The living world is a complex web of interactions between organisms and their environment. Biology, especially ecology, helps us understand how organisms adapt, interact, and survive in nature. The first unit of ecology in Class 12 NCERT, Organisms and Populations, explains how organisms respond to abiotic and biotic factors, population dynamics, and adaptations for survival.
This chapter is highly important for both board exams and competitive exams like NEET, as it builds the foundation of ecological concepts. In this article, we will cover every point in detail with clear notes format so that students can revise easily.
Organisms and Populations Class 12 Notes | MCQs, Short & Long Questions, FAQs
Organisms and Their Environment
Levels of Biological Organization
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Organism – Individual living being (e.g., a deer, a plant).
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Population – Group of individuals of the same species in a given area.
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Community – Different populations living together in an area.
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Ecosystem – Community plus the physical environment.
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Biome – Large regions with characteristic climate, flora, and fauna (e.g., deserts, tundra).
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Biosphere – Global sum of all ecosystems.
Major Abiotic Factors Affecting Organisms
Temperature
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Most important ecological factor.
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Influences metabolism, physiology, reproduction.
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Temperature tolerance decides distribution of organisms.
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Categories:
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Eurythermal organisms – Tolerate wide range of temperatures (e.g., cat, dog).
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Stenothermal organisms – Tolerate narrow range of temperatures (e.g., penguins, corals).
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Water
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Essential for all forms of life.
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Distribution of organisms depends on water availability.
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Examples:
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Desert animals show adaptations like water conservation.
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Aquatic organisms are influenced by salinity.
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Categories:
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Euryhaline – Can tolerate wide range of salinity (e.g., salmon).
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Stenohaline – Can tolerate narrow range of salinity (e.g., goldfish).
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Light
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Important for photosynthesis, which sustains life.
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Influences photoperiodism (flowering, reproduction).
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Regulates diurnal and seasonal rhythms in organisms.
Soil
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Nature and quality of soil affects vegetation.
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Properties: soil composition, grain size, pH, mineral content, water-holding capacity.
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Determines type of flora and fauna in an area.
Responses of Organisms to Environment
Homeostasis
Organisms try to maintain a constant internal environment despite external changes.
Types of Responses
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Regulators
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Maintain constant internal environment.
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Example: Humans maintain constant body temperature (37°C) through sweating, shivering.
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Conformers
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Do not regulate internal conditions.
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Most animals and plants fall in this category.
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Their body temperature or osmotic concentration changes with environment.
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Partial Regulators
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Regulate at certain conditions but conform under extreme conditions.
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Migrate
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Temporary movement to more suitable environment.
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Example: Siberian birds migrate to India in winter.
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Suspend
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Enter dormant stage to avoid unfavorable conditions.
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Examples:
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Bears hibernate in winter.
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Seeds and spores go dormant.
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Adaptations in Organisms
Adaptations are special features that help organisms survive in specific environments.
Types of Adaptations
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Structural adaptations – Morphological changes (e.g., thick fur in polar bears).
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Physiological adaptations – Functional changes (e.g., kangaroo rat concentrates urine to conserve water).
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Behavioral adaptations – Change in behavior (e.g., desert lizards bask in sun to regulate body temperature).
Examples of Adaptations
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Desert plants: thick cuticle, sunken stomata, CAM pathway.
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Polar animals: small extremities (Allen’s rule), thick fat layer (blubber).
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Altitude: high RBC count, increased breathing rate to adapt to low oxygen.
Population Ecology
Definition of Population
A population is a group of individuals of the same species living in a defined geographical area, sharing a common gene pool, and capable of interbreeding.
Population Attributes
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Population size (Density)
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Number of individuals per unit area or volume.
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Population growth
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Increases due to natality (birth rate) and immigration.
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Decreases due to mortality (death rate) and emigration.
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Population age distribution
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Pre-reproductive, reproductive, and post-reproductive individuals.
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Predicts future growth trends.
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Population sex ratio
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Ratio of males to females.
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Population Growth
1. Exponential Growth
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When resources are unlimited.
