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State Board · Class 12 · Biology

Lesson 13: Organisms and Populations

7 concepts · 56 practice questions with answers · Free to read

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Ecological hierarchy, habitat and niche

Link to Std 9–10

Last lesson covered biotechnology; here we learn how organisms and their groups fit their environment.

From daily life

Class, standard, school — nature too has levels.

Simple definition

Ecology = study of organisms and their environment. Habitat = where it lives; niche = its role.

Ecological levelsEcological hierarchy, habitat, nicheOrganismunitPopulationone-species groupCommunitymany speciesBiomeclimatic zoneLevels: organism → population → community → biomeEcology: E. Haeckel (1865); ecosystem: Tansley (1935)Habitat = address (place); niche = profession (role)Niche: Grinnell (1917); unique to each speciesNiche types: spatial, trophic, hypervolumeFundamental niche (no competitor) / realized niche (with competition)Biosphere = all ecosystems of the earth

In the figure : Levels, habitat and niche.

Rule 1 : Levels — Four levels: organism (basic unit) → population (same species) → community (populations of many species) → biome (large terrestrial unit of a climatic zone). All ecosystems together = biosphere. The word ecology: E. Haeckel (1865); ecosystem: Tansley (1935). Climate (temperature, rainfall) decides the biome — desert, tundra, grassland, rain forest.

  • Easy : Population = one species.
  • Medium : Community = many species.
  • Exam-type (CET) : Biome = climatic region.

Rule 2 : Habitat and niche — Habitat = physical place (pond, river, ocean); may hold many niches; not species-specific. Niche (Grinnell, 1917) = a species' role — food, shelter, place in energy flow; unique to each species. Analogy: habitat = address; niche = profession. Types: spatial, trophic, hypervolume. Fundamental niche (no competitors) and realized niche (with competition, more realistic). Different niches reduce competition.

  • Easy : Habitat = address.
  • Medium : Niche = profession.
  • Exam-type (CET) : Realized = with competition.
Remember

Address = habitat; profession = niche.

Common mistake and CET trap

‘Two species share one niche’ is wrong. Trap: ‘niche = place’ is wrong.

Shortcut

Memory clue : "H = home; N = nature of job".

Spot the difference

TermMeaning
HabitatPlace
NicheRole
BiomeClimatic zone

Practice questions

Easy
  1. 1Who named ecology?
  2. 2Ecosystem coined by?
  3. 3Niche first used by?
Medium
  1. 4A habitat example?
  2. 5Realized niche?
  3. 6Largest level?
Challenging
  1. 7A niche type?
  2. 8Community is?
Show answers (try them yourself first)
  1. 1E. Haeckel.
  2. 2Tansley.
  3. 3Grinnell.
  4. 4Pond / river.
  5. 5Niche with competition.
  6. 6Biome / biosphere.
  7. 7Trophic.
  8. 8Populations of many species.

Major abiotic factors and responses

From daily life

In summer we seek shade and sweat — the body responds to the environment.

Simple definition

Abiotic factors = non-living factors: temperature, water, light, soil. Homeostasis = constant internal environment.

Abiotic factorsMajor abiotic factorsTemperatureaffects enzymesWatersalinity pptLightphotoperiodSoilpH, grain sizeTemperature: eurythermal (wide) / stenothermal (narrow)Water: fresh <5 ppt; sea 30–35 ppt; hypersaline up to 100 pptEuryhaline (wide salinity) / stenohaline (narrow)Light: photosynthesis, photoperiod — migration, breeding cuesSoil: grain size, pH, water holding; decides vegetationLife in hot springs up to 80–100°CHomeostasis = constant internal environment

In the figure : Abiotic factors.

Rule 1 : Four factors — Temperature: most important; affects enzymes; falls from equator to poles; eurythermal (wide tolerance) / stenothermal (narrow). Water: salinity — fresh below 5 ppt, sea 30–35 ppt, some lagoons 100 ppt; euryhaline / stenohaline. Light: photosynthesis; photoperiod cues breeding and migration. Soil: grain size, pH, minerals — decides vegetation and hence animals.

  • Easy : Eury = wide.
  • Medium : Steno = narrow.
  • Exam-type (CET) : Ocean 30–35 ppt.

