(B) All interacting organisms in an area together with the non-living constituents of the environment.

The textbook explicitly defines an ecosystem as "all the interacting organisms in an area together with the non-living constituents of the environment." Options A and C describe only parts (biotic or abiotic), and D describes just one component (food chains). Always remember: ecosystem = biotic + abiotic components together.

Source: Chapter 13, Section 13.1

(D) Crop-field

Crop-fields are human-made (artificial) ecosystems. Forests, ponds and lakes are natural ecosystems.

Source: Chapter 13, Section 13.1

The textbook explicitly states: "forests, ponds and lakes are natural ecosystems while gardens and crop-fields are human-made (artificial) ecosystems." For MCQs, simply writing the correct option with a one-line justification is sufficient for full marks.

(B) 10%

An average of 10% of the food eaten by an organism at one trophic level is turned into its own body mass and made available for the next trophic level.

The 10% law is a key concept in energy flow through ecosystems. The source passage explicitly states: "An average of 10% of the food eaten is turned into its own body and made available for the next level of consumers." The rest is lost as heat, used in digestion, or for work. This is why food chains are limited to 3–4 steps. Remember: green plants capture ~1% of sunlight energy, but 10% of food energy passes between trophic levels — don't confuse the two.

Source: Chapter 13, Section 13.1.1 Food Chains and Webs

(B) Grass → Goat → Human

In a food chain, energy flows from producer to consumer. Grass (producer) is eaten by goat (primary consumer), which is eaten by human (secondary consumer).

A food chain always begins with a producer (green plant/grass) and energy flows in one direction toward higher trophic levels. The source passage (Chapter 13, Exercise Q2) directly lists "Grass, goat and human" as a correct food chain. Options A and C reverse the order; option D skips the goat as intermediate consumer.

Source: Chapter 13 (Our Environment), Exercises Q2 & Section 13.1.1

(B) Biological magnification

The progressive accumulation of non-degradable chemicals at each successive trophic level in a food chain is called biological magnification.

The key phrase is "progressive accumulation at each successive trophic level." Biological magnification (biomagnification) is the correct term — concentrations of harmful chemicals like pesticides increase as we move up the food chain. Eutrophication refers to excess nutrients in water bodies; biodegradation and decomposition are breakdown processes, not accumulation. This concept is directly covered in Chapter 13 (Our Environment).

(B) Plastic bags, CFCs, glass bottles

Plastic bags, CFCs, and glass bottles are all non-biodegradable — they cannot be broken down by biological processes and persist in the environment for a long time.

• Options A, C, and D each contain at least one biodegradable item (vegetable peels, fruit peels, paper cups, cotton, wood, grass, leather — all broken down by microorganisms).
• Option B is correct because plastic bags, CFCs, and glass bottles are all human-made, non-biodegradable substances that resist biological breakdown.
• Key trick: eliminate options with any biodegradable item to find the all-non-biodegradable group.

(B) Shielding the Earth's surface from ultraviolet radiation

The ozone layer absorbs harmful UV radiation from the Sun, protecting organisms from damage such as skin cancer in humans.

Source: Chapter 13, Section 13.2.1

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The passage explicitly states: "ozone performs an essential function. It shields the surface of the earth from ultraviolet (UV) radiation from the Sun." Options A, C, and D are incorrect — ozone does not provide oxygen for respiration (that is O₂), does not primarily absorb infrared radiation, and does not filter visible light.

(B) Chlorofluorocarbons (CFCs)

CFCs, used as refrigerants and in fire extinguishers, are the synthetic chemicals primarily responsible for the depletion of the ozone layer.

The textbook (Chapter 13) explicitly states that the decrease in ozone levels "has been linked to synthetic chemicals like chlorofluorocarbons (CFCs)." DDT is a pesticide, CO₂ causes global warming, and SO₂ causes acid rain — none deplete the ozone layer.

(C) 1%

Green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy.

