Energy & Environmental Engineering (ES-301) - Unit 2 Short Notes

How unit 2 is examined

This unit covers what an ecosystem is, its components, energy flow, succession, food chains and pyramids, and the forest, grassland, desert and aquatic ecosystems. Marks sit on structure and function, energy flow, pyramids and food chains, the terrestrial ecosystems and aquatic ecosystems.

Concept of an ecosystem

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Definition. <mark>An ecosystem is a self-sustaining functional unit of nature in which living organisms interact with one another and with their non-living physical environment through energy flow and nutrient cycling.</mark> The term was coined by A. G. Tansley (1935); ecology is the study of these relationships.

Key points.

1. An ecosystem can be as small as a pond or as large as a forest or an ocean, and all of them together form the biosphere.
2. It is an open system: it takes in solar energy and matter and gives out heat and waste.
3. Ecosystems are natural (forest, pond) or artificial (crop field, aquarium).
4. Each species survives only within a range of conditions, as Shelford's law of tolerance states (see Structure and function).

Structure and function of an ecosystem

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Definition. <mark>An ecosystem is a community of living organisms (biotic) interacting with their non-living environment (abiotic) as one functional unit, with energy flowing through it and nutrients cycling within it.</mark> Ecology is the science that studies the relationship between organisms and their environment.

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u2-01" viewBox="0 0 388 338" width="388" height="338" role="img" aria-label="Ecosystem schematic. Sun = solar energy, Abi = abiotic environment (nutrients, air, water, soil), Prod = producers, Con = consumers, Dec = decomposers. Nutrients cycle Dec to Abi to Prod; energy flows one way from Sun and leaves as heat."><style>#dsfig-u2-01 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u2-01 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u2-01 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u2-01 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u2-01 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u2-01 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u2-01 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u2-01 .t{fill:#16181D;font-weight:500}#dsfig-u2-01 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u2-01 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u2-01 .dot{fill:#16181D}#dsfig-u2-01 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u2-01 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u2-01 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u2-01 .ah{fill:#454C5A}#dsfig-u2-01 .ah.hi{fill:#2340B8}#dsfig-u2-01 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u2-01 .wl .t{font-size:12px;font-weight:700}#dsfig-u2-01 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u2-01 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u2-01 .e{stroke:#B1B7C3}html.dark #dsfig-u2-01 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u2-01 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u2-01 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u2-01 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u2-01 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u2-01 .t{fill:#E6E8ED}html.dark #dsfig-u2-01 .t.inv{fill:#0F1115}html.dark #dsfig-u2-01 .kd{stroke:#E6E8ED}html.dark #dsfig-u2-01 .dot{fill:#E6E8ED}html.dark #dsfig-u2-01 .ann{fill:#8FA3FF}html.dark #dsfig-u2-01 .lbl{fill:#858D9C}html.dark #dsfig-u2-01 .ptr{fill:#8FA3FF}html.dark #dsfig-u2-01 .ah{fill:#B1B7C3}html.dark #dsfig-u2-01 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u2-01 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u2-01 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u2-01 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah4" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh4" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M59,40 L184,40" marker-end="url(#ah4)"/><path class="e" d="M55.2,157.6 L189.6,56.8" marker-end="url(#ah4)"/><path class="e" d="M230.4,58.4 L326.2,154.2" marker-end="url(#ah4)"/><path class="e" d="M212,66 L212,277" marker-end="url(#ah4)"/><path class="e" d="M327.6,182.4 L226.8,283.2" marker-end="url(#ah4)"/><path class="e" d="M196.8,286.6 L56.8,181.6" marker-end="url(#ah4)"/><circle class="n" cx="40" cy="169" r="18"/><text class="t" x="40" y="169" dy=".35em" text-anchor="middle">Abi</text><rect class="n" x="187" y="25" width="50" height="30" rx="15"/><text class="t" x="212" y="40" dy=".35em" text-anchor="middle">Prod</text><circle class="n" cx="341" cy="169" r="18"/><text class="t" x="341" y="169" dy=".35em" text-anchor="middle">Con</text><circle class="n" cx="212" cy="298" r="18"/><text class="t" x="212" y="298" dy=".35em" text-anchor="middle">Dec</text><circle class="n" cx="40" cy="40" r="18"/><text class="t" x="40" y="40" dy=".35em" text-anchor="middle">Sun</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Ecosystem schematic. Sun = solar energy, Abi = abiotic environment (nutrients, air, water, soil), Prod = producers, Con = consumers, Dec = decomposers. Nutrients cycle Dec to Abi to Prod; energy flows one way from Sun and leaves as heat.</figcaption></figure>

Key points.

