NCERT Exemplar Class 12 Biology Ch 12 Ecosystem Solutions PDF

Correct option: (c) ii and iii.

Concept used. A decomposer is an organism that breaks down dead organic matter (detritus) into simpler inorganic substances. Nutritionally it is classified by two independent labels: (a) source of carbon (autotroph = self-fixing CO₂; heterotroph = takes carbon from organic matter), and (b) mode of feeding (saprotroph = absorbs nutrients from dead matter through extracellular digestion).

1. Fungi and bacteria such as Aspergillus, Mucor and Pseudomonas cannot fix CO₂ via photosynthesis or chemosynthesis; they obtain carbon from dead leaves, wood and carcasses. They are therefore heterotrophs (statement ii is true).
2. They feed by secreting digestive enzymes onto the dead substrate and absorbing the soluble products through their cell wall: classic saprotrophic nutrition (statement iii is true).
3. Statement i (autotrophs) is false: autotrophs make their own food from CO₂, which decomposers cannot do.
4. Statement iv (chemo-autotrophs) is false: chemo-autotrophs oxidise inorganic compounds (e.g. NH3, H2S, Fe^2+) and use the energy to fix CO₂. Decomposers do not fix CO₂.

Option (c): decomposers are heterotrophs and saprotrophs.

Correct option: (a) inorganic nutrients from humus.

Concept used. Decomposition proceeds in five overlapping steps: fragmentation (detritivores break detritus into fragments), leaching (water-soluble nutrients percolate down), catabolism (microbial enzymes degrade detritus into simpler substances), humification (accumulation of dark, amorphous, colloidal humus) and finally mineralisation (humus is further degraded by microbes to release inorganic nutrients such as NH4+, NO3-, PO4^3-, SO4^2- into the soil solution).

1. Identify the stage: ``mineralisation'' refers strictly to the final stage of decomposition, acting on humus, not on the original detritus. This rules out (b) and (d), which both mention detritus.
2. Identify the product: mineralisation releases inorganic nutrients (mineral ions), not organic ones. This rules out (c), which says organic nutrients.
3. Option (a) correctly pairs the substrate (humus) with the product class (inorganic nutrients).

Option (a): mineralisation releases inorganic nutrients from humus.

Correct option: (c) ii and iv (intended units: g m^-2 yr^-1 and kcal m^-2 yr^-1).

Concept used. Primary productivity is the rate at which producers fix solar energy into organic biomass. Because it is a rate of biomass per area per time, the SI-style units are: mass per area per time: g m^-2 yr^-1 energy per area per time: kcal m^-2 yr^-1 The NCERT Exemplar prints these with a typographic slip (g^-2yr^-1 for g m^-2yr^-1), but the intent is clear: area-based, not volume-based.

1. Productivity is per unit area (m^-2), not per unit volume (m^-3). This eliminates option (i), which uses m^-3.
2. Productivity needs both a mass/energy unit AND an area unit. Option (iii) has no area term, so it is dimensionally incomplete.
3. Option (ii) g m^-2 yr^-1 (printed as g^-2 in the Exemplar) is correct for mass-based productivity.
4. Option (iv) kcal m^-2 yr^-1 is correct for energy-based productivity.

Option (c): ii and iv (g m^-2 yr^-1 and kcal m^-2 yr^-1).

Correct option: (b) Marine.

Concept used. A pyramid of biomass stacks the dry biomass at each trophic level. It is normally upright (more biomass in producers than consumers). But in a marine ecosystem the producers are microscopic phytoplankton with a very short life span (turnover of days), while the consumers (zooplankton, fish) are larger and longer-lived. At any instant the standing crop of zooplankton/fish exceeds that of phytoplankton, giving an inverted pyramid.

1. Identify producer biomass in each option. Forests, grasslands and tundra all have large, long-lived producers (trees, grasses), so producer biomass dominates and the pyramid is upright.
2. In oceans the producers are phytoplankton: tiny (∼ 10 μm), short-lived, grazed almost as fast as they are produced. Standing biomass is therefore tiny.
3. Consumer biomass (zooplankton + fish) accumulates because these organisms live longer and store biomass between reproductive events.
4. Net effect: consumer biomass > producer biomass, so the pyramid inverts.

Option (b): marine ecosystem.

Correct option: (b) Agaricus.

Concept used. A producer (autotroph) makes its own food, almost always by photosynthesis using chlorophyll, and forms the first trophic level. Anything without photosynthetic machinery is a consumer or decomposer.

1. Spirogyra: filamentous green alga with spiral chloroplasts – photoautotrophic producer.
2. Agaricus: a basidiomycete fungus (the common edible mushroom). It has no chlorophyll, cannot photosynthesise, and absorbs nutrients saprotrophically from decaying organic matter. It is a decomposer, not a producer.
3. Volvox: colonial green alga with chlorophyll – photoautotrophic producer.
4. Nostoc: a filamentous cyanobacterium with chlorophyll a, capable of photosynthesis and nitrogen fixation – photoautotrophic producer.

Option (b): Agaricus (a fungus, not a producer).

Correct option: (b) Tropical rain forests.

Concept used. Net primary productivity (NPP) is the rate at which producers fix energy in excess of their own respiratory losses: NPP = GPP - R. Standard NCERT figures (per unit area):

• Tropical rain forest: ∼ 2200 g m^-2 yr^-1 (highest among terrestrial systems).
• Estuary: ∼ 1500 g m^-2 yr^-1 (high among aquatic systems).
• Open ocean: ∼ 125 g m^-2 yr^-1 (low: nutrient limited).
• Desert: ∼ 90 g m^-2 yr^-1 (lowest: water limited).

1. Identify limiting factors per ecosystem. Tropical rain forests have abundant sunlight, water, warmth and nutrients year-round, so producers can fix CO₂ at near-maximum rates.
2. Estuaries are productive (high nutrient inflow + tidal mixing) but cover small area, and their per-unit-area productivity is still below tropical rain forest.
3. Oceans are nutrient-limited (low N, P) – their high total NPP comes from their vast area, not high per-area rate.
4. Deserts are water-limited and have low producer cover.
5. Per unit area: tropical rain forest > estuary > ocean > desert.

Option (b): tropical rain forests.

Correct option: (c) Either upright or inverted.

Concept used. A pyramid of numbers stacks the number of individuals at each trophic level. Shape depends on the chain:

• Grassland: many grass plants → fewer grasshoppers → fewer frogs → fewer snakes → upright.
• Single tree (parasitic chain): 1 tree → many fruit-eating birds → many lice on each bird → many bacteria on each louse → inverted.

1. In a grazing food chain the producer is small and abundant; the pyramid is upright.
2. In a parasitic food chain the producer is a single large organism (a tree) that supports many parasites, which in turn support more hyper-parasites; the pyramid is inverted.
3. Therefore the pyramid of numbers can be either upright or inverted depending on the ecosystem.

Option (c).

Correct option: (b) 2–10%.

Concept used. Photosynthesis converts incident photosynthetically active radiation (PAR, ∼ 400–700 nm) into chemical bond energy in glucose. Most solar energy is either reflected by the leaf, transmitted through it, absorbed and re-emitted as heat, or lost as fluorescence. NCERT states that only 2–10% of incident PAR ends up as captured chemical energy.

1. Less than 50% of incident solar radiation is PAR (the rest is UV, IR and visible outside chlorophyll absorption).
2. Of the PAR that hits a leaf, much is reflected by the cuticle or transmitted through.
3. Of the absorbed PAR, only a fraction is photochemically used; the rest dissipates as heat or fluorescence.
4. Field measurements converge on 2–10% of incident solar energy as actually captured into glucose.

Option (b): 2–10%.

Correct option: (a) Tropical rain forest.

Concept used. Decomposition rate depends on three abiotic factors: temperature (warm > cold), moisture (moist > dry), and aeration (well-aerated > anoxic). Microbial enzymes work fastest in warm, moist, oxygen-rich soils.