Source: Chapter 13, Section 13.1.1 – Food Chains and Webs

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The passage explicitly states: "The green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy." Do not confuse this with the 10% law, which refers to energy transfer between trophic levels (i.e., only 10% of food eaten by one level passes to the next). These are two different figures and are frequently mixed up in MCQs.

(B) Unidirectional — energy flows from autotrophs to consumers and is not reversed

The flow of energy is unidirectional. Energy captured by autotrophs does not revert to solar input, and energy passed to herbivores does not return to autotrophs.

The textbook explicitly states: "the flow of energy is unidirectional" — energy moves from producers → primary consumers → secondary consumers → tertiary consumers and is never reversed. At each step, energy is also lost as heat, so it cannot cycle back. This is a frequently tested concept; remember it contrasts with the cycling of matter (which does cycle), making option B the only correct choice.

(C) Fruit peels, cake and lime juice

Fruit peels, cake, and lime juice are all derived from natural/biological sources and can be broken down by biological processes, making them biodegradable. Plastic, glass, and DDT are non-biodegradable.

The textbook defines biodegradable substances as those broken down by biological processes (bacteria/saprophytes). Items of natural origin — food waste, plant matter, natural fibres — are biodegradable. Plastic, glass, and DDT resist biological breakdown and are non-biodegradable. Note: the correct option in the source passage is (c) Fruit-peels, cake and lime-juice, which matches Option C here.

Source: Chapter 13, Section 13.2.2 (Exercise Q.1)

(B) Food web

When branching food chains interconnect instead of forming a straight line, the resulting network is called a food web.

The textbook (Chapter 13) explicitly states: "instead of a straight line food chain, the relationship can be shown as a series of branching lines called a food web." Trophic pyramid refers to energy/number levels; "Energy chain" and "Decomposition chain" are not defined terms in the syllabus.

(B) Greater accumulation of persistent waste in the environment

Non-biodegradable packaging cannot be broken down by biological processes and persists in the environment for a long time, causing serious accumulation of waste.

The passage directly states: "Changes in packaging have resulted in much of our waste becoming non-biodegradable" and non-biodegradable substances "persist in the environment for a long time or may harm the various members of the eco-system." Options A, C, and D are unrelated to non-biodegradable waste disposal. Always link "non-biodegradable" → "persists" → "accumulates."

(A) Both A and R are true and R is the correct explanation of A.

Non-degradable pesticides accumulate at each trophic level through biological magnification, so the highest trophic level organisms accumulate maximum concentration.

Source: Chapter 13, Section 13.1.1 (Food Chains and Webs)

The textbook explicitly states: "As these chemicals are not degradable, these get accumulated progressively at each trophic level. As human beings occupy the top level in any food chain, the maximum concentration of these chemicals get accumulated in our bodies." This directly confirms both A and R, and R correctly explains A. Option (A) is the answer.

(A) Both A and R are true and R is the correct explanation of A.

Only about 10% of energy passes to the next trophic level; the rest is lost as heat. So very little usable energy remains beyond the fourth level, limiting food chain length to 3–4 steps.

The textbook explicitly states: "Since so little energy is available for the next level of consumers, food chains generally consist of only three or four steps. The loss of energy at each step is so great that very little usable energy remains after four trophic levels." This directly confirms both A and R, and R correctly explains why A is true. The 10% law is the key concept here.

(A) Both A and R are true and R is the correct explanation of A.

Ozone is a deadly poison at ground level but shields Earth from harmful UV radiation at higher altitudes, so R correctly explains A.

The textbook explicitly states that ozone "is a deadly poison" but at higher atmospheric levels "shields the surface of the earth from ultraviolet (UV) radiation." The Reason directly explains why the Assertion is true — making option (A) the correct choice. Examiners expect students to verify both the truth of A and R, and whether R logically explains A.

Biodegradable substances: Substances that are broken down by biological processes (action of bacteria or other saprophytes) are called biodegradable substances.