1. Structure means the components: abiotic (physical factors such as light, temperature, rainfall and soil; chemical factors such as nutrients, oxygen, carbon dioxide and pH) and biotic (producers, consumers, decomposers).
2. Producers are green plants and algae that make food by photosynthesis; consumers eat other organisms; decomposers such as bacteria and fungi break dead matter into nutrients.
3. Function means the processes: energy flow, nutrient (biogeochemical) cycling, productivity, food chains and webs, and regulation of populations.
4. Energy flows in one direction from the sun to producers to consumers and is lost as heat, while nutrients are used again and again.
5. Biotic and abiotic parts depend on each other: plants take nutrients from soil, animals return them through waste and death, and decomposers restore them to the soil.
6. Ecosystems self-regulate through feedback, for example predators keep prey populations in check.
7. Structure decides function: the number of species and trophic levels present sets the productivity and stability.

Shelford's law of tolerance. Every species has a minimum, an optimum and a maximum for each factor; between the limits it survives, near the optimum it thrives, and outside the limits it dies. Species differ in their tolerance ranges (wide range: eurytopic; narrow: stenotopic), so different species occupy different niches and share resources along an environmental gradient. This partitioning lets many species coexist in one area, which increases biodiversity.

Ecosystem health. Diagnosis uses indicators: bioindicators (lichens for air, plankton for water), species diversity, pollutant levels, and soil and water quality. Prognosis uses ecological modelling, remote sensing, GIS and trend analysis to predict decline. Therapy is restoration: habitat restoration, reforestation, bioremediation and conservation policy.

Answer frame. Open with the definition and the two components; draw the schematic; then develop abiotic, producers, consumers, decomposers, energy flow, nutrient cycling; close with the line that structure and function together keep the system self-sustaining. For tolerance, draw a bell curve with optimum and stress zones.

Asked: [14 marks] (Nov 2022, Dec 2025) Describe the fundamentals of an ecosystem. Explain the relationship among the different parts with a schematic diagram / Define an ecosystem and explain its structure and functions / Define ecology and ecosystem and give a brief account of its components. Asked: [7 marks] (Dec 2023) What is the meaning of law of tolerance? Explain how tolerance can increase biodiversity? Asked: [7 marks] (Dec 2023) What are the indicators, tools and techniques of diagnosis, prognosis and therapy (treatment) of ecosystem health?

Producers, consumers and decomposers

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Definition. <mark>Producers make their own food, consumers depend on others for food, and decomposers break down dead organic matter into simple nutrients.</mark>

Key points.

1. Producers (autotrophs) are green plants, algae and some bacteria that fix solar energy by photosynthesis and start every food chain.
2. Consumers (heterotrophs) are herbivores (primary), carnivores (secondary and tertiary) and omnivores.
3. Decomposers (saprotrophs) such as bacteria and fungi digest dead remains and return nutrients to the soil.
4. Detritivores such as earthworms and termites break matter into pieces, which speeds up decomposition.