1. Tropical rain forest: warm year-round (25–30^∘C), very wet (>2000 mm rain/yr), well-aerated litter – all three factors favourable. Decomposition is fastest.
2. Antarctic: extremely cold; microbial activity nearly stops.
3. Dry arid region: warm but lacks moisture; microbes are water-limited.
4. Alpine region: cold and short growing season; microbes are temperature-limited.

Option (a): tropical rain forest.

Correct option: (b) 10%.

Concept used. On land, much of the NPP is in the form of woody tissue (bark, heartwood) that herbivores cannot digest, and much standing crop dies and falls to the detritivore food chain before any herbivore reaches it. The NCERT figure is that only about 10% of terrestrial NPP is actually consumed by herbivores; the remaining ∼ 90% enters the detritus food chain.

1. Total NPP available at the producer level is the ``buffet''.
2. Herbivore intake (terrestrial) ≈ 10% of NPP – small because much plant biomass is structural and indigestible (cellulose lignin in stems and bark).
3. Detritus intake ≈ 90% – fallen leaves, dead wood, un-eaten standing biomass.
4. This is sometimes confused with the Lindemann 10% rule (10% energy transferred to the next trophic level). Both numbers are 10% but they describe different things.

Option (b): 10%.

Correct option: (a) Orderly and sequential.

Concept used. Ecological succession is the predictable, directional change in community composition over time at a site, leading from a pioneer community through several seral stages to a stable climax community. ``Predictable, directional'' means the changes follow an orderly sequence: not random, not quick (it takes decades to thousands of years), and strongly influenced by the physical environment (substrate, climate).

1. ``Orderly and sequential'': pioneers → early seral → mid seral → climax. The sequence is repeatable in similar habitats.
2. ``Random'' is incorrect: each stage prepares the environment for the next, so the order is fixed by ecological mechanism.
3. ``Very quick'' is incorrect: succession spans decades-to-millennia.
4. ``Not influenced by environment'' is incorrect: substrate, moisture, temperature and disturbance regime drive the sequence.

Option (a): orderly and sequential.

Correct option: (b) equilibrium.

Concept used. A climax community is the final, stable stage of an ecological succession, in dynamic equilibrium with its physical environment: the rates of birth/colonisation balance death/emigration, and species composition stays near-constant in the absence of disturbance.

1. Climax = stable end-stage; species composition does not change significantly with time.
2. Population sizes fluctuate around long-term means, with gains balancing losses: this is equilibrium.
3. ``Disorder'' and ``constant change'' describe earlier seral stages, not climax.

Option (b): equilibrium.

Correct option: (d) All of the above.

Concept used. Aerobic respiration produces CO₂ and H₂O. So respiratory losses occur only in cycles whose chief gaseous form is CO₂: the carbon cycle. The phosphorus, sulphur and nitrogen cycles do not lose material as CO₂ through respiration.

1. Carbon cycle: respiration is the main return path (CO₂ to atmosphere). Excluded from the question.
2. Phosphorus cycle: reservoir is sedimentary rock; movement is by weathering, uptake, decomposition. No respiratory loss of P.
3. Nitrogen cycle: movement by fixation, nitrification, ammonification, denitrification. No N is lost as CO₂.
4. Sulphur cycle: movement by weathering of sulphide minerals, microbial oxidation/reduction. No S lost as CO₂.
5. All three lack respiratory losses, so (d).

Option (d): all of the above.

Correct option: (b) (NCERT-listed hydrarch sequence).

Concept used. Hydrarch succession is primary succession that begins in open water (a pond, lake) and proceeds through colonisation that progressively converts the water body into dry land. The NCERT sequence is: aligned phytoplankton &→ free-floating hydrophytes
&→ rooted hydrophytes → sedges
&→ grasses → trees. aligned

1. Pioneers in open water are the smallest photosynthetic forms: phytoplankton (microscopic algae).
2. As organic debris settles, the bottom becomes muddy enough to anchor free-floating hydrophytes (Lemna, Pistia).
3. Further sediment accumulation supports rooted floating-leaf plants (Nymphaea, Trapa).
4. As the water shallows, marshy sedges (Carex, Cyperus) move in.
5. Sedges trap more soil, drying the site enough for grasses, then woody shrubs, then forest: the climax.
6. Option (b) matches this sequence exactly.

Option (b).

Correct option: (b) atmosphere.

Concept used. Biogeochemical cycles are classified by the location of their reservoir:

• Gaseous cycles: reservoir in the atmosphere (carbon, nitrogen, oxygen, water).
• Sedimentary cycles: reservoir in the lithosphere (phosphorus, sulphur, calcium).

1. Identify the question: gaseous cycle.
2. Gaseous cycles by definition have their reservoir in the atmosphere (the lowermost layer where biota interact with air).
3. Stratosphere and ionosphere are upper-atmospheric layers and not the biological reservoir.
4. Lithosphere is the reservoir for sedimentary cycles.

Option (b): atmosphere.

Correct option: (c) tertiary consumer.

Concept used. Trophic levels count from producer (T1) outward: T1 producer → T2 primary consumer → T3 secondary consumer → T4 tertiary consumer. ``Passed through three species'' means three transfers after the producer.

1. Producer fixes carbon (T1).
2. Transfer 1: producer → primary consumer (T2).
3. Transfer 2: primary consumer → secondary consumer (T3).
4. Transfer 3: secondary consumer → tertiary consumer (T4).
5. After three transfers (passing through three species), the last species is at trophic level T4: tertiary consumer.

Option (c): tertiary consumer.

Correct option: (c) Desert.

Concept used. Climatic classification of ecosystems by rainfall:

• Rain forest: >2000 mm/yr.
• Grassland: 250–750 mm/yr.
• Shrubby (semi-arid) forest: 250–500 mm/yr.
• Desert: <250 mm/yr (true desert <100 mm/yr), evaporation exceeds precipitation.
• Mangrove: coastal tidal wetlands, not rainfall-defined.

1. Use rainfall threshold: <100 mm/yr is well below the grassland and shrubby-forest minima.
2. Use water balance: evaporation > precipitation defines an arid climate.
3. Both conditions match a desert.
4. Mangrove (d) is a coastal saline ecosystem dependent on tides, not on low rainfall; it does not fit.

Option (c): desert.

Correct option: (d) Littoral zone.

Concept used. The standard aquatic-zone vocabulary:

• Littoral zone: shore zone; alternately exposed to air at low tide / low water and submerged at high tide / high water.
• Pelagic zone: open-water column away from shore.
• Benthic zone: bottom of the lake/ocean.
• ``Lentic'' is not a zone but a system descriptor (lentic = standing water, e.g. lakes; lotic = flowing water, rivers).

1. Match the key phrase ``alternately exposed to air and immersed in water'' to the zone vocabulary.
2. Only the littoral zone, by definition, sits at the air-water interface and alternates with tides/water level.
3. Pelagic = open water: always submerged.
4. Benthic = bottom: always submerged.
5. Lentic is a system descriptor, not a zone.

Option (d): littoral zone.

Correct option: (b) Soil.

Concept used. Abiotic factors are grouped into climatic (light, temperature, water, wind, humidity) and edaphic (soil-related: texture, pH, mineral composition, moisture-holding capacity). The Greek ``edaphos'' = soil.

1. Recall the etymology: edaphos = soil.
2. Edaphic factor = any soil-related factor.
3. Water and humidity are climatic.
4. Altitude is a topographic factor.

Option (b): soil.

Correct option: (d) All of the above.

Concept used. Ecosystem services are benefits flowing to humans from natural ecosystems. The four standard categories are: provisioning (food, timber, water), regulating (climate, water purification, pollination), supporting (soil formation, nutrient cycling, primary production) and cultural (recreation, aesthetic, spiritual).

1. Nutrient cycling: a supporting service. Yes, an ecosystem service.
2. Soil-erosion prevention by plant cover and root mats: a regulating service. Yes.
3. Pollutant absorption (especially CO₂ uptake by forests moderating global warming): a regulating service. Yes.
4. All three (a, b, c) are valid ecosystem services, so (d) is correct.

Option (d): all of the above.

