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State Board · Class 12 · Biology

Lesson 14: Ecosystems and Energy Flow

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

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Structure and functions of an ecosystem

Link to Std 9–10

Last lesson covered populations and their relations; here we see how energy and nutrients move through an ecosystem.

From daily life

A pond — water, algae, fish, bacteria in the mud; all depend on each other.

Simple definition

An ecosystem = self-regulating, self-sustaining unit of living and non-living parts. Stratification = vertical layers; zonation = horizontal belts.

EcosystemStructure of an ecosystemTerrestrialforest, grasslandAquaticpond, oceanNaturalself-sustainingArtificialfarm, fish tankEcosystem: self-regulating, self-sustaining unit; Tansley (1935)Stratification = vertical layers (trees, shrubs, herbs); ocean: epi-, meso-, bathy-pelagic, benthicZonation = horizontal belts; shore: inter-tidal, littoral, sub-littoralFour functions: productivity, decomposition, nutrient cycling, energy flowPond: abiotic = water, soil; producers = phytoplankton, algaeConsumers = zooplankton, insects, fish; decomposers = fungi, bacteriaEnergy one-way; nutrients cyclic

In the figure : Ecosystem structure.

Rule 1 : Types and structure — Tansley (1935). Types: terrestrial (forest, grassland, desert) and aquatic (pond, river, ocean, estuary); natural (self-sustaining) and artificial (farm, fish tank — constant human inputs). Stratification: forest trees → shrubs → herbs; ocean epipelagic, mesopelagic, bathypelagic, benthic. Zonation: shore/wetland — inter-tidal, littoral, sub-littoral; clearest at the edge between two habitats.

  • Easy : Stratification = vertical.
  • Medium : Zonation = horizontal.
  • Exam-type (CET) : Farm = artificial.

Rule 2 : Four functions — (1) Productivity (inorganic → organic), (2) decomposition (breaking down dead matter), (3) nutrient cycling (storage and movement of nutrients), (4) energy flow (one-way: producer → consumer → heat). Pond: abiotic = water, minerals, sediment; producers = phytoplankton, algae, aquatic plants; consumers = zooplankton, insects, fish; decomposers = bottom fungi, bacteria.

  • Easy : Energy flows one way.
  • Medium : Nutrients cycle.
  • Exam-type (CET) : Decomposers = at the bottom.
Remember

PDNE: productivity, decomposition, nutrient cycling, energy flow.

Common mistake and CET trap

‘A farm is a natural ecosystem’ is wrong (artificial). Trap: ‘energy is cyclic’ is wrong (one-way).

Shortcut

Memory clue : "Stratification = stairs (vertical); zonation = zones (horizontal)".

Spot the difference

TermDirection
StratificationVertical
ZonationHorizontal
EnergyOne-way
NutrientsCyclic

Practice questions

Easy
  1. 1Who coined ecosystem?
  2. 2Artificial ecosystem?
  3. 3Zonation seen at?
Medium
  1. 4Pond producers?
  2. 5Pond decomposers?
  3. 6Number of functions?
Challenging
  1. 7An ocean layer?
  2. 8Energy flow is?
Show answers (try them yourself first)
  1. 1Tansley.
  2. 2Farm / fish tank.
  3. 3Shores/wetlands.
  4. 4Phytoplankton, algae.
  5. 5Fungi, bacteria.
  6. 6Four.
  7. 7Benthic.
  8. 8Unidirectional.

Productivity

From daily life

Total harvest versus what is left after seed and losses — gross versus net.

Simple definition

Primary productivity = rate of organic matter produced by photosynthesis. NPP = GPP − respiratory loss (R).

ProductivityProductivityGPPgrossRrespiratory lossNPPavailable biomassGSPsecondaryGPP − R = NPPUnit: g/m²/day (per area, per time)Biosphere NPP ≈ 170 billion tons (dry weight)Of this only 55 billion tons from oceans; rest from landFactors: plant species, nutrients, photosynthetic capacity, environmentSecondary productivity = new organic matter by consumersLimiting factors: light, nutrients (N, P), water

In the figure : GPP, R, NPP.

Rule 1 : GPP and NPP — Productivity = rate of biomass formation; unit g/m²/day or carbon mass. GPP = total production in photosynthesis; plants use part in respiration (R); GPP − R = NPP — biomass available to consumers and decomposers. E.g. GPP 100 and R 40 ⇒ NPP = 60. The biosphere's annual NPP ≈ 170 billion tons (dry weight); oceans only 55 billion tons, the rest from land.

