Organisms and Populations - Class 12 Biology

Class 12 Biology | Unit X — Ecology

Chapter 13: Organisms and Populations

Ecology • Adaptations • Population Attributes • Growth Models • Population Interactions

1. Ecology and Abiotic Factors

Ecology: Scientific study of the relationships of living organisms to each other and to their environment (abiotic + biotic). Term coined by Ernst Haeckel (1869). Broadly studied at 4 levels: Organism, Population, Community, Biome/Ecosystem (Biosphere).

1.1 Major Abiotic Factors

Factor	Key Concept / Effect
Temperature	Most important abiotic factor. Affects enzyme activity, metabolic rate. Range: -70°C (fish in Antarctic ice) to 50°C (rajasthani desert). Most organisms (eurythermal) tolerate wide range; stenothermal tolerate narrow range. Bergmann's Rule: body size increases with colder climate (within same species).
Water	Life cannot exist without water. Productivity of habitats depends on water availability. Marine (high salinity), freshwater, estuarine. Salinities: >35 ppt = hypersaline (Red Sea). Euryhaline (wide salinity) vs stenohaline (narrow).
Light	Essential for photosynthesis. Photoperiod determines season-related behaviours (reproduction, migration, hibernation). Shade plants (sciophytes) vs sun plants (heliophytes). UV-B radiation can damage DNA.
Soil	Composition, grain size, aggregation, water-holding capacity, pH determine type of organisms. Sandy soils = low water retention. Clayey soils = high. pH affects nutrient availability.

1.2 Responses to Abiotic Factors

Response	Definition & Example
Regulate	Maintain constant internal temperature/osmolarity regardless of external conditions. Most birds and mammals (homeotherms). Energy-intensive but allows survival in wide range of conditions.
Conform	Internal environment changes with external changes. Most animals (99%) are conformers. No energy spent on thermoregulation. Examples: fish, amphibians, reptiles (poikilotherms).
Migrate	Move to more hospitable areas when conditions become unfavourable. Example: Siberian cranes migrate to Bharatpur (Keoladeo) in winter. Arctic terns make the longest migration.
Suspend	Reduce metabolic activity to survive unfavourable conditions.

1.3 Types of Suspension

Hibernation: Winter sleep to escape cold. Example: bears, some bats. Metabolic rate drops.
Aestivation: Summer dormancy to escape heat and desiccation. Example: snails, lungfish, some amphibians.
Diapause: Stage of suspended development in insects/zooplankton to survive unfavourable conditions (e.g., in eggs or pupal stage). Example: Daphnia (zooplankton).

⚠️ NEET Focus (2013, 2016, 2019, 2022): Hibernation = winter dormancy. Aestivation = summer dormancy (lungfish, snails). Diapause = suspended development in insects/zooplankton. Ecology = Haeckel (1869). Allen's Rule: appendages shorter in cold climate. Bergmann's Rule: body size larger in cold climate. Eurythermal = wide temperature range. Stenothermal = narrow.

2. Adaptations

Adaptation: Any attribute of an organism — morphological, physiological, or behavioural — that enables it to survive and reproduce in its habitat.

2.1 Desert Adaptations

Kangaroo rat (Dipodomys): nocturnal; never drinks water (metabolic water from seeds); concentrated urine; dry faeces; long loops of Henle in kidney.
Cacti: Succulent; CAM photosynthesis (stomata open at night); thick waxy cuticle; spines instead of leaves; shallow widespread roots.
Desert lizards: bask in sun to warm body (thermoregulate behaviourally); shade/burrow during heat.

2.2 High-Altitude Adaptations

Low O₂ at high altitude → acclimatisation: increased RBC count, increased haemoglobin, deeper breathing, heart rate changes.
Occurs over days to weeks if person moves to high altitude. Reversible on return to lower altitude.

3. Population Attributes

Population: Group of individuals of the same species living in a defined area at a given time that can interbreed.

3.1 Key Population Attributes

Attribute	Description
Population size (N)	Total number of individuals in population at a given time. Measured by census (direct count, mark-recapture method).
Population density	Number of individuals per unit area or volume. Can also be measured as biomass, percentage cover, etc.
Birth rate (Natality)	Number of births per individual per unit time. Increases population size.
Death rate (Mortality)	Number of deaths per individual per unit time. Decreases population size.
Age structure	Distribution of individuals among different age groups. Three pyramids: growing (young-heavy, wide base), stable (uniform), declining (old-heavy, narrow base).
Sex ratio	Ratio of males to females. Important for reproductive success.