Energy flow in the ecosystem

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Definition. <mark>Energy flow is the one-way transfer of solar energy from producers through successive trophic levels to decomposers, with a large part lost as heat at every step.</mark>

Diagram. <figure class="ds-fig" style="margin:1.4rem 0;overflow-x:auto"><svg xmlns="http://www.w3.org/2000/svg" id="dsfig-u2-02" viewBox="0 0 534.2 338" width="534.2" height="338" role="img" aria-label="Energy flow. P = producers, PC = primary consumers (herbivores), SC = secondary consumers, TC = tertiary consumers, Dec = decomposers. Heat is lost at every level."><style>#dsfig-u2-02 .e{stroke:#454C5A;stroke-width:1.4;fill:none}#dsfig-u2-02 .e.hi{stroke:#2340B8;stroke-width:2.6}#dsfig-u2-02 .n{fill:#FFFFFF;stroke:#16181D;stroke-width:1.4}#dsfig-u2-02 .n.hi{fill:#E3E9FC;stroke:#2340B8;stroke-width:2.2}#dsfig-u2-02 .n.rb-b{fill:#16181D;stroke:#16181D}#dsfig-u2-02 .n.rb-r{fill:#BD3227;stroke:#BD3227}#dsfig-u2-02 text{font-family:"JetBrains Mono",ui-monospace,Menlo,Consolas,monospace;font-size:13px}#dsfig-u2-02 .t{fill:#16181D;font-weight:500}#dsfig-u2-02 .t.inv{fill:#FFFFFF;font-weight:700}#dsfig-u2-02 .kd{stroke:#16181D;stroke-width:1.2}#dsfig-u2-02 .dot{fill:#16181D}#dsfig-u2-02 .ann{fill:#2340B8;font-size:11px;font-weight:700}#dsfig-u2-02 .lbl{fill:#6F7787;font-family:system-ui,-apple-system,sans-serif;font-size:12px;font-weight:700}#dsfig-u2-02 .ptr{fill:#2340B8;font-size:12px;font-weight:700}#dsfig-u2-02 .ah{fill:#454C5A}#dsfig-u2-02 .ah.hi{fill:#2340B8}#dsfig-u2-02 .wl rect{fill:#FFFFFF;stroke:#DCE0E7}#dsfig-u2-02 .wl .t{font-size:12px;font-weight:700}#dsfig-u2-02 .wl.hi rect{fill:#2340B8;stroke:#2340B8}#dsfig-u2-02 .wl.hi .t{fill:#FFFFFF}html.dark #dsfig-u2-02 .e{stroke:#B1B7C3}html.dark #dsfig-u2-02 .e.hi{stroke:#8FA3FF}html.dark #dsfig-u2-02 .n{fill:#161920;stroke:#E6E8ED}html.dark #dsfig-u2-02 .n.hi{fill:#1E2748;stroke:#8FA3FF}html.dark #dsfig-u2-02 .n.rb-b{fill:#E6E8ED;stroke:#E6E8ED}html.dark #dsfig-u2-02 .n.rb-r{fill:#FF7E71;stroke:#FF7E71}html.dark #dsfig-u2-02 .t{fill:#E6E8ED}html.dark #dsfig-u2-02 .t.inv{fill:#0F1115}html.dark #dsfig-u2-02 .kd{stroke:#E6E8ED}html.dark #dsfig-u2-02 .dot{fill:#E6E8ED}html.dark #dsfig-u2-02 .ann{fill:#8FA3FF}html.dark #dsfig-u2-02 .lbl{fill:#858D9C}html.dark #dsfig-u2-02 .ptr{fill:#8FA3FF}html.dark #dsfig-u2-02 .ah{fill:#B1B7C3}html.dark #dsfig-u2-02 .ah.hi{fill:#8FA3FF}html.dark #dsfig-u2-02 .wl rect{fill:#161920;stroke:#2A2E37}html.dark #dsfig-u2-02 .wl.hi rect{fill:#8FA3FF;stroke:#8FA3FF}html.dark #dsfig-u2-02 .wl.hi .t{fill:#0F1115}</style><defs><marker id="ah5" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah" d="M0,1 L9,5 L0,9 z"/></marker><marker id="ahh5" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse"><path class="ah hi" d="M0,1 L9,5 L0,9 z"/></marker></defs><path class="e" d="M59,126 L130.8,126" marker-end="url(#ah5)"/><path class="e" d="M170.8,126 L242.6,126" marker-end="url(#ah5)"/><path class="e" d="M282.6,126 L354.4,126" marker-end="url(#ah5)"/><path class="e" d="M394.4,126 L466.2,126" marker-end="url(#ah5)"/><path class="e" d="M164.1,140.5 L284.4,282" marker-end="url(#ah5)"/><path class="e" d="M267.3,144.6 L293.9,277.4" marker-end="url(#ah5)"/><path class="e" d="M367.6,143.3 L306.6,278.8" marker-end="url(#ah5)"/><path class="e" d="M473.1,138.8 L313.5,283.9" marker-end="url(#ah5)"/><path class="e" d="M168.2,116.4 L273.9,54.2" marker-end="url(#ah5)"/><path class="e" d="M270.7,108.4 L287.6,66" marker-end="url(#ah5)"/><path class="e" d="M362.7,111.9 L316.7,60.8" marker-end="url(#ah5)"/><circle class="n" cx="40" cy="126" r="18"/><text class="t" x="40" y="126" dy=".35em" text-anchor="middle">Sun</text><circle class="n" cx="151.8" cy="126" r="18"/><text class="t" x="151.8" y="126" dy=".35em" text-anchor="middle">P</text><circle class="n" cx="263.6" cy="126" r="18"/><text class="t" x="263.6" y="126" dy=".35em" text-anchor="middle">PC</text><circle class="n" cx="375.4" cy="126" r="18"/><text class="t" x="375.4" y="126" dy=".35em" text-anchor="middle">SC</text><circle class="n" cx="487.2" cy="126" r="18"/><text class="t" x="487.2" y="126" dy=".35em" text-anchor="middle">TC</text><circle class="n" cx="298" cy="298" r="18"/><text class="t" x="298" y="298" dy=".35em" text-anchor="middle">Dec</text><rect class="n" x="273" y="25" width="50" height="30" rx="15"/><text class="t" x="298" y="40" dy=".35em" text-anchor="middle">Heat</text></svg><figcaption style="font-size:.82em;opacity:.72;margin-top:.45rem">Energy flow. P = producers, PC = primary consumers (herbivores), SC = secondary consumers, TC = tertiary consumers, Dec = decomposers. Heat is lost at every level.</figcaption></figure>