  • Easy : NPP = GPP − R.
  • Medium : Ocean 55 billion tonnes.
  • Exam-type (CET) : Total 170 billion tonnes.

Rule 2 : Secondary productivity and factors — Secondary productivity = rate at which consumers form new organic matter (assimilate food energy). Factors for primary productivity: plant species, light, water, nutrients, temperature, photosynthetic capacity; so it differs between ecosystems. Oceans are less productive because light falls with depth and nutrients are limited.

  • Easy : Consumer = secondary.
  • Medium : Light, nutrients.
  • Exam-type (CET) : Ocean is lower.
Remember

Subtract R from GPP to get NPP.

Common mistake and CET trap

‘NPP = GPP + R’ is wrong. Trap: ‘oceans are more productive’ is wrong (less).

Shortcut

Memory clue : "Net = gross − respiration (N = G − R)".

Spot the difference

TermMeaning
GPPTotal production
RRespiratory loss
NPPAvailable biomass

Practice questions

Easy
  1. 1GPP 80, R 30, NPP?
  2. 2Unit of productivity?
  3. 3Biosphere NPP?
Medium
  1. 4Ocean NPP?
  2. 5Secondary productivity by?
  3. 6NPP available to?
Challenging
  1. 7A factor?
  2. 8Why oceans less?
Show answers (try them yourself first)
  1. 150.
  2. 2g/m²/day.
  3. 3≈170 billion tons.
  4. 4≈55 billion tons.
  5. 5Consumers.
  6. 6Consumers, decomposers.
  7. 7Light / nutrients.
  8. 8Limited light and nutrients.

Decomposition

From daily life

Fallen leaves vanish into soil within weeks — unseen cleaners.

Simple definition

Decomposition = decomposers convert dead organic matter into CO₂, water and nutrients. Detritus = dead remains, faeces.

DecompositionDecompositionFragmentdetritivoreLeachingwashed downCatabolismenzymesHumushumificationDetritus: dead remains, faeces — raw material for decomposition1 Fragmentation: earthworms break detritus up2 Leaching: soluble salts percolate down3 Catabolism: bacterial/fungal enzymes make simple compounds4 Humification: humus (dark, colloidal, slow breakdown)5 Mineralisation: humus → inorganic nutrientsWarm, moist, oxygen = fast; cold/anaerobic = slow

In the figure : Steps of decomposition.

Rule 1 : Five steps — (1) Fragmentation: detritivores (earthworms) break detritus, increasing surface. (2) Leaching: water-soluble salts move down and can become unavailable salts. (3) Catabolism: bacterial and fungal enzymes turn complex matter into simple inorganic compounds. (4) Humification: dark, amorphous, colloidal humus; resists decay; nutrient reservoir; raises soil water-holding capacity. (5) Mineralisation: humus is broken down; inorganic nutrients released. All steps run simultaneously.

  • Easy : Humus = slow decomposition.
  • Medium : Earthworm = fragmentation.
  • Exam-type (CET) : Stages run together.

Rule 2 : Factors deciding the rate — Decomposition needs oxygen. Temperature and soil moisture are main factors. Warm, moist conditions speed it up; low temperature and anaerobic conditions slow it. Chemistry of the detritus matters too (nitrogen-rich breaks down fast; lignin- and chitin-rich slowly).

  • Easy : Warm + moist = fast.
  • Medium : Cold = slow.
  • Exam-type (CET) : O₂ required.
Remember

F-L-C-H-M: fragment, leach, catabolise, humify, mineralise.

Common mistake and CET trap

‘Humus decays quickly’ is wrong (very slowly). Trap: ‘decomposition is faster in cold’ is wrong.

Shortcut

Memory clue : "F L C H M = ‘Fine Leaves Change Hard Matter’".

