual ecological impact and resource use better than count ✓
  • C. Because plants cannot be counted accurately
  • D. Because carrot grass has no ecological value

Answer: B — Biomass or percentage cover captures the actual contribution to community structure and resource allocation; the banyan's canopy and root system have far greater ecological influence than 200 small plants.

Q5. A population initially has 50 individuals. During one generation, 15 new individuals are born and 8 die. What is the net change in population size?

  • A. +7 individuals (birth rate = 0.3, death rate = 0.16) ✓
  • B. +23 individuals (net reproduction exceeds mortality)
  • C. +15 individuals (births only counted)
  • D. -7 individuals (deaths exceed births)

Answer: A — Net change = births − deaths = 15 − 8 = +7; birth rate = 15/50 = 0.3 per capita; death rate = 8/50 = 0.16 per capita.

Q6. Which statement about sex ratio in populations is correct?

  • A. Sex ratio is always 1:1 in all populations
  • B. Sex ratio is an individual attribute, not a population attribute
  • C. Sex ratio affects reproductive capacity and population growth rate ✓
  • D. Sex ratio is independent of ecological factors

Answer: C — Sex ratio is a population-level attribute; a higher proportion of females increases reproductive potential and population growth, while skewed ratios limit breeding success.

Q7. Both A and R statement analysis: Assertion (A): Population ecology links ecology to evolution. Reason (R): Natural selection operates at the population level, and population genetics determines trait frequencies. Which is correct?

  • A. Both A and R are true, and R correctly explains A ✓
  • B. Both A and R are true, but R does not explain A
  • C. A is true but R is false
  • D. Both A and R are false

Answer: A — Population ecology bridges ecology and evolution because populations, not individuals, experience natural selection and genetic change, making population genetics and ecology inseparable.

Q8. If a stable population has an age pyramid with a columnar (rectangular) shape, which of the following is NOT true?

  • A. Birth rate approximately equals death rate
  • B. Each age group has roughly similar numbers
  • C. The population is rapidly growing and will double soon ✓
  • D. Reproductive age individuals are balanced relative to young and old

Answer: C — A columnar age pyramid indicates a stable population with equal age distribution, low growth rate, and balanced birth-death rates; rapid growth is shown by pyramidal shape.

Q9. In a pond, there are 100 lotus plants. Over 3 months, 30 plants grow from seeds (births) and 10 plants die. Calculate the death rate (per lotus per 3 months) and identify what is measured.

  • A. Death rate = 0.3, measuring per capita mortality
  • B. Death rate = 0.1, measuring total mortality in absolute terms
  • C. Death rate = 10, measuring absolute number of deaths only
  • D. Death rate = 0.1, measuring per capita mortality correctly ✓

Answer: D — Death rate = (deaths) / (population size) = 10 / 100 = 0.1 per lotus per 3 months; this is per capita (per individual) mortality, not absolute count.

Q10. A researcher monitoring two populations finds: Population X has 1,000 individuals with 100 births and 50 deaths per year; Population Y has 500 individuals with 50 births and 50 deaths per year. Which statement correctly interprets both populations?

  • A. Both have equal growth because both produced 50 net offspring
  • B. Population X has birth rate = 0.1 and is growing faster than Y (birth rate = 0.1, net zero growth)
  • C. Population Y is stable (birth rate = death rate = 0.1), while X grows (net rate = 0.05 per capita) ✓
  • D. Population X is declining while Y is growing

Answer: C — Population X: birth rate = 100/1000 = 0.1, death rate = 50/1000 = 0.05, net rate = 0.05 (growing); Population Y: birth rate = 50/500 = 0.1, death rate = 50/500 = 0.1, net rate = 0 (stable).

Flashcards

What is a population in ecological terms?

A group of organisms of the same species living in a defined geographical area, sharing resources, potentially interbreeding, and constituting a reproductive unit.

Define birth rate and give its formula.

Birth rate is the number of offspring produced per individual per unit time; calculated as (number of births)/(population size).

Define death rate and give its formula.

Death rate is the number of deaths per individual per unit time; calculated as (number of deaths)/(population size).

What is sex ratio in a population?

Sex ratio is the proportion or percentage of males to females in a population at any given time, e.g., 40% males and 60% females.

What is age structure (age pyramid)?

Age structure is the distribution of individuals across different age groups in a population, shown graphically as an age pyramid that reflects growth status.

What does population density (N) measure?

Population density is the total number, biomass, or percentage cover of a population per unit area or volume of habitat.