Rule 2 : Four responses — Regulate: physiological/behavioural control — birds, mammals (thermo-, osmoregulation). Conform: most animals and plants; body follows outside level. Migrate: temporary move to better place (birds). Suspend: dormancy — seeds; in animals hibernation (winter, polar bear) and aestivation (summer, snails, some fish).

  • Easy : Regulate = birds, mammals.
  • Medium : Hibernation = winter.
  • Exam-type (CET) : Aestivation = summer.
Remember

R-C-M-S: regulate, conform, migrate, suspend.

Common mistake and CET trap

‘All animals regulate temperature’ is wrong. Trap: ‘aestivation is in winter’ is wrong.

Shortcut

Memory clue : "Hibernation = winter; aestivation = summer".

Spot the difference

ResponseExample
RegulateBirds
MigrateMigratory birds
SuspendSeeds, polar bear

Practice questions

Easy
  1. 1Eurythermal?
  2. 2Freshwater salinity?
  3. 3Sea salinity?
Medium
  1. 4Euryhaline?
  2. 5Photoperiod?
  3. 6Conformers?
Challenging
  1. 7Hibernation occurs in?
  2. 8Dormancy example?
Show answers (try them yourself first)
  1. 1Tolerates wide temperature.
  2. 2<5 ppt.
  3. 330–35 ppt.
  4. 4Tolerates wide salinity.
  5. 5Light duration.
  6. 6Change with surroundings.
  7. 7Winter.
  8. 8Seeds.

Adaptation

From daily life

White clothes in summer, sweaters in winter — our small ‘adaptations’.

Simple definition

Adaptation = a trait (structure, function, behaviour) helping survival and reproduction in the habitat.

AdaptationResponses to the environmentRegulatebirds, mammalsConformmost organismsMigratebirdsSuspendseeds, hibernationRegulate: thermo/osmoregulation; conform: follows outside levelMigrate: temporary move to better place; suspend: dormancyHibernation (winter; polar bear) / aestivation (summer; snail, fish)Desert plants: thick cuticle, sunken stomata, CAM, Opuntia spinesAllen's rule: shorter ears, tail, limbs in cold climatesSeals: blubber — insulationLizards: bask / shade / burrow — behavioural

In the figure : Types of adaptation.

Rule 1 : In plants — Desert plants: thick cuticle, stomata in pits (less transpiration), CAM pathway (stomata closed by day), Opuntia has leaves reduced to spines while the flat stem photosynthesises. Aquatic plants: light, air spaces. Seeds survive bad seasons.

  • Easy : CAM = stomata closed by day.
  • Medium : Opuntia : stem does photosynthesis.
  • Exam-type (CET) : Sunken stomata.

Rule 2 : In animals — Allen's rule: mammals of cold climates have shorter snout, ears, tail, limbs — less heat loss. Polar seals: blubber (fat layer) under skin. Desert lizards: bask, move to shade, burrow — behavioural adaptation. Deep-sea animals tolerate crushing pressure. Adaptation = structural, physiological or behavioural.

  • Easy : Blubber = fat.
  • Medium : Allen = smaller body parts.
  • Exam-type (CET) : Lizard = behavioural.
Remember

Allen: small extremities in cold.

Common mistake and CET trap

‘Opuntia leaves photosynthesise’ is wrong (stem). Trap: ‘blubber produces heat’ is wrong (insulates).

Shortcut

Memory clue : "Cold → small ears (Allen)".

Spot the difference

AdaptationExample
StructuralSpines, blubber
PhysiologicalCAM
BehaviouralLizard basking

Practice questions

Easy
  1. 1Adaptation is?
  2. 2Allen's rule?
  3. 3Blubber is?
Medium
  1. 4CAM allows?
  2. 5Opuntia leaves?
  3. 6Lizard adaptation?
Challenging
  1. 7Why thick cuticle?
  2. 8Hibernation is?
Show answers (try them yourself first)
  1. 1Trait aiding survival.
  2. 2Smaller extremities in cold.
  3. 3Insulating fat layer.
  4. 4Stomata closed by day.
  5. 5Spines.
  6. 6Behavioural.
  7. 7Reduce water loss.
  8. 8Winter dormancy.