Spot the difference

StepRole
FragmentationBreak up
HumificationMakes humus
MineralisationReleases nutrients

Practice questions

Easy
  1. 1Detritus is?
  2. 2Fragmentation by?
  3. 3Leaching is?
Medium
  1. 4Humus is?
  2. 5Mineralisation gives?
  3. 6Fast conditions?
Challenging
  1. 7Effect of anaerobic?
  2. 8Catabolism by?
Show answers (try them yourself first)
  1. 1Dead remains, faeces.
  2. 2Detritivores (earthworm).
  3. 3Salts washed down.
  4. 4Dark, colloidal, slow.
  5. 5Inorganic nutrients.
  6. 6Warm, moist.
  7. 7Slows.
  8. 8Bacterial/fungal enzymes.

Energy flow, food chains and the 10% law

From daily life

Energy leaks at every step, like heat from a charger.

Simple definition

A trophic level = feeding level in a food chain. The 10% law: only about 10% of energy passes to the next level.

Energy flowEnergy flow and food chainsProducer1st levelPrimaryherbivoreSecondarycarnivoreTertiarytopSun = only source (except deep sea); PAR < 50%Plants capture only 2–10% of PAREnergy flow one-way: sun → producer → consumer → heatChains: grazing (GFC), detritus (DFC), parasiticE.g. grass → deer → leopard (GFC)10% law: Lindeman (1942) — only 10% energy passes on1000 J → 100 → 10 → 1; so 4–5 levels; food web adds stability

In the figure : Energy flow.

Rule 1 : Energy source and chains — The sun is the only source (except deep-sea ecosystems). Of incident solar radiation PAR is under 50%; plants capture only 2–10% of PAR. Flow is one-way — energy can be used only once. Grazing food chain: grass → deer → leopard. Detritus food chain: dead matter → decomposers (saprotrophs). Third: parasitic. Linked chains form a food web; omnivores (crow, man) occupy several levels.

  • Easy : PAR < 50%.
  • Medium : Plants 2–10%.
  • Exam-type (CET) : DFC = dead matter.

Rule 2 : 10% law — Lindeman (1942): only 10% of net energy passes to the next level; rest lost in respiration/heat. E.g. from 1000 J of light the producer holds 100 J, herbivore 10 J, carnivore 1 J. So chains usually have 4–5 levels; very little energy at the top. Shorter chains give more energy. Levels: producer (1st), primary consumer (2nd), secondary (3rd), tertiary/top (4th).

  • Easy : Lindeman = 10%.
  • Medium : 1000 → 100 → 10 → 1.
  • Exam-type (CET) : 4–5 levels.
Remember

The 10 rule: 1000 → 100 → 10 → 1.

Common mistake and CET trap

‘All energy passes on’ is wrong. Trap: ‘energy is cyclic’ is wrong.

Shortcut

Memory clue : "Lindeman = little energy (10%)".

Spot the difference

LevelExample
1st (producer)Grass
2nd (primary)Deer
3rd (secondary)Leopard

Practice questions

Easy
  1. 1PAR is?
  2. 210% law by?
  3. 3Herbivore from 1000 J?
Medium
  1. 4DFC starts with?
  2. 5Levels in chains?
  3. 6An omnivore?
Challenging
  1. 7Energy direction?
  2. 8Why food webs?
Show answers (try them yourself first)
  1. 1Photosynthetically active radiation.
  2. 2Lindeman 1942.
  3. 310 J.
  4. 4Dead matter.
  5. 54–5.
  6. 6Crow / man.
  7. 7One-way.
  8. 8Stability.

Ecological pyramids

From daily life

A broad base of soldiers and a few officers on top — the pyramid shape.

Simple definition

An ecological pyramid shows numbers, biomass or energy across trophic levels (C. Elton, 1927); base = producers.

PyramidsEcological pyramidsNumbernumbersBiomassbiomassEnergyalways uprightPyramid: C. Elton (1927); base = producersBiomass e.g.: grass 900 kg, mice 100, snakes 50, owls 10Number e.g.: 500 000 / 5000 / 5 (three levels)Ocean biomass inverted (little phytoplankton, more zooplankton)Insects–birds–parasites on a tree: number invertedEnergy pyramid never inverted (10% law, heat loss)Limits: no webs, no decomposers, one species at several levels

In the figure : Three pyramids.

Live figure

Ecological pyramids

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

Open in the app

Rule 1 : Three types — Number: individuals per area. E.g. 500 000 (grass) → 5000 → 5. Biomass: biomass per area. E.g. grass 900 kg, mice 100, snakes 50, owls 10. Energy: stored energy. Usually all three are upright. Exceptions: ocean biomass pyramid is inverted (small phytoplankton biomass, larger zooplankton); insects → small birds → parasites on one tree give an inverted number pyramid.