Concept used. In an aquatic food chain, the trophic order is phytoplankton (T1) → zooplankton (T2 primary consumer) → small fish (T3 secondary consumer / primary carnivore) → large predatory fish (T4 tertiary consumer / secondary carnivore). A secondary carnivore eats a primary carnivore.

1. Identify trophic level: secondary carnivore = T4.
2. Example: Wallago attu (an Indian catfish), or Channa (snakehead) eating smaller fish that have eaten zooplankton-eaters. Other classic examples are Lates calcarifer (sea bass) in marine systems.

Catfish (Wallago) or snakehead (Channa) act as secondary carnivores in fresh-water ecosystems.

Concept used. An ecological pyramid stacks trophic levels with producers at the base and the top carnivore at the apex. The base is the widest tier because producers fix the most energy (or biomass, or numbers) before losses up the chain.

1. Recall the pyramid layout: T1 (producers) at bottom, T2, T3, T4 above.
2. The base tier = T1 = producers (green plants on land, phytoplankton in water).

The base tier represents producers (autotrophs / T1).

Concept used. Succession is usually directional, but a disturbance (natural or human) can reset the community to an earlier seral stage. This is called retrogressive succession.

1. List disturbances that reset succession: forest fire, flood, landslide, volcanic eruption, deforestation, over-grazing, mining.
2. Each removes the dominant community and exposes the substrate, returning the site to a pioneer or early seral stage.
3. Other resets: severe pollution or invasive-species outbreaks that kill the climax community.

A stage reverts to an earlier seral stage when a disturbance (fire, flood, landslide, deforestation, overgrazing, mining, severe pollution) removes the dominant community.

Concept used. A forest shows vertical stratification: distinct horizontal layers of vegetation by height. Bottom-up: ground layer (grasses, tiny herbs) → herb layer (taller herbs like Amaranthus) → shrub layer → canopy (trees like teak).

1. Place Grass (tiny, ground-hugging) at the lowest stratum.
2. Place Amaranths (Amaranthus, an annual herb up to 1–1.5 m) just above grasses.
3. Place Shrubby plants (woody, 1–4 m) above herbs.
4. Place Teak (Tectona grandis, a deciduous tree up to 40 m) at the top stratum (canopy).

Grass → Amaranths → Shrubby plants → Teak (ground → herb → shrub → tree).

Concept used. An omnivore eats both plant and animal matter. Crows are textbook examples: they feed on grain, fruit, insects (grazing food chain) and also scavenge carrion and animal waste (decomposer / detritus food chain).

1. Identify the criterion: same species must appear in both chains.
2. Crows eat fresh insects and grains → part of grazing chain.
3. Crows also scavenge dead animals and food refuse → part of decomposer / detritus chain.

The crow (Corvus splendens); cockroaches and rats are also valid examples.

Concept used. A producer is any organism that synthesises its own organic food from inorganic CO₂ via photosynthesis. The pitcher plant (Nepenthes) has green photosynthetic leaves; it traps insects only as a supplementary source of nitrogen (the soils it grows on are nitrogen-poor), not as its main energy source. Its carbon and energy still come from photosynthesis.

1. Identify chlorophyll: pitcher plant leaves are green and contain chloroplasts.
2. Identify photosynthesis: it carries out the normal 6CO2 + 6H2O ->[light] C6H12O6 + 6O2 pathway.
3. Identify the role of insectivory: insects are digested for their nitrogen (and trace minerals), not carbon. Pitcher plants in N-rich soil can survive without trapping insects.
4. Conclude: because carbon and energy come from photosynthesis, the plant is a producer at the first trophic level, regardless of its insectivorous habit.

Pitcher plants synthesise their own food via photosynthesis and use insects only as a nitrogen supplement; they remain T1 producers.

Concept used. Omnivores can occupy more than one trophic level depending on what they eat in a given meal: as T2 when eating plants, as T3 when eating herbivores, as T4 when eating carnivores.

1. Example 1: Humans (Homo sapiens) eat grains and vegetables (T2), chicken and fish (T3), and large predatory fish like tuna (T4).
2. Example 2: Sparrows eat seeds and grains (T2) and also caterpillars and small insects (T3).

Humans and sparrows (other valid answers: crows, cockroaches, foxes).

Concept used. Secondary succession is succession that begins on a site where the original community has been destroyed but soil and seed bank remain intact (e.g. after fire, abandoned cropland). It is faster than primary succession because the substrate is already fertile.

1. In jhum (slash-and-burn) cultivation, the forest is cleared by fire, cropped for a season, then abandoned. The soil and dormant seeds remain.
2. Pioneer colonisers (grasses and weeds) re-establish quickly from the seed bank within the first year.
3. Shrubs and pioneer tree species (Macaranga, Trema) follow over a few years.
4. The site re-tracks the seral stages toward the original forest – this is secondary succession.
5. Climax forest is reached faster than in primary succession because soil is already present (no need for soil-building lichens/mosses).

Regrowth after jhum is an example of secondary succession: pre-existing soil and seed bank let pioneers and later seral stages re-establish rapidly toward the climax forest.

Concept used. Primary succession starts on a bare, lifeless substrate (rock, lava, sand) with no soil. The first 1000+ years are spent on soil formation by lichens and mosses. Secondary succession starts where soil already exists (after fire or abandonment), so the slow soil-building phase is skipped.

1. Primary succession sequence: bare rock → lichens (crustose, foliose) → mosses → herbs → shrubs → trees. Total: 1000–10000 years.
2. Secondary succession sequence: weeds → grasses → shrubs → trees. Total: 50–200 years.
3. Key difference: secondary succession starts at the herb / shrub seral stage because soil and seed bank already exist.
4. The soil-formation step (slowest in primary succession) is absent.

Secondary succession is faster because soil and a viable seed bank already exist, skipping the long soil-formation phase.

Concept used. Xerarch (xeric) succession starts on bare, dry rock. Pioneer species must survive desiccation and attach to bare rock. Crustose lichens (a mutualism of alga + fungus) are the classic pioneers: they secrete acids that slowly weather rock into the first thin soil layer. Bryophytes (mosses, liverworts) follow once a little soil is present. Ferns arrive much later, in herb-shrub seral stages.

1. Bare rock has no soil, very little water, full sun.
2. Lichens tolerate desiccation, fix rock by hyphae, secrete acids that crumble the rock surface.
3. Bryophytes need at least a thin soil film for rhizoid attachment; they cannot colonise bare rock first.
4. Ferns need moist, partially shaded soil; they arrive after lichens and bryophytes have built soil.

Lichens are the pioneer species in xeric (xerarch) succession.

Concept used. All energy flowing through an ecosystem ultimately enters via photosynthesis, in which producers fix solar radiation into chemical bonds. Other apparent ``sources'' (food for herbivores, dead matter for decomposers) are downstream stores of the same captured solar energy.

1. Sunlight provides the photon flux that drives photosynthesis: 6CO2 + 6H2O ->[h] C6H12O6 + 6O2.
2. Glucose passes up the food chain to herbivores, carnivores and decomposers.
3. At each transfer, ∼ 90% of energy is lost as heat; the remaining ∼ 10% is built into next-level biomass.
4. There is no other significant primary energy input (chemo- synthetic ecosystems near hydrothermal vents are a rare exception).

The Sun – solar radiation – is the ultimate energy source for nearly all ecosystems.

Concept used. An autotroph fixes CO₂ using sunlight or chemical energy. A heterotroph obtains carbon from organic matter made by other organisms. Mushrooms (Agaricus) lack chlorophyll and cannot fix CO₂; they absorb nutrients from dead organic matter (decaying wood, leaf litter, compost).

1. Mushrooms have no chlorophyll: no photosynthesis.
2. No chemo-autotrophic pathway either.
3. They feed by secreting enzymes onto dead organic substrate and absorbing soluble products (saprotrophic nutrition).
4. Saprotrophic nutrition is a form of heterotrophy.

The edible mushroom is a heterotroph (specifically, a saprotrophic decomposer).