3.2 Survivorship Curves

Type	Pattern	Example
Type I (Convex)	High survival early in life; most die in old age.	Humans, large mammals, Dall sheep.
Type II (Diagonal)	Constant death rate at all ages.	Birds, many lizards, rodents.
Type III (Concave)	Very high infant mortality; survivors live long.	Oysters, fishes, many invertebrates, most plants.

4. Population Growth Models

4.1 Exponential Growth

When resources are unlimited, a population grows exponentially. Produces a characteristic J-shaped curve.

Geometric growth: N_t = N₀ e^rt
Where: N_t = population size at time t | N₀ = initial population size | e = Euler's number (2.71828) | r = intrinsic rate of natural increase

Differential form: dN/dt = rN
r = (b − d) where b = birth rate, d = death rate.

4.2 Logistic Growth

In nature, resources are finite. Population grows fast initially but slows as it approaches carrying capacity (K). Produces a characteristic S-shaped (sigmoid) curve.

Logistic growth equation (Verhulst-Pearl logistic equation):
dN/dt = rN [(K − N) / K]

Where: N = population size | K = carrying capacity | r = intrinsic rate of increase
When N = K: dN/dt = 0 (no more growth). When N << K: dN/dt ≈ rN (exponential).

4.3 Comparison of Growth Models

Feature	Exponential Growth	Logistic Growth
Curve shape	J-shaped	S-shaped (sigmoid)
Resources	Unlimited	Limited
Carrying capacity K	Not incorporated	Incorporated
Real world?	Not realistic (long-term)	More realistic
Equation	dN/dt = rN	dN/dt = rN[(K−N)/K]
Darwin's idea	Yes — basis for natural selection	Modified view

⚠️ NEET Focus (2014, 2016, 2018, 2020, 2022): J-shaped = exponential (unlimited resources). S-shaped = logistic (limited resources, carrying capacity K). dN/dt = rN vs dN/dt = rN[(K-N)/K]. r = intrinsic rate of increase. N_t = N₀e^rt. Most wildlife populations follow logistic model. Darwin's common observation = exponential growth potential.

5. Population Interactions

Interaction	Species 1	Species 2	Definition & Examples
Mutualism	+ (benefit)	+ (benefit)	Both species benefit. Examples: Lichens (fungi + algae); mycorrhiza (fungi + roots); Rhizobium + legumes; fig-fig wasp (Ficus + Agaonidae); orchid + bee; clownfish + sea anemone; Cattle egret + cattle.
Competition	− (harm)	− (harm)	Both species are harmed. Intraspecific (same species) > Interspecific (different species). Competitive exclusion principle (Gause): no two species can share the same niche indefinitely — one will eliminate the other or one will adapt to use different resources (resource partitioning). Example: Abingdon tortoises vs goats in Galapagos.
Predation	+ (predator)	− (prey)	Predator kills and eats prey. Examples: Tiger-deer; Lion-zebra; Sparrow-insect; Drosophila-yeast. Indirect benefit: controls prey population, herbivory controls plant biomass. Counteradaptations: camouflage, warning colouration (aposematism), mimicry (Batesian), spines, thorns, toxic chemicals.
Parasitism	+ (parasite)	− (host)	Parasite benefits, host is harmed (but usually not killed quickly). Ectoparasites: lice, ticks, mites (live on body surface). Endoparasites: tapeworm, roundworm, liver fluke (live in organs). Brood parasite: Cuckoo lays eggs in crow's nest. Cuscuta (dodder) on hedge plants (plant parasite).
Commensalism	+ (benefit)	0 (no effect)	One benefits, other is neither helped nor harmed. Examples: Orchid growing on tree branch (epiphyte); Barnacles on whale; Egret-cattle (cattle egret = mutualism actually!); Clownfish-anemone; Pseudomyrmex ants on Acacia.
Amensalism	0 (no effect)	− (harm)	One species is harmed; other is unaffected. Example: Penicillium secretes penicillin → inhibits bacterial growth. Aspergillus secreting aflatoxin. Large tree shading out smaller plants below it.

5.1 Co-evolution Examples

Fig and fig wasps: Ficus species pollinated exclusively by specific species of Agaonidae wasps. Wasp lays eggs in fig; fig provides shelter and food — obligate mutualism.
Orchids and bees: Ophrys orchid mimics female bee sexually — male bee attempts to mate, picks up pollen (sexual deceit = pseudocopulation). Bee gets nothing = not mutualism!
Orchid (Angraecum sesquipedale): Morgan predicted a moth with a 30 cm long proboscis would pollinate this orchid before it was discovered. Found 130 years later — Xanthopan morgani praedicta.