Key points.

1. The sun is the only source; producers capture only about 1 percent of incident solar energy in photosynthesis.
2. Energy passes along the food chain producer, herbivore, carnivore, top carnivore, and each step is a trophic level.
3. Flow is unidirectional: energy cannot be recycled, because heat lost by respiration cannot be used by plants again.
4. Lindeman's 10 percent law: only about 10 percent of the energy at one level reaches the next, and the rest is lost as respiration, heat and waste.
5. Example: if producers hold 10,000 J, herbivores get 1,000 J, carnivores 100 J and top carnivores 10 J, which is why food chains have only 4-5 links.
6. Dead matter at every level goes to decomposers, which release the remaining energy as heat and return nutrients.
7. Energy flow follows the laws of thermodynamics: energy is conserved, and each transfer degrades some of it to heat.
8. Flow is shown by a single-channel model or a Y-shaped model with grazing and detritus channels.

Answer frame. Open with the definition; draw the flow diagram with heat arrows; then develop the sun, trophic levels, unidirectional flow, 10 percent law with the numerical example, decomposers; close with the point that energy flows while nutrients cycle. For "energy flow and pyramids", add the pyramid of energy from the next topic.

Asked: [14 marks] (Nov 2019) Give a flow diagram of energy flow in ecosystem. Also explain about desert ecosystem. Asked: [7 marks] (Dec 2025) Describe energy flow and ecological pyramids in an ecosystem.

Ecological succession

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Definition. <mark>Ecological succession is the orderly, gradual replacement of one biotic community by another in an area until a stable climax community is reached.</mark>

Key points.

1. Primary succession starts on bare, lifeless surfaces such as rock or lava, while secondary succession starts where soil exists, for example after a fire or an abandoned field.
2. Each temporary community is a seral stage; the whole sequence is a sere, and the first colonisers are pioneer species such as lichens.
3. Rock sequence: lichens, mosses, herbs, shrubs, trees (climax), with soil building up at each stage.
4. The climax community is stable, has high diversity and is in balance with the climate.