• Biodegradable waste (home example): Vegetable peels / spoilt food / used tea leaves.
• Non-biodegradable waste (home example): Plastic bags / empty medicine strips / bubble packs.

Source: Our Environment, Section 13.2.2

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• The definition must include the key idea: broken down by biological processes — not just "broken down naturally." Examiners look for this specific phrase from the textbook.
• One mark is for the definition; one mark is split between the two examples (both must be correct to score).
• Common mistakes: writing "decomposed by nature" (too vague) or giving examples that don't clearly belong to home-generated waste. Stick to kitchen/household items like vegetable peels (biodegradable) and plastic/medicine strips (non-biodegradable).

Decomposers are microorganisms (bacteria and fungi) that break down the complex organic matter of dead plants and animals into simple inorganic substances, which are returned to the soil.

In their absence, dead organic matter would keep accumulating, nutrients would not be recycled back into the soil, and producers (plants) would be deprived of minerals — disrupting the entire ecosystem.

• 1 mark for defining decomposers (mention microorganisms + breaking down dead matter + releasing nutrients).
• 1 mark for the consequence — nutrient cycling stops, minerals are not returned to soil, producers cannot grow, ecosystem collapses.
• Examiners look for the key phrase "nutrient recycling" or equivalent. Avoid vague answers like "the environment will get dirty."

Pesticides sprayed on crops enter the body of herbivores (e.g., grasshoppers, cattle) when they eat the crops. When humans consume these animals or their products (milk, meat), the pesticides enter our bodies. This process is called biological magnification — the concentration of harmful chemicals increases at each trophic level of the food chain.

The key concept here is biological magnification (biomagnification). Examiners expect you to mention: (1) pesticides entering herbivores via crops, (2) transfer to humans through the food chain, and (3) the term "biological magnification" with the idea that concentration increases at higher trophic levels. Avoid mixing this with nervous system content from Ch. 6 — this question is purely from Ch. 13 (Our Environment).

Food Chain: A linear sequence where each organism eats the one before it.
Example: Grass → Goat → Human

Food Web: An interconnected network of multiple food chains in an ecosystem.
Example: Grass → Insect → Frog → Snake → Eagle (linked with other chains like Grass → Rabbit → Eagle)

Source: Chapter 13, Section 13.1.1

• Examiners expect a definition + example for each — both parts are needed for full marks (1 mark each).
• Key distinction: food chain = single straight-line sequence; food web = several interlinking food chains.
• Use examples directly from the textbook figures (forest, grassland, pond) for safety.
• Do not write long paragraphs — two crisp definitions and two examples are sufficient for 2 marks.

Trophic levels are the steps or levels in a food chain, each occupied by organisms sharing the same position in the flow of energy.

| Organism | Trophic Level |
|---|---|
| Grass | 1st (Producer) |
| Grasshopper | 2nd (Primary consumer) |
| Frog | 3rd (Secondary consumer) |
| Snake | 4th (Tertiary consumer) |
| Eagle | 5th (Quaternary consumer) |

Eagle has the least available energy. At each trophic level, only 10% of the energy is transferred to the next level; the rest is lost as heat. By the 5th trophic level, energy has been reduced to a tiny fraction of what was originally fixed by the grass.

Source: Chapter 13, Section 13.1.1 — Food Chains and Webs

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• Define first, then apply — examiners expect the definition before the example.
• The 10% law is the key scientific reason for least energy at the top; always state it explicitly.
• A table is an efficient way to show trophic levels clearly and saves words — examiners appreciate it.
• Note: NCERT text shows food chains of 3–4 steps; a 5-step chain is valid but uncommon — identify Eagle as the top/quaternary consumer confidently.
• Do not write long paragraphs; this is 3 marks ≈ 60–90 words, so be concise.

At each trophic level, only 10% of the energy is transferred to the next level. The rest is lost as heat, used in digestion, and for life processes. Therefore, energy decreases rapidly at each successive trophic level, supporting fewer individuals.