⚠️ NEET Focus (2015, 2017, 2019, 2021, 2022): Competitive exclusion = Gause's principle. Mutualism = both +. Competition = both −. Predation = + / −. Parasitism = + / −. Commensalism = + / 0. Amensalism = 0 / −. Brood parasite = cuckoo. Ophrys orchid = sexual deceit (NOT mutualism). Resource partitioning = coexistence of competing species by dividing resources.

🎓 Key NEET Questions

Q1. [NEET 2022] In logistic growth, when does the rate of growth become maximum?

Answer The rate of growth (dN/dt) is maximum when N = K/2 (half the carrying capacity). At this point the derivative of dN/dt with respect to N equals zero (inflection point of the S-curve). Both below and above K/2, the rate is lower.

Q2. [NEET 2021] What is aestivation? Give two examples.

Answer Aestivation is summer dormancy adopted by animals to escape heat and desiccation during the summer months. Metabolic activity is greatly reduced. Examples: Lungfish (Protopterus) — encases itself in mucous cocoon in dry riverbeds; Snails — seal shell opening with mucus/epiphragm during summer.

Q3. [NEET 2020] Which of the following is an example of brood parasitism?

(a) Cuckoo laying eggs in crow's nest (b) Clownfish in sea anemone (c) Tapeworm in human gut (d) Barnacles on whale

Answer: (a) Brood parasitism: Cuckoo (Cuculus canorus) lays its eggs in the nest of crows — crow raises cuckoo chick as its own. The cuckoo chick may even push out crow eggs. Classic example of parasitism where the host raises the parasite's offspring.

Q4. [NEET 2019] The population growth curve that is described by Verhulst-Pearl equation is:

(a) J-shaped (b) S-shaped (sigmoid) (c) Exponential (d) Linear

Answer: (b) The Verhulst-Pearl logistic growth equation [dN/dt = rN(K−N)/K] produces an S-shaped (sigmoid) curve. Population grows slowly at first, then rapidly, then slows as it approaches carrying capacity K.

Q5. [NEET 2018] Gause's competitive exclusion principle states that:

Answer Gause's Competitive Exclusion Principle: Two species competing for the same resource and niche cannot coexist indefinitely. The more efficient competitor will eliminate (or displace) the other. However, coexistence is possible through resource partitioning (dividing the resource between species so each uses a different part).

💡 Rapid Revision — Key Points

Ecology coined by Haeckel (1869). 4 levels: Organism → Population → Community → Biome.
Hibernation = winter dormancy | Aestivation = summer dormancy | Diapause = insects/zooplankton.
J-curve = exponential (dN/dt = rN) | S-curve = logistic (dN/dt = rN[(K-N)/K])
Maximum growth rate in logistic = at N = K/2
Mutualism (+/+) | Commensalism (+/0) | Amensalism (0/−) | Competition (−/−) | Predation and Parasitism (+/−)
Competitive exclusion = Gause's principle (two species, same niche → one eliminated)
Brood parasite = cuckoo in crow's nest. Ophrys = sexual deceit (NOT mutualism).
Type I survivorship = humans | Type II = birds | Type III = oysters/fish

NCERT Solutions - Organisms and Populations - Class 12

CLASS 12 BIOLOGY | NCERT SOLUTIONS

Chapter 13 — Organisms and Populations

All NCERT Exercise Questions with Detailed Solutions

📋 Note: All questions from NCERT Class 12 Biology Chapter 13 Exercise. Growth models, interaction tables, and ecological examples as per NCERT.

NCERT Exercise Questions & Solutions

2 MarksQ1. How is diapause different from hibernation?

✓ Answer

Feature	Diapause	Hibernation
Definition	Stage of suspended development in insects and zooplankton to overcome unfavourable conditions	Winter dormancy in warm-blooded animals to escape cold
Organisms	Insects (pupae, eggs), zooplankton (Daphnia)	Mammals (bears, hedgehog), some bats
Season	Any unfavourable season (not only winter)	Winter (cold season)
Stage	Specific developmental stage (e.g., pupal, egg)	Whole organism goes into dormancy
Trigger	Unfavourable environmental cues (photoperiod, temperature, food)	Dropping temperature + reduced food

3 MarksQ2. If a population growing exponentially doubles in size in 3 years, what is the intrinsic rate of increase (r) of the population?