Concept used. Oceanic NPP per unit area (∼ 125 g m^-2 yr^-1) is among the lowest because three factors limit producer growth: (i) shortage of dissolved macro-nutrients (especially nitrogen, phosphorus, iron), (ii) light availability falls exponentially with depth so photosynthesis is confined to the top ∼ 200 m (the photic zone), and (iii) producers are tiny phytoplankton, grazed quickly.

1. Nutrient limitation: surface waters are depleted of N and P because dead plankton sink to the deep, removing nutrients from the photic zone.
2. Light limitation: below the photic zone (200 m), there is no photosynthesis.
3. High grazing pressure: zooplankton consume phytoplankton almost as fast as they grow.
4. Net result: producer biomass stays very small per square metre.

Oceans are least productive per unit area because surface waters lack N/P, light penetrates only ∼200 m, and phytoplankton are grazed rapidly.

Concept used. Primary productivity is the rate of energy fixation by producers (T1). Secondary productivity is the rate at which energy is assimilated by consumers (T2 onward) into their own biomass. Because herbivores are the first consumer level, the energy they assimilate counts as secondary productivity (the second step in energy capture, after producers).

1. Producers fix solar energy: this is primary productivity (PP, with GPP and NPP variants).
2. Herbivores eat producers and assimilate part of that energy into their tissues.
3. This second step of energy capture, downstream of producers, is called secondary productivity.
4. Hence herbivore-level energy assimilation = secondary productivity.

Herbivore-level energy assimilation is the second step in the energy-capture chain (after producers), so it is called secondary productivity.

Concept used. Nutrient cycles move atoms (C, N, P, S, O, H, K, Ca) through three compartments: the biosphere (living organisms), the geosphere (soil, rocks, sediments) and the chemical environment (atmosphere, hydrosphere). The cycling is chemical (atoms change chemical form between ions, gases, organic compounds). The name combines all three: bio-geo-chemical.

1. ``Bio'' = organisms participate (uptake by roots, excretion, decomposition).
2. ``Geo'' = soil and rock reservoirs (lithosphere) participate.
3. ``Chemical'' = atoms change chemical form (e.g. N2 -> NH3 -> NO3- -> amino acids).
4. All three appear in the same cycle, so the name combines them.

Because nutrient atoms cycle through biological, geological and chemical compartments.

Concept used. Xerarch succession starts on dry substrates. Two named sub-types in NCERT: lithosere (on bare rock) and psammosere (on sand dunes). A third example is halosere (on saline soil), sometimes classed as a xerosere.

1. Lithosere: succession on bare rock; pioneer = crustose lichens; passes through mosses, herbs, shrubs, trees.
2. Psammosere: succession on sand dunes; pioneer = sand-binding grasses (Spinifex); passes through herbs, shrubs, trees.

Two examples: lithosere (rocky substrate) and psammosere (sandy substrate).

Concept used. Self-sustainability of an ecosystem is its capacity to maintain its own structure (species composition, biomass) and function (energy flow, nutrient cycling) indefinitely from internal resources, without depending on external subsidies.

1. A self-sustaining ecosystem captures its own energy (solar, via producers).
2. It recycles its own nutrients via decomposers, so no external nutrient input is needed.
3. It maintains population balance through internal predator-prey interactions and reproduction.
4. Natural ecosystems (forest, grassland, ocean) are self-sustaining; aquaria and crop fields are not.

Self-sustainability is the ability of an ecosystem to maintain its structure and function indefinitely from internal energy and nutrient flows.

Concept used. The figure shows a sparsely vegetated landscape: small thorny trees and scrub grass standing on a wide sandy floor with no tall canopy. Such conditions indicate very low rainfall (<250 mm/yr), high temperatures and sandy soil: the diagnostic features of a desert ecosystem.

1. Note the substrate: bare sand, no dense ground cover.
2. Note the vegetation: scattered short, thorny trees and tussock grasses adapted to low water.
3. Conclude ecosystem type: desert (specifically an arid/semi-arid Indian desert such as the Thar).
4. Characteristic plants of Thar / Indian deserts: Prosopis cineraria (khejri), Calotropis procera (aak), Acacia nilotica (babool / kikar), Capparis decidua (kair), Opuntia (prickly pear), and grasses like Cenchrus and Spinifex.

(i) Desert ecosystem; (ii) Prosopis cineraria (khejri) or Acacia nilotica (babool) or Opuntia.

Concept used. All four organisms feed on dead organic matter or wastes and play roles in decomposition:

• Earthworm: detritivore that fragments dead leaves and aerates soil.
• Mushroom (Agaricus, a fungus): saprotrophic decomposer that catabolises complex organic matter.
• Soil mites: micro-detritivores that shred fine litter.
• Dung beetle: detritivore on animal waste, returning nutrients to soil.

1. All four feed on dead/waste organic matter.
2. All four operate in the detritus food chain, not the grazing food chain.
3. All four release inorganic nutrients back to the soil and thereby maintain nutrient cycles.

All four belong to the detritus food chain: detritivores or saprotrophic decomposers that recycle dead organic matter into inorganic nutrients.

Concept used. Lindemann's 10% law (1942) states that only about 10% of the energy entering one trophic level is passed to the next as new biomass. The remaining ∼ 90% is lost as metabolic heat (respiration), excretion (faeces, urine), and unused biomass that dies and enters the detritus chain.

1. Suppose producers fix 10000 kJ in a unit area per year (T1).
2. Herbivores (T2) assimilate ∼ 10% of T1: 0.10 × 10000 = 1000 kJ.
3. Primary carnivores (T3) assimilate ∼ 10% of T2: 0.10 × 1000 = 100 kJ.
4. Secondary carnivores (T4) assimilate ∼ 10% of T3: 0.10 × 100 = 10 kJ.
5. Energy at T4 is only 0.1% of the original producer energy. So higher trophic levels have far less energy.

Each trophic transfer loses ∼90% of incoming energy as heat, so higher levels have exponentially less energy available.

Concept used. By Lindemann's 10% law (above), energy at level T_n is only (0.1)^n-1 of producer energy. By T₅, only 0.01% remains, which is too little to support a viable predator population at that level.

1. Producers fix energy E₁.
2. Apply E_n = E₁ (0.1)^n-1.
3. At T₄, energy is 10^-3 E₁ (0.1%).
4. At T₅, energy is 10^-4 E₁ (0.01%); insufficient to feed a viable population.
5. Hence most ecosystems show 4–5 trophic levels at most.
6. Other limits: time and area required for a top predator to find enough prey; high specific metabolic cost of larger predators.

Trophic levels are limited to ∼4–5 because energy lost at each transfer leaves too little (<0.01%) to support higher predators.

Concept used. A complete ecosystem contains all four functional components – producers, consumers, decomposers and abiotic environment – and is self-sustaining (captures its own energy, recycles its own nutrients, regulates its own populations). A home aquarium typically falls short on several of these.

1. Producers: usually present (aquatic plants, algae).
2. Consumers: present (fish, snails).
3. Decomposers: present but usually sparse and not enough to keep nutrient cycling balanced – ammonia builds up unless a filter is added.
4. Self-regulation: absent. The owner adds food, removes waste, changes water, controls temperature and light.
5. Energy and nutrient cycling: incomplete; external inputs (food pellets, electricity for light/heater) are continuous.
6. Therefore an aquarium is an incomplete (artificial) ecosystem: it lacks self-sustainability and depends on external subsidies.

No. An aquarium is an incomplete, artificial ecosystem; it depends on external food, light and water-change inputs and lacks self-regulation.

Concept used. Decomposition rate is governed by three abiotic factors: temperature, moisture and aeration. In the tropics:

• Mean temperature is high (25–30 ^∘C), close to the optimum for microbial enzymes.
• Rainfall is high, keeping the soil moist year-round.
• Litter is rich in nitrogen and sugar (short-lived broad leaves), easy to decompose.