Working:

• Producers (T1) = 10,000 units
• Primary consumers (T2) = 10% of 10,000 = 1,000 units
• Secondary consumers (T3) = 10% of 1,000 = 100 units
• Tertiary consumers (T4) = 10% of 100 = 10 units

∴ Energy available at the fourth trophic level = 10 units.

Source: Chapter 13, Section 13.1.1 (Food Chains and Webs)

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• The examiner expects the 10% law to be stated clearly — this is the key concept.
• The calculation must show all three steps (T2, T3, T4) with working, not just the final answer.
• "Loss as heat/digestion/life processes" is the reason — one line is enough; don't over-explain.
• Always state the final answer explicitly. Skipping the working loses marks even if the answer is correct.

Biological Magnification: The progressive increase in the concentration of harmful chemicals (like pesticides) at each successive trophic level of a food chain is called biological magnification.

Example:
$$\text{Plants} \rightarrow \text{Insects (herbivore)} \rightarrow \text{Frog} \rightarrow \text{Snake} \rightarrow \text{Eagle (carnivore)}$$

Pesticides are absorbed by plants from soil/water. Since these chemicals are non-degradable, they are not broken down and keep accumulating in the body of each organism. A herbivore eats many plants, collecting all their pesticide content. The top carnivore then eats many herbivores and intermediate consumers, so the chemicals concentrate further at each level. Thus, the top carnivore accumulates the maximum concentration of pesticides.

Source: Chapter 13, Section 13.1.1 – Food Chains and Webs

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• 1 mark for defining biological magnification correctly.
• 1 mark for a valid food chain with labels.
• 1 mark for the explanation of why concentration increases — key idea is that chemicals are non-degradable and each consumer eats many organisms from the level below, so chemicals keep accumulating upward.
• Do not just say "carnivores eat more" — you must link it to the non-degradable nature of chemicals and their progressive accumulation at each trophic level.

Formation of Ozone:
UV radiation from the Sun splits oxygen molecules (O₂) into free oxygen atoms (O). These free atoms then combine with other oxygen molecules to form ozone:

$$O_2 \xrightarrow{UV} O + O$$
$$O + O_2 \rightarrow O_3 \text{ (Ozone)}$$

Chemicals responsible for depletion:
Chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers.

Two harmful consequences:

1. Increased UV radiation reaching Earth causes skin cancer in humans.
2. UV radiation is highly damaging to all living organisms/ecosystems.

Source: Chapter 13, Section 13.2.1

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• Examiners expect the two chemical equations for ozone formation — writing them earns key marks.
• "CFCs" must be written in full at least once: Chlorofluorocarbons.
• The textbook explicitly mentions skin cancer as a consequence; always include it. For the second consequence, any reference to harm to organisms/UV damage is acceptable.
• Do not confuse ground-level ozone (pollutant) with stratospheric ozone (protective layer).

Three environment-friendly practices to reduce non-biodegradable waste:

1. Carry cloth bags while shopping — This avoids the use of plastic carry bags, which are non-biodegradable and persist in the environment for a very long time.

1. Avoid disposable plastic items — Using reusable containers, cups, and bottles reduces the generation of plastic waste that cannot be broken down by biological processes.

1. Choose products with minimal/biodegradable packaging — Changes in packaging have resulted in much of our waste becoming non-biodegradable, so selecting eco-friendly packaging directly reduces such waste.

Source: Our Environment, Section 13.2.2

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• The question asks for three practices + justification, so each point must name the practice AND briefly explain why it works — that earns 1 mark each.
• The textbook directly states that "changes in packaging have resulted in much of our waste becoming non-biodegradable" and that non-biodegradable substances "persist in the environment for a long time or may harm the various members of the ecosystem." Use these ideas in justifications.
• Exercise Q3 lists cloth bags, switching off lights, and walking — carry bags is the most directly relevant to non-biodegradable waste. Build the other two from the chapter's discussion on plastics and packaging.
• Keep each justification to one sentence; don't over-explain.