Food chains, food webs and ecological pyramids

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Definition. <mark>A food chain is the sequence of organisms in which each eats the one before it, transferring energy from producers to top consumers; each step is a trophic level. A food web is many interlinked food chains.</mark> An ecological pyramid is the graphic representation of trophic structure, with producers at the base.

Key points.

1. Grazing food chain (GFC) starts with green plants and passes to herbivores and carnivores, for example grass, grasshopper, frog, snake, hawk.
2. Detritus food chain (DFC) starts with dead organic matter eaten by detritivores and decomposers, for example leaf litter, earthworm, bird; it is the main channel in forests.
3. Parasitic food chain runs from a large host to smaller parasites, for example tree, bird, lice, bacteria.
4. A food web is more stable than a single chain because an animal with several food sources survives the loss of one.
5. Pyramid of numbers counts individuals: upright in grassland, but inverted in a tree ecosystem (one tree supports many insects) and in a parasitic chain.
6. Pyramid of biomass (dry weight): upright on land, but inverted in ponds and oceans since phytoplankton is small in mass yet reproduces fast.
7. Pyramid of energy is always upright because energy is lost at each step by the 10 percent law.

Energy (J/m2/yr):  Tertiary 10 | Secondary 100 | Primary 1,000 | Producers 10,000
Numbers, grassland: hawk 1 | snake 5 | frog 50 | grasshopper 5,000 | grass 50,000 (upright)
Biomass, pond: zooplankton and fish outweigh phytoplankton at a moment in time (inverted)

Answer frame. For food chains: define, draw a GFC and a DFC, add the parasitic chain, close with the link to webs. For pyramids: define, sketch the three pyramids side by side, note upright or inverted cases, close that energy is always upright.

Asked: [7 marks] (Nov 2022, Dec 2025) What is Food Chain? Explain various types of food chains with suitable examples. Asked: [7 marks] (Dec 2024) Explain the concept of ecological pyramids and also describe its various types.

Forest ecosystem, Grassland ecosystem and Desert ecosystem: types, features, structure and function

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Definition. <mark>A forest ecosystem is dominated by trees, a grassland by grasses with few trees, and a desert by very low rainfall (below about 25 cm a year) with sparse, drought-adapted life.</mark>

Key points.

1. Forest types are tropical rain, deciduous, coniferous (taiga) and temperate forests, and they have layers: canopy, understorey, shrub and ground.
2. Forest producers are trees; consumers are herbivores (deer), carnivores (tiger) and omnivores; decomposers are fungi and bacteria that work fast on the litter.
3. Grassland types are tropical savanna, temperate prairie and polar tundra, and grasses regrow after grazing because growth is from the base.
4. Grassland producers are grasses, consumers are grazers and predators, and fire and grazing keep the trees out.
5. Desert plants have xerophytic adaptations: deep or wide roots, thick waxy cuticle, spines instead of leaves and stems that store water.
6. Desert animals are nocturnal or burrowing, excrete concentrated urine and get water from food; the camel stores fat in its hump.
7. Trees do attract rain, which is scientific: transpiration adds moisture, shade cools the air, and forest aerosols act as cloud condensation nuclei (the biotic pump idea).
8. Forest value: economic (timber, non-timber products, medicines, valued by market price and contingent valuation) and social (carbon sequestration, soil and water conservation, recreation, culture and livelihoods of tribal people).

Answer frame. Open with the definition of each ecosystem; give the comparison table; develop climate, flora, fauna, then function; close with the roles in balance and productivity. For desert questions write climate, xerophytes, nocturnal animals.

Asked: [14 marks] (Nov 2022) Write brief notes on any two: a) Desert ecosystem (noise pollution, Environment Protection Act, wind energy belong to other units). Asked: [7 marks] (Dec 2023) A collection of trees is attractive to rainfall. Is it a scientific thought? How to evaluate the economic and social values of a forest? Asked: [7 marks] (Dec 2025) Explain forest and grassland ecosystems highlighting their characteristics.