Population attributes

From daily life

A school's strength changes with admissions, dropouts, transfers in and out.

Simple definition

Population = individuals of one species in an area. Density = number per unit space. Natality = birth rate; mortality = death rate.

PopulationPopulation attributesSize/DensityNNatalitybirth rateMortalitydeath rateSex ratio1 : 1 ESSNₜ₊₁ = Nₜ + [(B + I) − (D + E)]B births, I immigration (+); D deaths, E emigration (−)E.g. 200 carp + 800 new = 800/200 = 4 offspring/carp/yearAbsolute natality > realized; absolute mortality < realizedAge pyramid: 0–14 pre-reproductive, 15–44 reproductive, 45–85+ postShape: broad base = rapid growth; straight = zero; narrow base = declineDensity: number, biomass or relative (catch per trap)

In the figure : Population attributes.

Live figure

Population attributes

In the app you can move this figure with your finger.

Open in the app

Rule 1 : Size and change — Size/density (N) is measured as number, biomass or relative (catch per trap; tiger census from pug marks). Change: Nt+1 = Nt + [(B + I) − (D + E)]; B births, I immigration raise; D deaths, E emigration lower. E.g. 200 carp + 800 new ⇒ birth rate = 800/200 = 4 per carp per year. Absolute natality is always more than realized; absolute mortality is always less than realized.

  • Easy : B + I = increase.
  • Medium : D + E = decrease.
  • Exam-type (CET) : Absolute natality > realized.

Rule 2 : Sex ratio and age pyramid — Sex ratio = ratio of one sex to the other; 1 : 1 is the common evolutionary stable strategy (ESS). Age pyramid: three groups — pre-reproductive 0–14, reproductive 15–44, post-reproductive 45–85+. Broad base (Kenya, Nigeria) = rapid growth; USA, Australia = slow; Denmark, Italy = zero; Germany, Hungary = decline.

  • Easy : Three age groups.
  • Medium : Wide base = growth.
  • Exam-type (CET) : 1 : 1 = ESS.
Remember

BIDE: birth, immigration, death, emigration.

Common mistake and CET trap

‘Immigration reduces density’ is wrong (raises). Trap: ‘absolute mortality is higher’ is wrong.

Shortcut

Memory clue : "BIDE (B, I = +; D, E = −)".

Spot the difference

FactorEffect
Natality, immigrationRaise density
Mortality, emigrationLower density
Broad-base pyramidRapid growth

Practice questions

Easy
  1. 1Formula for Nₜ₊₁?
  2. 2Emigration?
  3. 3200 → +800 birth rate?
Medium
  1. 4Reproductive age group?
  2. 5Common sex ratio?
  3. 6Why biomass?
Challenging
  1. 7Kenya pyramid?
  2. 8Relative density?
Show answers (try them yourself first)
  1. 1Nₜ + (B + I) − (D + E).
  2. 2Leaving the habitat.
  3. 34.
  4. 415–44.
  5. 51 : 1.
  6. 6Numbers can mislead.
  7. 7Broad base.
  8. 8Catch per trap.

Population growth

From daily life

Two, four, eight... fast at first; but once the room fills, growth stops.

Simple definition

Carrying capacity (K) = maximum number a habitat can sustain. Exponential = unlimited; logistic = limited resources.

Growth modelsPopulation growth modelsExponentialJ-curveLogisticS-curveKcarrying capacityExponential: unlimited resources; geometric growth; J-shapedLogistic (Verhulst–Pearl): lag → acceleration → deceleration → asymptoteK = carrying capacity — maximum a habitat can supportIn nature the logistic model is more realisticDarwin: even slow elephants could reach huge numbersr-strategy: many small young (oysters); K-strategy: few large (birds, mammals)Beyond K: competition, rising deaths

In the figure : Exponential and logistic growth.

Live figure

Population growth

In the app you can move this figure with your finger.

Open in the app

Rule 1 : Two models — Exponential (geometric): unlimited food and space; J-shaped curve. Darwin: even a slow breeder like the elephant would reach huge numbers with unlimited resources. Logistic (Verhulst–Pearl): limited resources; lag → acceleration → deceleration → asymptote; S (sigmoid) curve; levels off at K. The logistic is more realistic in nature.