1. Temperature: enzymes work faster at higher temperature (Q₁₀ rule: rates double for every 10 ^∘C up to ∼ 40 ^∘C). Tropical temperatures sit near the optimum.
2. Moisture: high rainfall keeps litter and soil wet, promoting both microbial activity and fragmentation by detritivores.
3. Litter quality: tropical broadleaf litter is N-rich and low in lignin; microbes degrade it quickly. (Temperate coniferous litter is the opposite – waxy, low N, slow to decompose.)
4. Aeration: well-drained soils in tropical forests support aerobic microbes (fastest catabolism).

Warm temperature + high moisture + N-rich litter + good aeration combine to make decomposition fastest in the tropics.

Concept used. The carbon cycle balances photosynthesis (CO₂ → glucose) against respiration and combustion (glucose/fossil fuels → CO₂). Human activities that release additional CO₂ or remove CO₂-fixing producers disturb this balance and raise atmospheric CO₂.

1. Activity 1: Combustion of fossil fuels (coal, oil, natural gas) for energy, transport, industry. Releases ∼ 36 billion tonnes of CO₂/year worldwide, far beyond the rate at which producers and oceans can absorb.
2. Activity 2: Deforestation (slash-and-burn, logging). Removes producers that fix CO₂; the burned biomass releases stored carbon back to the atmosphere as CO₂.
3. Other examples: cement manufacture, land-use change, intensive livestock (methane).

Two interfering activities: combustion of fossil fuels and deforestation; both raise atmospheric CO₂.

Concept used. Energy enters an ecosystem as sunlight, gets fixed by producers, and flows up the food chain. At each transfer, ∼ 90% is lost as heat (second law of thermodynamics). The heat dissipates to space and cannot be recaptured by any organism. Hence energy moves one-way (sun → producer → consumer → heat) and never cycles.

1. Producers capture sunlight: chemical-energy fixation by photosynthesis.
2. Energy moves from T1 to T2 to T3 to T4 through grazing/ predation. Direction is always upward.
3. At each transfer, 90% of incoming energy is dissipated as metabolic heat through respiration. Heat cannot be re-used for photosynthesis.
4. Decomposers also use the energy stored in dead biomass but again lose it as heat – not recycled back to producers.
5. Net effect: producers must keep capturing fresh solar energy continuously; energy is non-cyclic.
6. Contrast with nutrients (C, N, P, S): atoms do cycle because they are conserved.

Energy is unidirectional and non-cyclic: at every trophic transfer, ∼90% is lost as heat to space, so producers must continuously capture fresh solar energy.

Concept used. Microbes (bacteria, fungi, archaea, protists) include both autotrophs and heterotrophs. The textbook classification splits them into two groups:

• Decomposers / saprotrophs: heterotrophs that absorb nutrients from dead organic matter (most fungi, many bacteria).
• Chemo-autotrophs and photo-autotrophs: microbes that fix CO₂ using chemical energy (nitrifying bacteria, sulphur bacteria) or sunlight (cyanobacteria).

1. Most microbes acting in decomposition are decomposers / saprotrophs. They secrete digestive enzymes onto dead organic matter (detritus, humus) and absorb the soluble breakdown products through their cell walls.
2. Chemo-autotrophic microbes (e.g. Nitrosomonas oxidising NH4+ to NO2-; Nitrobacter oxidising NO2- to NO3-) obtain energy by oxidising inorganic compounds and use it to fix CO₂.
3. Photo-autotrophic microbes (cyanobacteria like Nostoc, Anabaena) photosynthesise like plants.

Most microbes are referred to as decomposers (saprotrophs). They obtain energy by absorbing nutrients from dead organic matter through extracellular digestion; chemo-autotrophic microbes additionally derive energy from oxidising inorganic compounds.

Concept used. A top carnivore (tiger) regulates the population of herbivores by predation. Removing the apex predator causes trophic cascade: prey populations boom, overgraze vegetation, reduce producer biomass, and shift the whole community.

1. Without tigers, deer and wild boar populations grow unchecked.
2. Overgrazing by herbivores reduces ground cover, sapling survival and biodiversity at the producer level.
3. Reduced producer biomass lowers ecosystem productivity and soil-erosion protection.
4. Smaller predators (leopards) may multiply too, disturbing the rest of the food web.
5. Loss of an umbrella species also threatens many other organisms whose habitat depends on tiger-protected reserves.

Tiger loss triggers a trophic cascade: herbivore boom → overgrazing → producer decline → biodiversity loss and ecosystem destabilisation.

Concept used. Lindemann's 10% law: only 10% of energy assimilated at one trophic level is transferred to the next as new biomass. A lion (T3 carnivore) builds 1 kg of body mass only by consuming 10 kg of deer (T2 herbivore), because 90% of the deer energy is lost as heat, faeces and unused biomass.

1. Assume the lion assimilates the deer's biomass at the 10% efficiency: lion biomass gained = 0.10 × deer biomass consumed.
2. Solve for the deer biomass required for 1 kg of lion: 1 kg = 0.10 × m_deer ⇒ m_deer = 10 kg.
3. So 10 kg of deer biomass yields just 1 kg of lion biomass.
4. Hence the energy stored in 10 kg of deer meat is equivalent to the energy stored in 1 kg of lion flesh.

By the 10% law, 1 kg of lion biomass requires 10 kg of deer biomass to build, so the two are energetically equivalent.

Concept used. Primary productivity is the rate of biomass production by producers. It depends on (i) availability of solar energy (PAR), (ii) availability of water, (iii) availability of nutrients (especially N and P), (iv) temperature and (v) plant community type (annual herbs vs. perennial trees). Each ecosystem has its own mix of these factors, so productivity varies.

1. Tropical rain forest: warm + wet + nutrient-rich + dense canopy → very high NPP (∼ 2200 g m^-2 yr^-1).
2. Estuary: high nutrient inflow + tidal mixing → high NPP (∼ 1500 g m^-2 yr^-1).
3. Grassland: moderate rainfall + warm summers → moderate NPP (∼ 500–1000 g m^-2 yr^-1).
4. Tundra: cold, short growing season → low NPP (∼ 140 g m^-2 yr^-1).
5. Open ocean: N/P limited → low NPP (∼ 125 g m^-2 yr^-1).
6. Desert: water limited → very low NPP (∼ 90 g m^-2 yr^-1).

Productivity varies because climate (light, temperature, water), nutrients and plant community differ between ecosystems; rain forests are highest, deserts and open oceans lowest.

Concept used. A pre-climax (or sub-climax / dis- climax) community is a seral stage held below the true climatic climax by some persistent biotic or abiotic factor: grazing pressure, recurring fire, frost pocket, edaphic limitation. The true climax would be reached if the limiting factor were removed.

1. Yes, I agree. A persistent disturbance can prevent succession from reaching the climatic climax.
2. Example 1 (biotic): heavy grazing by livestock keeps a grassland from progressing to woodland/forest, even though rainfall would support trees. The grassland is a plagio-climax maintained by grazing.
3. Example 2 (abiotic): recurring annual fires in savanna regions keep the community as fire-tolerant grasses, not woody trees – a fire climax.
4. Example 3 (edaphic): waterlogged soils keep a community as marsh sedges instead of forest.

Yes. Persistent grazing, recurring fire or edaphic constraints can hold a community at a pre-climax (sub-climax) stage. Example: livestock-grazed grassland that would otherwise become forest.

Concept used. An incomplete ecosystem lacks one or more of the standard ecosystem components (producers, consumers, decomposers, abiotic environment) in functional amounts and is not self-sustaining without external inputs.

1. A complete ecosystem has all four functional components in balanced proportions and recycles its own energy and nutrients.
2. An incomplete ecosystem lacks at least one component or is sustained by continuous external inputs.
3. Example 1: A cave ecosystem. There are no producers (no light for photosynthesis). All energy enters as detritus carried in by bats or water; consumers and decomposers depend on this external organic input.
4. Example 2: An aquarium. Producers and consumers present but decomposers are sparse, light/heat is supplied, food is added by the keeper. Without these subsidies the system collapses.
5. Example 3: A deep-sea hydrothermal vent (no photoautotrophs but driven by chemo-autotrophs; sometimes classified separately).