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Population grows at a constant rate (J-shaped curve).
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Formula: dN/dt = rN
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N = population size
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r = intrinsic rate of increase
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2. Logistic Growth
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When resources are limited.
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Growth slows and stabilizes at carrying capacity (K).
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S-shaped curve (Sigmoid curve).
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Formula: dN/dt = rN (K-N)/K
Population Interactions
Organisms interact with other organisms of same or different species.
Positive Interactions
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Mutualism (+/+) – Both species benefit (e.g., pollination in flowers).
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Commensalism (+/0) – One benefits, other unaffected (e.g., orchids on mango tree).
Negative Interactions
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Predation (+/–) – Predator benefits, prey harmed (e.g., lion and deer).
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Parasitism (+/–) – Parasite benefits, host harmed (e.g., tapeworm in human intestine).
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Competition (–/–) – Both species harmed due to struggle for same resources (e.g., flamingos and fishes competing for zooplanktons).
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Amensalism (0/–) – One harmed, other unaffected (e.g., penicillin secreting fungus inhibits growth of bacteria).
Key Examples for NEET and Boards
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Gause’s Competitive Exclusion Principle – Two species competing for the same resources cannot coexist indefinitely.
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Lichen – Example of mutualism (algae + fungi).
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Cuckoo in crow’s nest – Example of brood parasitism.
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Mycorrhiza – Mutualism between fungi and plant roots.
MCQs – Organisms and Populations
Q1. Which of the following is the most important ecological factor influencing the distribution of organisms?
a) Soil
b) Light
c) Temperature
d) Water
Answer: The correct answer is c) Temperature, as it directly affects metabolism, growth, and reproduction of organisms.
Q2. Organisms that can tolerate a wide range of temperatures are called:
a) Stenothermal
b) Eurythermal
c) Endothermal
d) Homeothermal
Answer: The correct answer is b) Eurythermal, as they can survive in a wide temperature range.
Q3. Kangaroo rat conserves water by:
a) Producing very dilute urine
b) Drinking plenty of water
c) Producing highly concentrated urine
d) Sweating
Answer: The correct answer is c) Producing highly concentrated urine, which helps it survive without drinking water.
Q4. Which of the following is an example of mutualism?
a) Lichen
b) Orchid on mango tree
c) Lion and deer
d) Cuckoo in crow’s nest
Answer: The correct answer is a) Lichen, where algae and fungi live together mutually.
Q5. Gause’s Competitive Exclusion Principle states:
a) Two species can never compete
b) Two species competing for the same resource cannot coexist indefinitely
c) Competition always benefits both species
d) Competition leads to extinction of ecosystems
Answer: The correct answer is b) Two species competing for the same resource cannot coexist indefinitely.
Q6. Which of the following is an example of commensalism?
a) Algae and fungi in lichen
b) Cuckoo in crow’s nest
c) Orchid on mango tree
d) Lion and hyena
Answer: The correct answer is c) Orchid on mango tree, where orchid benefits while mango remains unaffected.
Q7. The logistic growth curve of a population is:
a) J-shaped
b) S-shaped
c) U-shaped
d) Straight line
Answer: The correct answer is b) S-shaped, showing growth followed by stabilization at carrying capacity.
Q8. Population density can be measured as:
a) Number per unit volume
b) Number per unit area
c) Both a and b
d) None of the above
Answer: The correct answer is c) Both a and b, depending on the type of population studied.
Q9. Which is a regulator organism?
a) Plants
b) Fishes
c) Amphibians
d) Humans
Answer: The correct answer is d) Humans, as they maintain constant body temperature and internal environment.
Q10. Altitude sickness is caused due to:
a) High CO₂ levels
b) Low O₂ levels
c) High humidity
d) High temperature
Answer: The correct answer is b) Low O₂ levels, leading to nausea, headache, and breathlessness.
Q11. The intrinsic rate of natural increase is represented by:
a) K
b) r
c) N
d) dt
Answer: The correct answer is b) r, which represents the growth rate of a population.