  • Easy : Elton = 1927.
  • Medium : Ocean biomass inverted.
  • Exam-type (CET) : Tree number inverted.

Rule 2 : Energy pyramid and limits — The energy pyramid is always upright, never inverted — because only 10% passes on at each level and most is lost as heat. (100% → 10% → 1% → 0.1%). Limits: assumes a simple chain, not a web; gives decomposers no place; one species can occupy several levels (sparrow — primary when eating seeds, secondary when eating insects).

  • Easy : Energy = always upright.
  • Medium : No decomposers.
  • Exam-type (CET) : Sparrow in two levels.
Remember

Energy never inverts — heat is lost.

Common mistake and CET trap

‘No pyramid is ever inverted’ is wrong (biomass/number can be). Trap: ‘energy pyramid inverted’ is wrong.

Shortcut

Memory clue : "E for Energy = always upright".

Spot the difference

PyramidInverted?
NumberCan be
BiomassIn ocean
EnergyNever

Practice questions

Easy
  1. 1Pyramid concept by?
  2. 2Ocean biomass pyramid?
  3. 3Energy pyramid is?
Medium
  1. 4Why?
  2. 5Biomass example top?
  3. 6A limitation?
Challenging
  1. 7Sparrow occupies?
  2. 8Base represents?
Show answers (try them yourself first)
  1. 1C. Elton.
  2. 2Inverted.
  3. 3Always upright.
  4. 410% law, heat loss.
  5. 5Owls 10 kg.
  6. 6No decomposers.
  7. 7Two levels.
  8. 8Producers.

Nutrient cycles: carbon and phosphorus

From daily life

Like refilling a bottle, nutrients are used again and again.

Simple definition

A nutrient (biogeochemical) cycle = nutrients move non-living → living → non-living. Reservoir = store: atmosphere for gaseous cycles; rocks for sedimentary.

Nutrient cyclesNutrient cyclesGaseousC, NSedimentaryPReservoirstoreNutrient cycle = biogeochemical cycleGaseous: reservoir atmosphere (C, N); sedimentary: reservoir rocks (P)Carbon: 71% in oceans; processes: photosynthesis, respiration, decomposition, combustionFossil fuel burning: 5.5 billion tons C/yr; 3.3 billion stays in airPhosphorus: rock → soil → plants → animals → decompositionP: tiny atmospheric input; no respiratory releaseExcess P runoff → eutrophication (algal bloom)

In the figure : Carbon and phosphorus cycles.

Live figure

Nutrient cycles: carbon and phosphorus

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

Open in the app

Rule 1 : Carbon cycle — A gaseous cycle; all life is carbon-based. 71% of global carbon is dissolved in oceans. Processes: photosynthesis (CO₂ → food), respiration, decomposition, sedimentation, combustion. Rocks, coal, oil = long-term stores (sinks). Human impact: burning fossil fuels releases about 5.5 billion tons of carbon a year; 3.3 billion tons stay in air; rest dissolves in sea (shell carbonate). Deforestation, volcanoes, fires also add CO₂.

  • Easy : 71% in the ocean.
  • Medium : 5.5 / 3.3 billion tonnes.
  • Exam-type (CET) : Combustion = CO₂ rise.

Rule 2 : Phosphorus cycle — A sedimentary cycle; reservoir = rock (phosphates). Weathering → soil → plant roots → herbivores → carnivores → dead matter → phosphate-solubilising bacteria → soil again. Needed for membranes, nucleic acids, ATP, bones, teeth. Differences (from carbon): tiny input from rain; no respiratory release to air; so P is often limiting. Sudden P from fertilisers/sewage ⇒ algal bloom ⇒ eutrophication — aquatic life dies.

  • Easy : Reservoir = rock.
  • Medium : No atmospheric P.
  • Exam-type (CET) : Excess P = eutrophication.
Remember

C = in air (gaseous); P = in rock (sedimentary).

Common mistake and CET trap

‘Phosphorus is released by respiration’ is wrong. Trap: ‘carbon's main store is the atmosphere’ is wrong (ocean 71%).

Shortcut

Memory clue : "C = cloud (air); P = pebble (rock)".