An incomplete ecosystem lacks one or more functional components and cannot sustain itself without external input. Examples: cave ecosystems (no producers, energy imported as detritus) and aquaria (decomposers sparse, food/light added externally).

Concept used. Ecological pyramids simplify trophic structure but ignore several real-world complications:

1. Assigns each organism to one trophic level only. Omnivores (humans, sparrows) occupy multiple levels simultaneously; pyramids cannot show this.
2. Saprophytes and decomposers ignored. Most pyramids in textbooks plot only the grazing chain; the detritus chain (carrying ∼ 90% of terrestrial energy) is omitted.
3. Inverted pyramids are possible for number and biomass. An aquatic pyramid of biomass can invert, and a parasitic pyramid of numbers can invert; standard upright diagrams misrepresent these.
4. No representation of nutrient cycling. Pyramids show one-way energy flow but not the cyclic movement of C, N, P.
5. Does not capture food-web complexity. Real ecosystems are food webs, not chains; a pyramid cannot show multiple feeding links.
6. Time scales hidden. Pyramids show standing crop at one moment, not the dynamic production rates over time.

Pyramids ignore omnivory, decomposer/detritus chain, food-web complexity, nutrient cycling, and time dynamics; biomass and number pyramids can also invert.

Concept used. Both are stages of decomposition but act on different substrates and yield different products:

• Humification: the accumulation of dark, amorphous, colloidal organic matter (humus) from partially decomposed detritus. The humus is highly resistant to further microbial attack.
• Mineralisation: the further microbial breakdown of humus to release inorganic nutrients (NH₄⁺, NO₃^-, PO₄^3-, K⁺, Ca²⁺) back into the soil solution.

1. Substrate: humification acts on detritus fragments; mineralisation acts on humus.
2. Product: humification yields humus (still organic, colloidal); mineralisation yields inorganic ions.
3. Agents: humification is largely done by fungi and soil arthropods; mineralisation by specific microbes (nitrifying bacteria, etc.).
4. Rate: humification is slow (∼ years); mineralisation is even slower for humus but releases nutrients steadily.
5. Role: humus improves soil structure and water retention; mineralisation releases ions that producers re-absorb.

Humification builds humus from detritus (organic colloid); mineralisation degrades humus to release inorganic nutrients.

Concept used. The figure shows a linear grazing food chain emerging from the Sun: Sun → plant (with roots) → caterpillar (larva) → small bird (sparrow) → large bird of prey (eagle/owl). ``Heat'' arrows show energy lost as respiration at each level, and a parallel detritus loop converges on a soil detritivore (slug/centipede). The four numbered boxes are producer through tertiary consumer.

1. Box (1) – the green plant fed by sunlight: Producer (T1). It performs photosynthesis using solar energy.
2. Box (2) – the caterpillar that eats the plant: Primary consumer / herbivore (T2).
3. Box (3) – the small bird (sparrow) that eats the caterpillar: Secondary consumer / primary carnivore (T3).
4. Box (4) – the bird of prey that eats the sparrow: Tertiary consumer / secondary carnivore (T4).
5. Heat arrows leaving each level show the ∼ 90% energy loss as respiration at each trophic transfer.

(1) Producer (plant); (2) Primary consumer / herbivore (caterpillar); (3) Secondary consumer / primary carnivore (sparrow); (4) Tertiary consumer / secondary carnivore (bird of prey).

Concept used. Decomposition is a microbial process. The rates of microbial enzyme action and substrate-microbe contact depend on five abiotic factors:

1. Temperature. Microbial enzymes follow the Q₁₀ rule (rate doubles per 10 ^∘C up to ∼ 40 ^∘C). Decomposition is fastest at warm (25–35 ^∘C), slowest near freezing.
2. Moisture / water availability. Microbes need moisture to swim and to exchange enzymes/products. Dry soils slow decomposition; waterlogged soils slow it differently (anaerobic).
3. Oxygen / aeration. Aerobic decomposition is faster and more complete (CO₂, H₂O as end products). Anaerobic conditions (waterlogging) slow decomposition and produce CH₄, H₂S, NH₃ instead.
4. Soil pH. Most decomposer bacteria prefer neutral to slightly alkaline soil (pH 6.5–7.5). Highly acidic soils (e.g. coniferous forest pH < 4) slow bacterial action; fungi take over but more slowly.
5. Substrate chemistry. N-rich, low-lignin litter (broadleaf) decomposes faster than waxy, lignin-rich coniferous litter.
6. Net effect: warm + moist + aerobic + neutral pH + N-rich litter → fastest decomposition (tropical rain forest); cold + dry / waterlogged + acidic + lignin-rich → slowest (tundra, peat bog).

Decomposition rate rises with temperature, moisture and aeration; is highest near neutral pH; and depends strongly on substrate quality. Combined optimum: warm + moist + aerobic + neutral pH + N-rich litter.

Concept used. Productivity in ecology has a strict dimensional meaning: it is biomass per unit area per unit time. The three statements provide different combinations of mass, area and time and therefore convey different ecological information.

1. Statement (a): only mass (10 quintals). No area, no time. This is a standing crop / yield number, not a productivity. Tells us the absolute harvest at one moment in space but nothing per unit area or per unit time.
2. Statement (b): mass + area (10 quintals per acre) over a single instant ``today''. This gives yield per unit area but still no time dimension; it is not a productivity either, just a per-area yield.
3. Statement (c): mass + area + time (10 quintals per acre per 6 months). All three dimensions present. This is a true productivity measurement: Productivity = 10 quintals 1 acre · 0.5 yr = 20 quintals acre^-1 yr^-1.
4. No, the three statements are not equivalent: only (c) carries enough information to compute productivity.

No, the three statements are not equivalent. (a) gives only total yield; (b) gives yield per unit area; (c) is a complete productivity statement (mass per area per time), yielding 20 quintals acre^-1 yr^-1 here.

Concept used.

• Gross primary productivity (GPP): total rate at which producers fix solar energy into organic biomass, before any losses.
• Net primary productivity (NPP): GPP minus respiratory losses by producers (R): NPP = GPP - R.
• In a natural climax community, biomass accumulates over decades → R is high (lots of living tissue to maintain) → NPP is small even though GPP is large.
• Humans maximise NPP by growing fast-turnover annual crops (low R) that channel most GPP into harvestable yield.

1. In nature, succession proceeds toward a climax with increasingly tall and dense vegetation – forests, coral reefs. Producers capture more sunlight and CO₂, so GPP rises.
2. But the same climax community has huge standing biomass (bark, wood, root systems) requiring continuous respiration to stay alive. R rises in proportion.
3. In a climax: NPP = GPP - R ≈ 0, because the system is at steady state.
4. Humans plant annual crops (wheat, rice) instead of climax forest. Annual crops have small standing biomass (low R), intense photosynthesis (high GPP per area), and the harvest is taken before respiration consumes the stored biomass.
5. Result: human-managed agro-ecosystems achieve very high NPP (lots of harvestable grain per acre per season), even though their GPP per area may be lower than a rain forest's.
6. Numerical illustration. Suppose a rain forest has GPP = 3000 g m^-2 yr^-1, R = 2200 g m^-2 yr^-1: NPP = 3000 - 2200 = 800 g m^-2 yr^-1. A wheat field may have GPP = 1500 g m^-2 yr^-1, R = 400 g m^-2 yr^-1: NPP = 1500 - 400 = 1100 g m^-2 yr^-1. Higher NPP despite lower GPP.

Nature pushes succession toward climax where GPP is maximal but R is also high, leaving NPP near zero. Humans cultivate fast-turnover annual crops with low standing biomass, so R is small and harvestable NPP is large.

Correct answer: a young forest.