(i) Trophic level above: The organisms at the level immediately above would face a shortage of food, causing their population to decline or starve. For example, if all grasshoppers (primary consumers) are removed, frogs (secondary consumers) would have no food and their numbers would fall sharply.

(ii) Trophic level below: The organisms at the level immediately below would increase in population due to reduced predation. In the same example, removing grasshoppers would cause grass (producers) to grow unchecked, disturbing the ecosystem balance.

• Examiners expect both consequences clearly distinguished — starvation above, population boom below.
• A specific, named example (grasshopper, frog, grass) is required for full marks; a general statement alone will lose a mark.
• Keep terminology correct: use "trophic level," "food chain," "producers/consumers."
• The answer maps to the interdependence of ecosystem components (Chapter 13).

(a) An ecosystem is a system in which all living organisms in an area interact with each other and with the non-living constituents of the environment, maintaining a balance in nature.

| | Biotic Components | Abiotic Components |
|---|---|---|
| Definition | Living organisms | Physical/non-living factors |
| Examples | Plants, animals | Temperature, soil |

(b)

• Producers (green plants) synthesise food from inorganic substances using sunlight, making solar energy available to all other organisms.
• Consumers (herbivores, carnivores) depend on producers directly or indirectly for sustenance, transferring energy through trophic levels.
• Decomposers (bacteria, fungi) break down dead remains into simple inorganic substances, returning nutrients to the soil for reuse by plants.

(c) Green plants capture solar energy and convert it into chemical energy (food) through photosynthesis, making them the primary (first) source of energy for all other organisms.

If all producers were removed, consumers would have no food source and would die. Decomposers would also eventually perish. The entire ecosystem would collapse as energy flow would stop completely.

Source: Chapter 13, Section 13.1 & 13.1.1

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• (a): Give the textbook definition and clearly separate biotic/abiotic with two examples each — a small table saves words.
• (b): Each role needs one clear sentence; examiners award one mark per component, so don't merge them.
• (c): Two distinct points — why green plants are the primary energy source (photosynthesis/solar energy capture), and consequence of removing producers (collapse of food chain). Both are needed for full marks.
• Keep language concise; CBSE rewards accuracy and coverage over length.

(a) The ozone layer is a region in the upper atmosphere (stratosphere) where ozone (O₃) is present in higher concentrations. It shields Earth's surface from harmful ultraviolet (UV) radiation from the Sun.

(b) UV radiation splits O₂ molecules into free oxygen atoms, which then combine with O₂ to form ozone:

$$O_2 \xrightarrow{\text{UV}} O + O$$
$$O + O_2 \rightarrow O_3 \text{ (Ozone)}$$

(c) Synthetic chemicals called chlorofluorocarbons (CFCs), used in refrigerants and fire extinguishers, have caused ozone depletion, which began to drop sharply in the 1980s. Consequences include increased UV radiation reaching Earth's surface, which is highly damaging to organisms — for example, it causes skin cancer in human beings.

(d) In 1987, the United Nations Environment Programme (UNEP) forged an agreement to freeze CFC production at 1986 levels. It is now mandatory for all manufacturing companies worldwide to produce CFC-free refrigerators.

Source: Chapter 13, Section 13.2.1

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• The question has four sub-parts, so answer all four clearly labelled — examiners check this.
• Include both chemical equations for part (b); writing only one loses marks.
• For part (c), name CFCs specifically and give at least one consequence (skin cancer).
• For part (d), mention the year (1987), UNEP, and the CFC-free refrigerator mandate — these are the specific facts examiners look for.
• Keep language concise; do not repeat information across sub-parts.

(a) Biodegradable substances are broken down by biological processes (bacteria, saprophytes). Non-biodegradable substances are not broken down this way and persist in the environment.