Why is biomass sometimes a better measure than count for population density?

Biomass measures the actual ecological impact of a population; one large organism (e.g., banyan tree) has greater effect than many small ones (e.g., grass plants).

What do the three shapes of age pyramids indicate?

Pyramidal shape indicates growing population; columnar shape indicates stable population; urn-shaped indicates declining population.

How are populations different from individuals in terms of attributes?

Individuals possess traits (male/female, birth, death); populations possess statistical rates (birth rate, death rate, sex ratio, age structure) at the group level.

What is the ecological significance of studying populations?

Population ecology links ecology to evolution and genetics, allowing prediction of natural selection outcomes and understanding of how species respond to environmental change.

Important Board Questions

Define population and state two attributes that distinguish a population from an individual organism. [2 marks]

Definition must include 'same species, shared resources, geographical area, potential interbreeding.' Two attributes: birth/death rates (per capita, not absolute); sex ratio; age structure (population-level only).

A pond contains 200 lotus plants. During one year, 60 new plants germinate and 20 plants die. Calculate the birth rate and death rate, and explain what these rates tell us about population growth. [5 marks]

Birth rate = 60/200 = 0.3 per lotus per year; death rate = 20/200 = 0.1 per lotus per year. Rates show per capita contribution: net rate (0.2) indicates population is growing; compare rates to explain growth potential independent of absolute numbers.

Explain why biomass is sometimes a more appropriate measure of population density than absolute number count, using a forest ecosystem example. Discuss how age pyramid shapes reflect population growth status and relate this to population dynamics. [6 marks]

Explain: one large banyan tree (biomass, canopy area) has greater ecological impact than 200 grass plants (count misleading). Age pyramids: pyramidal (growing—many youth), columnar (stable—even distribution), urn (declining—few youth). Link to natality/mortality rates and evolutionary significance of population-level analysis in natural selection.

Assertion-Reason Questions for Organisms and Populations

Q1. Assertion (A):

A population can have a birth rate of 0.4 offspring per individual per year, while an individual organism cannot possess this attribute.

Reason (R):

Birth rate and death rate are per capita population attributes that only emerge when multiple individuals are studied together as a group.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The assertion correctly states that populations have birth rates (a collective measure) while individuals do not. The reason correctly explains WHY this is true—these are per capita measures that only apply to groups. This directly reflects the textbook's statement that 'An individual may have births and deaths, but a population has birth rates and death rates.' The reason adequately explains the assertion.

Q2. Assertion (A):

Sex ratio is a population attribute because it represents the percentage distribution of males and females within a group.

Reason (R):

Individual organisms cannot have a sex ratio because they are either male or female, not both.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason directly explains why sex ratio is a population attribute. An individual is singular in sex (either male OR female), so sex ratio as a concept (60% female, 40% male) only applies to populations. The reason provides the logical foundation for the assertion. This tests understanding of why certain attributes are population-level rather than individual-level.

Q3. Assertion (A):

Population density of an organism can be measured using total number, percent cover, or biomass depending on the circumstances.

Reason (R):

Total number is always the most accurate and meaningful measure of population density in all ecological situations.

Answer: C — A is true but R is false — The assertion is true—the textbook explicitly states that population density can be measured differently depending on circumstances. However, the reason is false. The textbook clarifies that total number is 'generally the most appropriate measure' but explicitly notes it is 'in some cases either meaningless or difficult to determine.' The reason contradicts the textbook's nuanced position. Students often think that raw numbers are always best without recognizing practical and biological limitations.

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Case-Based Questions (CBQ) for Organisms and Populations

Case Study:

A wildlife researcher monitored a Siberian crane population at Bharatpur National Park over 10 years. In year 1, the population was 8 individuals. Through careful protection measures, the population grew as follows: Year 1-3 (average 9 individuals), Year 4-6 (average 18 individuals), Year 7-10 (average 35 individuals). The sex ratio remained relatively stable at 45% male and 55% female throughout the period. Age structure analysis showed that in years 1-3, most birds were older than 5 years; by years 7-10, juveniles and young adults composed 60% of the population.

Q1. What does the change in age structure from years 1-3 to years 7-10 indicate about the population's reproductive success?

  • A. Birth rate decreased because juvenile proportion increased
  • B. Birth rate increased significantly, as juvenile and young adult proportion rose from low to 60% ✓
  • C. The sex ratio changed unfavorably for reproduction
  • D. The population's age structure became less favorable for growth

Answer: B

Q2. If the population continued to grow following the trend shown (approximately doubling every 3 years), what concern should conservationists address regarding the sustainable population size at Bharatpur?