Concept used. Net primary productivity (NPP) measures the rate at which producers store biomass (after their own respiratory losses). The candidate ecosystems differ in standing biomass, nutrient supply, light availability and respiratory load:

1. Young forest: rapid growth phase. New leaves and stems add biomass quickly. Standing biomass is still modest, so respiratory load (R) is moderate. Result: high NPP. This is the rising part of the succession curve.
2. Natural old forest (climax): huge standing biomass (tall trees, dense canopy). GPP is large but R is almost as large because all that biomass must be maintained. NPP ≈ 0 at steady state.
3. Shallow polluted lake: excess nutrients (eutrophication) drive a short algal bloom, but pollutants kill many producers and consumers; oxygen depletion limits decomposition. Sustained NPP is low.
4. Alpine meadow: short growing season, low temperature, thin soil. NPP is low.

A young forest has the highest primary productivity. It is in the rapid-growth phase: standing biomass is still modest (low R), but photosynthesis runs at maximum, so NPP = GPP - R is maximal.

Concept used. An ecological pyramid represents the trophic structure of an ecosystem graphically, with the producer level at the base and successive consumer levels stacked above. Three types exist, each plotting a different quantity per trophic level.

1. Pyramid of numbers. Plots the number of individuals at each trophic level. Conveys structural information about population sizes. Shape varies: upright in grassland (many grass plants → fewer grasshoppers → fewer frogs → fewer snakes), but inverted in a single-tree parasitic chain (1 tree → many fruit-eating birds → many lice → many bacteria).
2. Pyramid of biomass. Plots the standing crop biomass (dry weight) at each trophic level. Conveys information about biomass distribution (a functional snapshot of standing matter). Usually upright; inverted in marine ecosystems where phytoplankton biomass is smaller than zooplankton biomass at any instant.
3. Pyramid of energy. Plots the rate of energy flow (kcal m^-2 yr^-1) through each trophic level. Conveys functional / energetic information. Always upright (2nd law of thermodynamics: each level loses ∼ 90% of energy as heat). This is the most ecologically meaningful pyramid because it cannot invert.

Three pyramids: numbers (structure, can invert), biomass (standing matter, can invert in marine), and energy (functional, always upright by the 2nd law).

Concept used. The two pyramids are introduced under ``ecological pyramids''. Each pyramid stacks trophic levels with producers at the base. Their key features:

1. Pyramid of numbers. Counts individuals per trophic level.
   • In a grassland: many small grass plants form a wide base; fewer grasshoppers above; fewer frogs; fewer snakes at the top. Upright.
   • In a single tree (parasitic chain): one tree at the base, but many fruit-eating birds, many more lice on each bird, many more bacteria on each louse. Inverted.
   • Shape depends on relative body sizes and reproductive rates of the organisms in the chain.
2. Pyramid of biomass. Stacks the standing dry-mass biomass per unit area per trophic level.
   • Forest: tree biomass ≫ deer biomass ≫ tiger biomass. Upright.
   • Open ocean: phytoplankton biomass is tiny at any instant (very short turnover); zooplankton and fish biomass is large. Inverted.
   • Biomass pyramids depend on standing biomass at a point in time and ignore production rates.
3. Both pyramids can invert – a key contrast with the pyramid of energy, which is always upright.

Pyramid of numbers stacks individuals per level (upright in grassland, inverted in tree+parasite chain); pyramid of biomass stacks dry-mass per area (upright in forests, inverted in oceans).

Concept used. A food chain is a linear ``eat and be eaten'' sequence from producer to top predator. Multiple food chains crossing at shared species form a food web: the answer to ``what is this inter-linkage known as''.

1. Identify producers in the list: algae, hydrilla, maize plant, phytoplankton, Nymphaea, Spirogyra.
2. Identify primary consumers (herbivores): grasshopper, rat, squirrel, deer, rabbit, crustaceans, duck, sparrow (seeds), cockroach (mostly), elephant, goat.
3. Identify secondary consumers (primary carnivores): lizard (insects), toad (insects), spider, sparrow (insects), crow (omnivore), fish (small fish/crustaceans), crane (fish, frogs), snake (rats, frogs).
4. Identify tertiary / top carnivores: wolf, tiger, lion, leopard, whale, peacock (snakes, lizards).
5. Build sample chains (terrestrial):
   • maize → grasshopper → lizard → snake → peacock.
   • maize → rat → snake → peacock.
   • maize → deer → tiger.
   • maize → goat → leopard.
   • maize → rabbit → wolf.
   • maize → squirrel → snake.
   • maize → elephant (top herbivore, no predator).
   • maize → cockroach → toad → snake.
   • maize → sparrow → snake.
6. Aquatic chains:
   • phytoplankton → crustaceans → fish → crane.
   • phytoplankton → crustaceans → whale (baleen whales).
   • algae / Spirogyra → crustaceans → fish.
   • hydrilla / Nymphaea → duck → crane.
7. Cross-linkage: snake appears in many terrestrial chains (eats rat, frog, lizard, sparrow); fish appears in multiple aquatic chains. These shared nodes create a network of intersecting chains.
8. The interconnected network of all these food chains is called a food web.

Multiple ``eat and be eaten'' chains intersect at shared species (snake, fish, sparrow, etc.) to form a network called the food web.

Concept used. The second law of thermodynamics states that for any spontaneous process the entropy of the universe increases; equivalently, no real process is 100% efficient at converting one form of energy into another – some energy is always degraded into low-grade heat that cannot do useful work.

1. Producers absorb solar radiation: a high-grade, ordered form of energy. Photosynthesis stores some of it as chemical-bond energy in glucose, but ∼ 90% is lost as heat at the leaf surface and during the dark reactions.
2. Herbivores eat producers and assimilate ∼ 10% of the producer's stored energy. The rest is dissipated as heat (respiration), excreted as faeces or lost as uneaten biomass.
3. Carnivores at each successive level repeat the pattern: only ∼ 10% is built into new biomass; ∼ 90% becomes heat.
4. Numerical illustration. Starting with E₁ = 10⁴ kJ m^-2 yr^-1 at producers: E₂ = 0.10 × 10⁴ = 10³ kJ, E₃ = 0.10 × 10³ = 10² kJ, E₄ = 0.10 × 10² = 10 kJ. Cumulative loss to heat: 10⁴ - 10 = 9990 kJ, i.e. 99.9% of the original producer energy is gone after just three transfers.
5. The heat radiates away to space and cannot be re-used. Producers must keep capturing fresh solar input.
6. This irreversible degradation of usable energy to heat at every transfer is exactly the 2nd law in action.

At every trophic transfer ∼ 90% of incoming energy is dissipated as heat (respiration), so usable energy degrades irreversibly down the chain. This pattern is a direct consequence of the second law of thermodynamics, which forbids 100% conversion of one energy form to another.

Concept used. Each trophic level plays a specific role. Removing one level disrupts energy flow and population dynamics in predictable ways.

1. (a) All producers removed. Producers are the only entry point for solar energy into the ecosystem. Without producers:
   • No CO₂-to-biomass conversion: no new chemical energy enters the system.
   • Herbivores starve within days/weeks; their populations collapse.
   • Carnivores collapse next as their prey disappears.
   • Decomposers finish the existing dead matter and then starve themselves.
   • Net result: the entire ecosystem collapses within a few weeks/months.
2. (b) All herbivores eliminated. The grazing food chain breaks at T2.
   • Producers grow unchecked: vegetation density rises; some plant species (those favoured by grazing-induced disturbance) decline.
   • Carnivores that fed on herbivores starve.
   • Detritus food chain becomes the dominant route; decomposers handle the un-eaten producer biomass.
   • Net: grazing chain collapses; producer biomass and detritus chain inflate.
3. (c) Top carnivores removed. Trophic cascade.
   • Mesopredators (smaller carnivores) and herbivores multiply unchecked because their predator is gone.
   • Herbivores overgraze producers, lowering plant biomass.
   • Plant community shifts toward grazing-resistant species; biodiversity drops.
   • Ecosystem destabilises but typically does not collapse fully (cf. Yellowstone wolf removal example).

(a) Removing producers collapses the entire ecosystem (no energy entry). (b) Removing herbivores breaks the grazing chain, inflating producer biomass and shifting energy to the detritus chain. (c) Removing top carnivores triggers a trophic cascade: herbivore population booms, overgrazing reduces producer biomass and biodiversity.