Spot the difference

CycleReservoir
CarbonAtmosphere/ocean
NitrogenAtmosphere
PhosphorusRock

Practice questions

Easy
  1. 1% carbon in oceans?
  2. 2Type of P cycle?
  3. 3Carbon sinks?
Medium
  1. 4Annual C from fuels?
  2. 5Releases P?
  3. 6Eutrophication cause?
Challenging
  1. 7Why P limiting?
  2. 8Why carbon-based?
Show answers (try them yourself first)
  1. 171%.
  2. 2Sedimentary.
  3. 3Rocks, coal, oil.
  4. 45.5 billion tons.
  5. 5Phosphate-solubilising bacteria.
  6. 6Excess P / nutrients.
  7. 7Small input.
  8. 8Carbon in all organic compounds.

Ecological succession and ecosystem services

From daily life

In an abandoned field grass, shrubs, then trees appear — nature refills itself.

Simple definition

Succession = gradual, predictable change in species composition of an area. Climax community = final stable community.

SuccessionSuccession and ecosystem servicesPrimarybare rock/lavaSecondaryabandoned farmClimaxstableServicesbenefitsSuccession: gradual, predictable change; pioneer → climaxPioneer: crustose lichen on rock → moss → herbs → forestPrimary: no soil — thousands of years; secondary: soil present — fasterHydrarch (wet → mesic); xerarch (dry → mesic)Steps: nudation, invasion, ecesis, aggregation, competition, reaction, stabilisationServices (MEA 2005): supporting, provisioning, regulatingE.g. air/water purification, flood control, pollination

In the figure : Succession and services.

Rule 1 : Types and steps of succession — Primary: where no life existed — new volcanic island, cooled lava, bare rock, new pond; no soil so hundreds to thousands of years. Secondary: abandoned farm, burnt/felled forest, flood; soil present so faster. Pioneers: crustose lichens on rock (acids weather rock) → mosses → herbs → shrubs → climax forest. Steps: nudation, invasion, ecesis, aggregation, competition and co-action, reaction, stabilisation. Whole sequence = sere; intermediate = seral communities. Hydrarch (wet) and xerarch (dry) both lead to mesic climax.

  • Easy : Primary = no soil.
  • Medium : Pioneer = lichen.
  • Exam-type (CET) : All → mesic.

Rule 2 : Ecosystem services — Millennium Ecosystem Assessment (2005): benefits people obtain from ecosystems. Types: supporting (nutrient cycling, primary production, soil formation, habitat, pollination); provisioning (food, timber, water, medicines, genetic resources); regulating (carbon sequestration, predator control, decomposition, air/water purification, pest control, easing floods and droughts); plus cultural/aesthetic benefits. Healthy ecosystems are free services.

  • Easy : Supporting = nutrient cycling.
  • Medium : Provisioning = food.
  • Exam-type (CET) : Regulating = purification.
Remember

Pioneer → … → climax.

Common mistake and CET trap

‘Secondary succession on bare rock’ is wrong (primary). Trap: ‘pioneers are trees’ is wrong (lichens).

Shortcut

Memory clue : "Lichen → moss → herb → shrub → tree".

Spot the difference

TypeStart
PrimaryBare rock
SecondaryAbandoned farm
ClimaxStable forest

Practice questions

Easy
  1. 1Primary succession site?
  2. 2Pioneer on rock?
  3. 3Secondary site?
Medium
  1. 4Sere is?
  2. 5Mesic means?
  3. 6Hydrarch occurs in?
Challenging
  1. 7A provisioning service?
  2. 8MEA year?
Show answers (try them yourself first)
  1. 1New volcanic island.
  2. 2Crustose lichen.
  3. 3Abandoned farm.
  4. 4Entire sequence.
  5. 5Medium water.
  6. 6Wet areas.
  7. 7Food / timber.
  8. 82005.

Quick revision of the whole lesson

ConceptKey fact
EcosystemFour functions; Tansley 1935
ProductivityGPP − R = NPP
DecompositionF-L-C-H-M
Energy10% law; Lindeman 1942
PyramidsEnergy always upright
CyclesC gaseous; P sedimentary
SuccessionPioneer → climax (mesic)

🎯 Most important points for the exam

  • Ecosystem structure.
  • Productivity.
  • Decomposition.
  • Energy flow, 10% law.
  • Ecological pyramids.
  • Carbon, phosphorus cycles.
  • Succession and services.
Study mantra

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