  • A. The habitat's carrying capacity may be exceeded, requiring expansion of protected area or translocation ✓
  • B. The population is growing too slowly and faces extinction risk
  • C. The stable sex ratio of 55% female proves population sustainability
  • D. Exponential growth can continue indefinitely without environmental constraints

Answer: A

Q3. Comparing the total population numbers from year 1 (8 individuals) to year 10 (approximately 35 individuals), which statement BEST explains why this growth, while successful, still represents conservation challenge?

  • A. 35 individuals is still a small population vulnerable to stochastic events and genetic bottleneck effects ✓
  • B. The population doubled in size, making it no longer endangered
  • C. Sex ratio of 55% female means the population is female-biased
  • D. The increase from 8 to 35 indicates the species is fully recovered

Answer: A

Q4. What population attribute, beyond total number, should conservationists monitor to predict the population's continued growth or stability over the next 5 years?

  • A. Only the sex ratio, which must remain at 55% female
  • B. The age structure, particularly the proportion of reproductive-age females and juvenile recruitment rate ✓
  • C. The geographic distribution of individuals within the park
  • D. The evolutionary history of the protected population

Answer: B

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Previous Year Questions (PYQ) for Organisms and Populations

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 57/1/1 · 4 marks

The diagram below shows the distribution of two barnacle species, Chthamalus and Balanus on a rocky sea shore. When Balanus is experimentally removed, Chthamalus expands its range in lower intertidal zone.

(a) Identify and define the ecological phenomenon demonstrated by this observation. (1 mark)

CBSE 2026 · Set 57/1/3 · 4 marks

The diagram below shows the distribution of two barnacle species, Chthamalus and Balanus on a rocky sea shore. When Balanus is experimentally removed, Chthamalus expands its range in lower intertidal zone.

Identify and define the ecological phenomenon demonstrated by this observation.

CBSE 2026 · Set 57/2/1 · 2 marks

(a) In some cases, measuring population density by counting individuals may not be feasible. Mention two other parameters that can be used and justify their use with suitable examples.

CBSE 2026 · Set 57/2/2 · 1 mark

Which of the following is an important parameter for assessing the impact of any biotic or abiotic factor on population growth?

  • Birth rate
  • K-carrying capacity
  • Death rate
  • r-Intrinsic rate of natural increase

CBSE 2026 · Set 57/2/2 · 2 marks

In some cases, measuring population density by counting individuals may not be feasible. Mention two other parameters that can be used and justify their use with suitable examples.

CBSE 2026 · Set 57/2/3 · 2 marks

(a) Explain the type of interaction between: (i) Egret and Cattle (ii) Cuckoo and Crow

CBSE 2026 · Set 57(B) Biology · 3 marks

Answer the following questions: (a) Name the type of interaction seen between fig tree and wasps. (b) Mention the benefits that the female wasps desire from the fig tree from such an interaction. (c) Give one example where animal-plant interaction involves 'co-evolution' other than the relationship between fig tree and wasps.

CBSE 2026 · Set 57/3/1 · 3 marks

(a) What is the carrying capacity of a species in a habitat? (b) Explain the growth curve that takes this capacity into account.

CBSE 2026 · Set 57/3/2 · 3 marks

(a) What is the carrying capacity of a species in a habitat ? (b) Explain the growth curve that takes this capacity into account.

CBSE 2026 · Set 57/4/1 · 1 mark

Connell's elegant field experiments on the rocky seacoast of Scotland, found that larger barnacles Balanus dominated the intertidal area and removed the smaller barnacles Chathamalus. This happened due to:

  • Parasitism
  • Predation
  • Mutualism
  • Competition

CBSE 2026 · Set 57/4/1 · 3 marks

(a) Study the graph given below and answer the questions that follow: (i) Write the status of food and space in the curves a and b. (ii) In the absence of predators, which one of the two curves would appropriately depict the prey population? (iii) 'Time' has been shown on x-axis and there is a parallel dotted line above it. Give the significance of this dotted line.

CBSE 2026 · Set 57/4/3 · 1 mark

Assertion (A): The interaction between a whale and the small barnacles on its back is considered as commensalism. Reason (R): In the above association, neither of the species is benefitted.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

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Class 12 Biology Previous Year Questions, Chapter-wise

All 1074 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

See all past board questions →

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