Q12. Which of the following is an example of brood parasitism?
a) Orchid on mango tree
b) Cuckoo laying eggs in crow’s nest
c) Lichen
d) Tapeworm in human intestine
Answer: The correct answer is b) Cuckoo laying eggs in crow’s nest, where host bird raises cuckoo’s offspring.
Q13. Soil pH is important because:
a) It affects light intensity
b) It influences mineral availability and vegetation
c) It controls atmospheric pressure
d) It regulates photoperiodism
Answer: The correct answer is b) It influences mineral availability and vegetation, which impacts distribution of plants.
Q14. Which interaction represents both species being harmed?
a) Mutualism
b) Commensalism
c) Competition
d) Amensalism
Answer: The correct answer is c) Competition, as both species expend energy and resources.
Q15. Which of the following is an example of amensalism?
a) Orchid on mango tree
b) Penicillium secreting antibiotic killing bacteria
c) Cuckoo in crow’s nest
d) Lichen
Answer: The correct answer is b) Penicillium secreting antibiotic killing bacteria, where fungus is unaffected but bacteria are harmed.
Q16. Which of the following statements is true about polar animals?
a) They have long ears and tails
b) They have thick layer of fat (blubber)
c) They hibernate in summer
d) They are stenothermal but water-loving
Answer: The correct answer is b) They have thick layer of fat (blubber) to conserve heat.
Q17. Which of the following curves represents exponential growth?
a) S-curve
b) J-curve
c) Bell-shaped curve
d) Parabolic curve
Answer: The correct answer is b) J-curve, representing unlimited resource growth.
Q18. Migration of Siberian cranes to India is an adaptation to:
a) Avoid predation
b) Avoid cold climatic conditions
c) Search for new habitats permanently
d) Reduce competition
Answer: The correct answer is b) Avoid cold climatic conditions, as they migrate during winter.
Q19. Which of the following is not an attribute of population?
a) Birth rate
b) Mortality
c) Sex ratio
d) Metabolism
Answer: The correct answer is d) Metabolism, as it is a property of individuals, not populations.
Q20. Which rule states that polar animals have shorter extremities to minimize heat loss?
a) Bergmann’s rule
b) Allen’s rule
c) Gause’s principle
d) Hardy-Weinberg law
Answer: The correct answer is b) Allen’s rule, which relates to adaptation in cold climates.
Short Answer Questions
- Q1. Define population density.
Answer: Population density is the number of individuals of a species present per unit area or volume at a given time. For example, the number of tigers per square kilometer in a forest.
Q2. What are eurythermal organisms?
Answer: Eurythermal organisms are those that can tolerate and survive within a wide range of temperatures. For example, many fish and insects.
Q3. What is hibernation?
Answer: Hibernation is a type of dormancy seen in some animals during winter, where metabolic activities are reduced to conserve energy. For example, bears hibernate to survive extreme cold.
Q4. Mention two examples of mutualism.
Answer: Examples of mutualism are:
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Lichen – association between algae and fungi.
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Mycorrhiza – association between fungi and roots of higher plants.
Q5. Define carrying capacity.
Answer: Carrying capacity is the maximum number of individuals of a species that an environment can support sustainably, without degradation of resources.
Q6. What is brood parasitism?
Answer: Brood parasitism is a form of parasitism in which one organism lays its eggs in the nest of another, leaving the host to raise its offspring. Example: Cuckoo lays eggs in crow’s nest.
Q7. Name the four factors that affect population size.
Answer: The four factors affecting population size are:
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Natality (birth rate)
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Mortality (death rate)
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Immigration (entry of individuals)
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Emigration (exit of individuals)
Q8. What is Allen’s rule?
Answer: Allen’s rule states that animals of colder regions have shorter extremities like ears, tails, and legs to minimize heat loss, while animals of warmer regions have longer extremities.
Q9. Define amensalism with an example.
Answer: Amensalism is an interaction in which one species is harmed while the other remains unaffected. Example: Penicillium secretes an antibiotic that kills surrounding bacteria.
Q10. What is photoperiodism?