✓ Answer
Given: Population doubles in t = 3 years. So N_t = 2 N₀

Using: N_t = N₀ e^rt
2 N₀ = N₀ e^r×3
2 = e^3r
ln 2 = 3r
0.693 = 3r
r = 0.693 / 3 = 0.231 per year

This means the population grows at an intrinsic rate of 0.231 year⁻¹ (approximately 23.1% per year).

3 MarksQ3. Name important characteristics of a population and explain any one.

✓ Answer
Important characteristics of a population:

Population size (N)
Population density
Birth rate (natality)
Death rate (mortality)
Age structure / age pyramid
Sex ratio
Growth rate

Explanation of Age Structure:
The age structure is the proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive) in a population. It can be represented as an age pyramid:

Growing population: Wide base (many young); triangular pyramid (example: India formerly).
Stable population: Equal proportions at each age level; bell-shaped.
Declining population: Narrow base (few young); inverted pyramid (example: some European countries).

Age structure determines future population trends.

5 MarksQ4. Write a short note on the logistic population growth curve.

✓ Answer
Logistic Population Growth (Verhulst-Pearl Model):

In nature, no population can grow exponentially for long due to:

Limited food and space
Increased competition and disease at high density
Predation pressure

This environmental resistance causes the population to reach a maximum size called the Carrying Capacity (K).

Verhulst-Pearl Logistic Growth Equation:
dN/dt = rN [(K − N) / K]
N = population; K = carrying capacity; r = intrinsic rate of increase.

Phases of the S-shaped curve:

Lag phase: Slow initial growth. Few individuals, adaptation to environment.
Exponential phase (log phase): Rapid growth. N << K, so (K−N)/K ≈ 1, dN/dt ≈ rN.
Deceleration phase: Growth slows as N approaches K. Intraspecific competition increases.
Stationary phase (plateau): N = K. dN/dt = 0. Population stabilises at carrying capacity.

Key features:

Growth rate is maximum at N = K/2.
More realistic than exponential model.
Most wildlife populations follow this pattern.

5 MarksQ5. Select the statement which explains best parasitism.

✓ Answer
Parasitism is a type of interspecific interaction where one organism (parasite) lives in close association with another organism (host) and derives benefit from it, while the host is harmed but rarely killed immediately.

Best explaining statement: "In parasitism, the parasite depends on the host for food and shelter, deriving benefit while causing harm to the host over time without immediately killing it."

Types:

Ectoparasites: Live on body surface. Examples: lice, ticks, mites, bed bugs, Cuscuta (stem parasite on hedge plants).
Endoparasites: Live inside host's body. Examples: tapeworm (Taenia), roundworm (Ascaris), liver fluke (Fasciola), gut-dwelling Plasmodium.
Brood parasites: Cuckoo lays eggs in crow's nest — crow unwittingly raises cuckoo chick.

Co-evolutionary arms race: Parasites evolve strategies to exploit host while hosts evolve defence mechanisms, driving co-evolution.

5 MarksQ6. Describe the interactions between the following species pairs, giving the type of interaction: (a) Orchid growing on mango tree branch (b) Lichen (c) Clownfish and sea anemone (d) Mycorrhiza and plant roots.

✓ Answer

Pair	Type	Explanation
(a) Orchid on mango	Commensalism (+/0)	Orchid (epiphyte) uses mango branch for physical support and gains better access to sunlight and rain. Mango tree is neither helped nor harmed.
(b) Lichen	Mutualism (+/+)	Lichen = fungus + photosynthetic alga/cyanobacterium. Alga provides food (photosynthesis); fungus provides shelter, water, minerals. Both benefit obligately.
(c) Clownfish and sea anemone	Mutualism (+/+)	Clownfish lives among stinging tentacles of anemone (protected from predators). Fish drives away predators of anemone and provides nutrients via faeces.
(d) Mycorrhiza and plant	Mutualism (+/+)	Fungi help plant absorb phosphorus and other minerals from soil (large hyphal surface area). Plant provides carbohydrates (photosynthates) to fungi.

Facts Capsule - Organisms and Populations - Class 12

CLASS 12 BIOLOGY | NEET RAPID CAPSULE

Facts & High-Yield Points

Chapter 13 — Organisms and Populations | 20 Key Facts for NEET

🌎 Abiotic Factors & Adaptations

FACT #01 — Ecology

Term “Ecology” coined by Ernst Haeckel (1869). Studies relationships of organisms with biotic + abiotic environment. 4 levels: Organism → Population → Community → Biome/Ecosystem.