(d) Aquatic ecosystems (ponds, streams, lakes, rivers, oceans, estuaries)

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Definition. <mark>An aquatic ecosystem is one in which organisms live in water and depend on its physical and chemical properties; it is freshwater (below 0.5 percent salt) or marine (about 3.5 percent salt).</mark>

Key points.

1. Freshwater lentic (still) ecosystems are ponds and lakes; lotic (flowing) ones are streams and rivers.
2. Biotic groups are plankton (floating: phytoplankton and zooplankton), nekton (swimmers such as fish), benthos (bottom dwellers such as worms and snails) and neuston (surface life).
3. A pond has a littoral zone (shallow, rooted plants), limnetic zone (open water, plankton) and profundal zone (deep, no light); decomposers live at the bottom.
4. Pond food chain: phytoplankton, zooplankton, small fish, large fish; oxygen and light fall with depth.
5. Rivers and streams have fast current and high oxygen; life is attached or streamlined and depends on food washed in.
6. Oceans cover about 70 percent of the earth, with zones: intertidal, neritic (shallow, most productive), pelagic (open) and benthic; the coral reef has the greatest diversity.
7. An estuary is where river meets sea; brackish water, high nutrients and high productivity make it a nursery for fish and prawns, and mangroves grow there.
8. Human impact: nutrient runoff causes eutrophication and dead zones, industrial waste, plastics and microplastics poison life, overfishing and reef destruction cut biodiversity, and warm effluent and CO2-driven ocean acidification harm species.

Answer frame. Open with the definition and classification into freshwater and marine; draw a pond zonation sketch; develop lentic, lotic, ocean, estuary and biotic groups; close with importance and threats. For the human impact question, list the pollutant then its effect.

Asked: [7 marks] (Nov 2022, Dec 2025) Explain aquatic ecosystem / Discuss aquatic ecosystems with reference to ponds and oceans. Asked: [7 marks] (Dec 2023, Jun 2023) What impact do humans have on aquatic life zones?

Last-minute revision

• Ecosystem: living plus non-living parts as a functional unit; term by Tansley 1935.
• Components: abiotic (physical, chemical) and biotic (producers, consumers, decomposers).
• Energy flows one way; nutrients cycle.
• Lindeman's law: 10 percent of energy passes to the next level.
• GFC starts with plants, DFC with dead matter; food webs are more stable.
• Pyramid of energy is always upright; numbers and biomass can be inverted.
• Shelford's law: minimum, optimum, maximum limits of tolerance.
• Desert: below 25 cm rain, xerophytes, nocturnal animals.
• Lentic is still water, lotic is flowing water; plankton, nekton, benthos.
• Eutrophication comes from nutrient runoff and causes dead zones.

Memory hooks

• PCD: Producers make, Consumers eat, Decomposers recycle.
• Energy is a one-way street (flow), matter is a roundabout (cycle).
• Lentic = Lake stays; Lotic = Load moves.
• PNB: Plankton floats, Nekton swims, Benthos bottom.
• Succession: Pioneer to Seral to Climax.

Coverage checklist

• Concept of an ecosystem: definition; law of tolerance (Dec 2023).
• Structure and function of an ecosystem: fundamentals and schematic (Nov 2022, Dec 2025); ecosystem health (Dec 2023).
• Producers, consumers and decomposers: definition and roles.
• Energy flow in the ecosystem: flow diagram (Nov 2019); energy flow and pyramids (Dec 2025).
• Ecological succession: primary, secondary, climax.
• Food chains, food webs and ecological pyramids: food chain types (Nov 2022, Dec 2025); pyramids (Dec 2024).
• Introduction, types, characteristic features, structure and function of the following ecosystem (a.) Forest ecosystem (b) Grassland ecosystem (c) Desert ecosystem: desert notes (Nov 2022); forest and rain (Dec 2023); forest and grassland (Dec 2025).
• (d) Aquatic ecosystems (ponds, streams, lakes, rivers, oceans, estuaries): aquatic (Nov 2022, Dec 2025); human impact (Dec 2023, Jun 2023).