  • Easy : J = unlimited.
  • Medium : S = limited.
  • Exam-type (CET) : K = asymptote.

Rule 2 : Beyond K; reproductive strategies — Near K competition rises; weaker individuals are lost; growth stops. Beyond K food and space fall, deaths rise, numbers drop again. Strategies: some species produce many small young (oysters, marine fish); others few large young (birds, mammals) — according to selection pressure.

  • Easy : K = stable level.
  • Medium : Oyster = many young.
  • Exam-type (CET) : Mammals = few, larger young.
Remember

J = jungle, unlimited; S = struggle, limited.

Common mistake and CET trap

‘Logistic growth is unlimited’ is wrong. Trap: ‘K = minimum number’ is wrong (maximum).

Shortcut

Memory clue : "J = jump; S = slow down".

Spot the difference

ModelShape
ExponentialJ
LogisticS
KUpper limit

Practice questions

Easy
  1. 1Exponential growth?
  2. 2Logistic named after?
  3. 3K stands for?
Medium
  1. 4Asymptote at?
  2. 5First phase?
  3. 6Many small young?
Challenging
  1. 7Few large young?
  2. 8Darwin's example?
Show answers (try them yourself first)
  1. 1J-shaped.
  2. 2Verhulst–Pearl.
  3. 3Carrying capacity.
  4. 4K.
  5. 5Lag.
  6. 6Oysters.
  7. 7Birds, mammals.
  8. 8Elephant.

Interactions: mutualism and competition

From daily life

Two friends share notes (both gain); two rivals chase one prize (both lose).

Simple definition

Interspecific interaction = relation between two species. Signs: + benefit, − harm, 0 no effect.

InteractionsPopulation interactionsMutualism+ +Competition− −Commensalism+ 0Amensalism− 0Neutralism 0 0; protocooperation + + (optional)Mutualism: lichen (fungus + alga), pollination, co-evolutionCompetition: flamingo + fish (zooplankton); leopard–lionGause: close species, same resource — inferior eliminatedResource partitioning: different time/place — coexistenceParasitism + − ; predation + −+ benefit; − harm; 0 no effect

In the figure : Interactions: + − 0.

Rule 1 : Table of interactions — Neutralism 0/0; competition −/−; amensalism −/0; mutualism (symbiosis) +/+ (obligatory); protocooperation +/+ (optional); commensalism +/0; parasitism +/−; predation +/−. Mutualism: lichen (fungus + alga/cyanobacterium); flower–pollinator co-evolution; there are ‘cheaters’ (nectar thieves).

  • Easy : Mutualism + +.
  • Medium : Competition − −.
  • Exam-type (CET) : Lichen = fungus + alga.

Rule 2 : Competition — Unrelated species also compete: in Mumbai creeks flamingos and fish for zooplankton. Even with plenty of food a leopard avoids hunting near lions (interference). Gause's Competitive Exclusion Principle: closely related species using the same resource cannot coexist indefinitely — the inferior is eliminated. But with enough resources, resource partitioning (different time/place) allows coexistence. Remove the competitor and the restricted species expands.

  • Easy : Gause = exclusion.
  • Medium : Partitioning = coexistence.
  • Exam-type (CET) : Flamingo + fish.
Remember

+ + = friendship; − − = rivalry; + − = hunter/parasite.

Common mistake and CET trap

‘One gains in competition’ is wrong (both lose). Trap: ‘lichen = commensalism’ is wrong (mutualism).

Shortcut

Memory clue : "Gause: one stays, one goes".

Spot the difference

InteractionA / B
Mutualism+ / +
Competition− / −
Commensalism+ / 0
Amensalism− / 0

Practice questions

Easy
  1. 1Lichen shows?
  2. 2Gause principle?
  3. 3Resource partitioning?
Medium
  1. 4Amensalism sign?
  2. 5Flamingo and fish?
  3. 6Co-evolution example?
Challenging
  1. 7Neutralism?
  2. 8Protocooperation?
Show answers (try them yourself first)
  1. 1Mutualism.
  2. 2Competitive exclusion.
  3. 3Different time/place.
  4. 4− / 0.
  5. 5Competition.
  6. 6Flower–pollinator.
  7. 70 / 0.
  8. 8Optional + +.