FACT #02 — Temperature Terms

Eurythermal: tolerate wide temperature range. Stenothermal: narrow range only. Bergmann's Rule: body size larger in cold climates. Allen's Rule: appendages shorter in cold climates. Most important abiotic factor = temperature.

FACT #03 — Dormancy Types

Hibernation: winter. Bears, bats, hedgehog. Aestivation: summer (heat + desiccation). Lungfish, snails, earthworm. Diapause: suspended development. Insects (Daphnia). Any unfavourable season.

FACT #04 — Desert Adaptations

Kangaroo rat: nocturnal, metabolic water, never drinks, concentrated urine, long loop of Henle. Cacti: CAM photosynthesis, succulent stem, spine-leaves, shallow roots. Desert lizards: bask/burrow behavioural thermoregulation.

📊 Population Ecology

FACT #05 — Population Attributes

Key attributes: N (size), density, birth rate (natality), death rate (mortality), age structure, sex ratio. dN/dt = bN − dN. Population grows when b > d.

FACT #06 — Survivorship Curves

Type I (convex): most live long, few die young. Humans, large mammals. Type II (diagonal): constant death rate. Birds. Type III (concave): most die young, survivors live long. Oysters, fish, plants.

FACT #07 — Age Pyramids

Growing: wide base (many young), triangular. Stable: uniform = bell-shaped. Declining: narrow base (few young), inverted. Age pyramid determines future trajectory.

📈 Population Growth

FACT #08 — Growth Equations (Must Know!)

Exponential (J-curve, unlimited resources):
dN/dt = rN | N_t = N₀ e^rt | r = b − d

Logistic (S-curve, limited resources, Verhulst-Pearl):
dN/dt = rN [(K − N) / K]
K = carrying capacity. Max growth rate when N = K/2. At N=K: dN/dt = 0.

FACT #09 — r value

r = intrinsic rate of natural increase. r = b − d. If population doubles in t years: r = 0.693 / t (since ln2 = 0.693). Higher r = faster growth. r_max = under ideal conditions.

FACT #10 — J vs S Comparison

J-shaped: unlimited resources, no K, unrealistic. S-shaped: limited resources, has K, realistic, more common in nature. Darwin's theory based on exponential growth potential + limited resources → struggle for existence.

🤝 Population Interactions

FACT #11 — Interaction Signs Summary

Interaction	Species 1	Species 2	Examples
Mutualism	+	+	Lichen, mycorrhiza, Rhizobium+legume, fig+wasp, clownfish+anemone
Competition	−	−	Abingdon tortoise vs goats; interspecific resource competition
Predation	+	−	Tiger-deer, Sparrow-insect, Drosophila-yeast
Parasitism	+	−	Cuckoo-crow (brood), Cuscuta-hedge, tapeworm-human
Commensalism	+	0	Orchid on tree, barnacle on whale
Amensalism	0	−	Penicillium inhibiting bacteria, Aspergillus aflatoxin

FACT #12 — Competitive Exclusion

Gause's Principle: Two species occupying identical niches cannot coexist indefinitely. One out-competes the other. Solution: Resource partitioning (species evolve to use different parts of resource, enabling coexistence).

FACT #13 — Special Interactions

Brood parasitism: Cuckoo lays eggs in crow's nest. Ophrys orchid: sexual deceit, NOT mutualism (bee gets nothing). Fig-wasp: obligate mutualism. Angraecum sesquipedale: Morgan predicted 30 cm proboscis moth → found 130 yrs later.

🧠 Mnemonics — Remember Fast

Dormancy: “HAD” Hibernation = winter (Bears). Aestivation = summer (Lungfish, snails). Diapause = insects/zooplankton, any season.

Survivorship: “HBF” Humans = Type I (convex). Birds = Type II (diagonal). Fish/Oysters = Type III (concave). Type I = best survival.

r calculation trick: “ln2 = 0.693” Population doubles in t years → r = 0.693 / t. Doubles in 3 yrs → r = 0.231/yr. Doubles in 7 yrs → r = 0.099/yr. Same formula as half-life!

🔢 Critical Numbers — Never Forget

1869 — Haeckel coined “ecology” J-curve — exponential growth (dN/dt = rN) S-curve — logistic growth (Verhulst-Pearl) N = K/2 — maximum growth rate in logistic r = 0.693/t — doubling time formula ln 2 = 0.693 — key constant Gause — competitive exclusion principle Cuckoo — brood parasitism in crow's nest

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