Concept used. An artificial (man-made) ecosystem is one set up and maintained by humans for a particular purpose (food production, recreation, ornamentation). It depends on continuous external inputs of energy and matter and lacks the self-regulation of natural ecosystems.

1. Examples:
   • Agricultural cropland (rice paddy, wheat field, orchard).
   • Aquarium (home or laboratory).
   Other valid examples: gardens, dams/reservoirs, pisciculture ponds, urban parks.
2. Salient differences:
   • Species diversity: natural ecosystems have hundreds of species; man-made ecosystems are typically monocultures (one crop or one fish species).
   • Self-sustainability: natural ecosystems generate their own energy (sunlight via producers) and recycle nutrients internally. Artificial ecosystems depend on external inputs (fertilizers, pesticides, irrigation, fish food, electricity).
   • Stability / resilience: natural ecosystems buffer environmental fluctuations through species redundancy. Artificial ecosystems are fragile – a single pest or disease can wipe out a monoculture.
   • Genetic diversity: natural ecosystems have wild species with high genetic variability; artificial ones use selectively-bred high-yield varieties with low genetic diversity.
   • Productivity pattern: natural ecosystems have high GPP but low net export (most NPP cycles internally). Artificial ecosystems are designed for high net export (harvest).
   • Succession: natural ecosystems progress toward climax. Artificial ones are held in an early seral stage by continuous human intervention (ploughing, weeding).
   • Energy flow: natural ecosystems depend only on solar energy. Artificial ones receive an additional energy subsidy (fossil-fuel inputs for machinery, fertilizer manufacture, water pumping).

Examples: agricultural cropland and aquarium. They differ from natural ecosystems in low species/genetic diversity, lack of self-sustainability, need for external inputs (fertilizers, food, irrigation), high net export of biomass, and absence of succession (held in early seral stage by management).

Concept used. During succession, the community starts with a few pioneer species adapted to harsh, undeveloped substrate and progresses through several seral stages, accumulating species diversity until the climax is reached. The increase happens because of habitat development, niche differentiation and positive feedbacks among species.

1. Pioneer stage: very few species (often just lichens or grasses) can tolerate the extreme conditions of bare rock, sand or open water. Habitat is one-dimensional; few niches exist.
2. Habitat amelioration: pioneers modify the environment – lichens form soil from rock, mosses retain water, grasses add organic matter. More moderate conditions open the habitat to less hardy species.
3. Niche differentiation: as soil deepens and vegetation stratifies vertically (ground, herb, shrub, canopy), spatial heterogeneity increases. Each layer provides distinct niches for different species (canopy birds, shrub layer insects, ground beetles, soil organisms).
4. Mutualisms and food-web complexity: established plants attract pollinators, frugivores and seed dispersers. These animals bring more plant species. Predators arrive once prey is dense enough to support them. The food web widens at every step.
5. Microclimate buffering: at climax (e.g. tropical rain forest), the canopy buffers temperature and humidity, creating shaded, moist microhabitats that support shade- tolerant herbs, ferns, epiphytes and a rich invertebrate fauna.
6. Genetic and structural diversity feedback: more species → more interaction types → more selection pressures → more specialised species. Diversity begets diversity until limited by area, climate or nutrient availability.
7. Net result: species count, genetic variability, structural complexity and food-web complexity all peak at climax.

From pioneer to climax, habitat amelioration, niche differentiation, food-web complexity and microclimate buffering progressively widen the range of conditions and resources available, allowing many more specialised species to coexist.

Concept used. A biogeochemical cycle is the movement of an element through biological (bio), geological (geo) and chemical compartments of the biosphere. Each cycle has one or more reservoirs – large, slowly-cycling stores of the element – that buffer the cycle against rapid changes in the active pool. Sedimentary cycles (P, S, Ca) have reservoirs in the Earth's crust; gaseous cycles (C, N, O, H₂O) have atmospheric reservoirs.

1. Definition. A biogeochemical cycle is the cyclic movement of bio-essential elements between living organisms and the abiotic environment.
2. Two types.
   • Gaseous cycles: reservoir in the atmosphere (carbon, nitrogen, oxygen, water).
   • Sedimentary cycles: reservoir in the lithosphere (phosphorus, sulphur, calcium).
3. Role of reservoir. The reservoir:
   • Buffers the cycle: when the active pool is drawn down by uptake, the reservoir slowly releases more of the element; when the active pool surges (e.g. deforestation releases CO₂), the reservoir absorbs the excess.
   • Stabilises long-term concentration: over geological timescales, the reservoir keeps the cycle near steady state.
   • Stores the bulk of the element on Earth: e.g. 99% of crustal phosphorus is in rocks, only ∼ 1% is in the active pool.
4. Example of a sedimentary cycle: the phosphorus cycle.
   • Reservoir: phosphate-bearing rocks in the Earth's crust (apatite, Ca5(PO4)3(F,Cl,OH)).
   • Weathering of rock releases PO4^3- ions into soil solution.
   • Plants absorb PO4^3- via roots; build it into nucleic acids, ATP, phospholipids and bones.
   • Phosphorus moves up the food chain through herbivores and carnivores.
   • Death and excretion return P to the soil through decomposers, which mineralise organic P to inorganic PO4^3-.
   • Some P is carried by runoff to oceans where it settles as marine sediment, eventually lithified back into rock. Geological uplift may expose these rocks for re-weathering.
   • No significant gaseous form of phosphorus exists, so the cycle is entirely sedimentary.

A biogeochemical cycle is the cyclic movement of an element through biotic and abiotic compartments. The reservoir buffers and stabilises the cycle by absorbing surpluses and releasing during shortages. Phosphorus cycle is a sedimentary cycle with the reservoir in phosphate rocks of the Earth's crust.

Concept used. The P/R ratio is the ratio of gross primary production (P) to community respiration (R). It is a quick indicator of where an ecosystem sits in successional time:

• Pioneer community: P/R > 1 (production exceeds respiration; biomass accumulates).
• Climax community: P/R ≈ 1 (production equals respiration; biomass at steady state).
• Polluted / heterotrophic system: P/R < 1 (respiration exceeds production; biomass declines).

1. Pioneer community (P/R > 1). Standing biomass is small; few organisms means R is small. Producers are actively growing on new substrate, fixing more energy than the community needs for maintenance. Net biomass accumulates at every cycle.
2. Numerical illustration. If pioneer producers fix P = 1500 g m^-2 yr^-1 and the community respires R = 800 g m^-2 yr^-1: P/R = 1500 / 800 = 1.88. Net biomass accumulated: P - R = 700 g m^-2 yr^-1.
3. Climax community (P/R ≈ 1). Standing biomass is large (mature trees, dense canopy); R rises in proportion because every gram of living tissue needs continuous maintenance. Producers cannot indefinitely outpace this maintenance cost, so P and R approach equality at steady state.
4. Numerical illustration. If climax producers fix P = 3000 g m^-2 yr^-1 and the community respires R = 2900 g m^-2 yr^-1: P/R = 3000 / 2900 = 1.03. Net biomass accumulated: P - R = 100 g m^-2 yr^-1, approaching zero at full climax.
5. Explanation of the trend. As succession proceeds:
   • Standing biomass climbs (small pioneer plants → tall mature trees).
   • Respiration cost climbs in proportion to biomass.
   • Producer photosynthesis approaches a ceiling set by light, nutrients, water.
   • P grows more slowly than R; the ratio falls toward 1.
   • At climax, P and R balance and the ecosystem accumulates no net biomass: it is in steady state.
6. Why this matters for management. A polluted lake often has P/R < 1 (oxygen depleted, decomposers dominate); a healthy young forest has P/R ≫ 1 (biomass accumulating). Tracking P/R lets ecologists diagnose ecosystem health.

Pioneer community: P/R > 1 (biomass accumulating). Climax community: P/R ≈ 1 (steady state). The trend toward 1 reflects rising respiratory cost as standing biomass grows, until P just covers R.