Answer: Photoperiodism is the response of organisms, especially plants, to the duration of light and dark periods, which influences flowering, reproduction, and growth.
Long Answer Questions
Q1. Explain the different responses of organisms to abiotic factors.
Answer: Organisms respond to abiotic factors such as temperature, water, light, and soil in various ways. Some regulate their internal environment (regulators), like humans maintaining constant body temperature. Others conform (conformers), allowing their body conditions to change with the environment, like most fishes and plants. Migration is another strategy, where organisms move temporarily to favorable conditions, such as migratory birds. Suspension or dormancy, like hibernation in bears or seed dormancy in plants, helps survive unfavorable conditions. These responses ensure survival and continuation of species under varying environmental conditions.
Q2. Differentiate between exponential and logistic growth of a population.
Answer: Exponential growth occurs when resources are unlimited, and the population grows at a constant rate, producing a J-shaped curve. Logistic growth occurs when resources are limited; the population increases initially but stabilizes at carrying capacity, forming an S-shaped curve. Exponential growth is ideal but unrealistic in nature, whereas logistic growth is more practical as it considers environmental resistance.
Q3. Describe the major population interactions with examples.
Answer: Populations interact in different ways:
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Mutualism (+/+): Both benefit (e.g., lichens).
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Commensalism (+/0): One benefits, other unaffected (e.g., orchid on mango tree).
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Predation (+/–): Predator benefits, prey harmed (e.g., lion and deer).
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Parasitism (+/–): Parasite benefits, host harmed (e.g., tapeworm in humans).
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Competition (–/–): Both harmed due to struggle for resources (e.g., flamingos and fishes competing for zooplanktons).
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Amensalism (0/–): One harmed, other unaffected (e.g., penicillin fungus kills bacteria).
These interactions shape ecosystems and affect biodiversity.
Q4. What adaptations help desert plants and animals survive?
Answer: Desert plants have thick cuticles, reduced leaves, sunken stomata, and CAM photosynthesis to reduce water loss. Desert animals show behavioral adaptations like nocturnal activity and physiological adaptations like water conservation. For example, kangaroo rats excrete concentrated urine and obtain water from food oxidation. Lizards bask in the sun or hide to regulate body temperature. Such adaptations are vital for survival in arid environments.
Q5. Discuss the significance of population age distribution.
Answer: Age distribution is the proportion of individuals in pre-reproductive, reproductive, and post-reproductive stages. It predicts the growth trend of a population. A population with more young individuals indicates positive growth, equal numbers suggest stable growth, and more old individuals indicate declining growth. Age distribution helps ecologists and policymakers in predicting population dynamics and planning resource management.
Frequently Asked Questions (FAQs)
Q1. What is the difference between regulators and conformers?
Answer: Regulators maintain constant internal conditions despite external changes, like humans. Conformers change their body conditions with the environment, like most fishes and reptiles.
Q2. Why are small animals rarely found in polar regions?
Answer: Small animals lose body heat quickly due to their large surface area to volume ratio. Since they cannot retain heat efficiently, survival in cold regions becomes difficult.
Q3. What is the ecological importance of predation?
Answer: Predation regulates prey population, maintains ecological balance, and promotes natural selection by removing weak and unfit individuals from a population.
Q4. How does migration differ from hibernation?
Answer: Migration is the seasonal movement of organisms from one region to another in search of favorable conditions, like Siberian cranes migrating to India. Hibernation is dormancy during unfavorable winter conditions, like bears sleeping through winter.
Q5. What is the role of carrying capacity in population growth?
Answer: Carrying capacity limits population growth by restricting the number of individuals that can be supported by resources in an ecosystem. It helps maintain ecological balance.
Conclusion
The chapter Organisms and Populations highlights how life thrives in diverse conditions due to interactions and adaptations. By studying abiotic factors, responses, adaptations, and population dynamics, we gain insights into survival strategies and ecological balance. Understanding these concepts is not only important for exams but also for appreciating the complexity of nature.
Complete Ecosystem Notes for Class 12 Biology | MCQs, Short & Long Questions