Parasitism, predation and commensalism

From daily life

A cat hunts a mouse; a tick clings to a dog — both ‘+ −’ relations.

Simple definition

A parasite feeds on a host and harms it. A predator kills and eats prey. A commensal gains; the host is unaffected.

+ − relationsParasitism, predation, commensalismParasiteendo/ectoPredatorhunterCommensalone gainsEctoparasite: lice, ticks, Cuscuta; endoparasite: PlasmodiumBrood parasitism: koel lays eggs in crow's nestPredators control prey, keep diversity; invasive species lack enemiesDefence: camouflage, Monarch (toxic, warning colour)Plants: thorns (Acacia), Calotropis cardiac glycosidesCommensalism: orchid on mango; cattle egret; clown fish + anemoneSecondary metabolites: nicotine, caffeine, quinine, strychnine, opium

In the figure : Parasitism, predation, commensalism.

Rule 1 : Parasitism — Ectoparasites (outside): lice, ticks, copepods, Cuscuta (parasitic plant without chlorophyll). Endoparasites (inside): Plasmodium — reduced sense organs, suckers, no digestive system, high reproduction; intermediate host/vector. Brood parasitism: the koel lays eggs in a crow's nest; eggs resemble the host's; koel chicks hatch first. Parasite and host co-evolve.

  • Easy : Cuscuta = parasitic plant.
  • Medium : Koel = brood parasite.
  • Exam-type (CET) : Plasmodium = endoparasite.

Rule 2 : Predation and commensalism — Predators control prey numbers, maintain diversity, basis of biocontrol. Over-hunting kills both — hence ‘prudent’. Invasive species (e.g. zebra mussel) lack natural enemies. Defences: camouflage, Monarch butterfly (toxic after eating a poisonous weed as caterpillar; warning colour). Plants: thorns (Acacia, cactus), Calotropis cardiac glycosides; nicotine, caffeine, quinine, strychnine, opium are secondary metabolites. Commensalism: orchid on mango; cattle egret + cattle; clown fish + sea anemone.

  • Easy : Predator = controller.
  • Medium : Monarch = warning colour.
  • Exam-type (CET) : Cattle egret = commensal.
Remember

Parasite lives on host; predator pounces.

Common mistake and CET trap

‘Cuscuta is a commensal’ is wrong (parasite). Trap: ‘clown fish–anemone is parasitism’ is wrong (commensalism).

Shortcut

Memory clue : "Egret follows cattle: E-C".

Spot the difference

RelationExample
EctoparasiteLice, ticks
Brood parasitismKoel–crow
CommensalismCattle egret

Practice questions

Easy
  1. 1Cuscuta is a?
  2. 2Brood parasite bird?
  3. 3Monarch is?
Medium
  1. 4Calotropis contains?
  2. 5Orchid on mango?
  3. 6Role of predators?
Challenging
  1. 7Invasive species?
  2. 8Clown fish–anemone?
Show answers (try them yourself first)
  1. 1Parasitic plant.
  2. 2Koel.
  3. 3Toxic, warning colour.
  4. 4Cardiac glycosides.
  5. 5Commensalism.
  6. 6Control prey.
  7. 7Lacks natural enemies.
  8. 8Commensalism.

Quick revision of the whole lesson

ConceptKey fact
LevelsOrganism → population → community → biome
Habitat/NicheAddress / profession
AbioticTemperature, water, light, soil
PopulationNₜ₊₁ = Nₜ + (B + I) − (D + E)
GrowthJ = exponential; S = logistic; K
Interactions+ +, − −, + 0, − 0, + −
GauseCompetitive exclusion

🎯 Most important points for the exam

  • Levels, habitat and niche.
  • Abiotic factors and responses.
  • Adaptation.
  • Population attributes.
  • Population growth.
  • Mutualism and competition.
  • Parasitism, predation, commensalism.
Study mantra

Understand the concept → draw the figure → memorise the key points → solve questions yourself.