• Biodegradable examples: Vegetable peels, spoilt food
• Non-biodegradable examples: Plastic bags, empty medicine strips

(b) Two environmental problems caused by non-biodegradable substances:

1. They persist in the environment for a very long time, polluting soil and water bodies.
2. They may cause biological magnification — harmful chemicals accumulate and increase in concentration at higher trophic levels, damaging organisms including humans.

(c) Yes, even biodegradable waste can harm the environment. If generated in large quantities, it produces foul smell and harmful gases during decomposition, may contaminate water bodies, and can disturb the balance of nutrients in the soil.

(d) Two practical steps for municipal bodies:

1. Collect biodegradable and non-biodegradable wastes separately using different bins and treat them by composting and recycling respectively.
2. Ensure proper sewage treatment so that untreated waste does not pollute local water bodies and soil.

Source: Chapter 13, Section 13.2.2 — Managing the Garbage we Produce

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• (a) The distinction must use the key phrase "broken down by biological processes." The textbook explicitly defines both terms this way — use these definitions, not vague ones.
• (b) Biological magnification is the textbook's own example of a specific problem; include it for full marks. Persistence in the environment is the other core point.
• (c) This tests critical thinking beyond definitions. The textbook asks this exact question (Exercise Q8) and the answer is that large quantities of even biodegradable waste cause pollution — gas production, water contamination, etc.
• (d) Activity 13.7 in the textbook explicitly asks students to check whether local bodies treat biodegradable and non-biodegradable wastes separately — make that your anchor point for a strong answer.

(i) The phenomenon responsible is Biological Magnification.

(ii) Increasing order of pesticide concentration:

Algae < Small fish < Large fish < Fish-eating birds

(iii) Pesticides are non-biodegradable chemicals. When algae absorb them from water, small fish eat large amounts of algae, accumulating more pesticides. Large fish eat many small fish, concentrating the chemical further. Fish-eating birds, being at the top trophic level, consume the most large fish, so the maximum concentration accumulates in their bodies. Since the pesticides cannot be broken down or excreted, they build up progressively at each trophic level — a process called biological magnification — resulting in the highest levels in fish-eating birds, even though the water has very little of the chemical.

Source: Chapter 13, Section 13.1.1 — Food Chains and Webs

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• (i) is a straight one-mark recall — just name the term correctly.
• (ii) is one mark for the correct sequence; the food chain order directly gives the order of increasing pesticide concentration.
• (iii) is 2 marks — examiners expect two key ideas: (a) non-biodegradable nature causes accumulation at each trophic level, and (b) organisms at higher levels eat many organisms below, so concentration keeps increasing. Mentioning "biological magnification" again here strengthens the answer. Keep it concise — roughly 50–60 words for 2 marks.

(i) The materials in Set A are called biodegradable substances. They are mainly broken down by microorganisms (bacteria and fungi/saprophytes) present in the soil.

(ii) The materials in Set B are non-biodegradable. Bacteria and other saprophytes do not possess the specific enzymes needed to break down human-made materials like plastic and polythene, so they remained unchanged.

(iii) Two harmful effects of non-biodegradable materials:

1. They persist in the environment for a very long time, accumulating in soil and water bodies, disrupting the ecosystem.
2. They may harm living organisms in the ecosystem — for example, through biological magnification if toxic substances enter food chains.

Source: Chapter 13, Section 13.2.2 — Managing the Garbage we Produce

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• (i) The key term "biodegradable" and the agents (bacteria/fungi/saprophytes) are both needed for the 1 mark.
• (ii) The reason must refer to the absence of specific enzymes in microorganisms — this is exactly what the textbook states. Just saying "plastic doesn't decompose" is too vague.
• (iii) The textbook states non-biodegradable substances either "persist in the environment for a long time" or "harm the various members of the ecosystem." Mention both distinct effects. Biological magnification is a valid second point from the same chapter.

(i) Energy available to the Lion (4th trophic level):

Using the 10% law:

• Grass (1st trophic level) = 1,00,000 units
• Rabbit (2nd trophic level) = 10% of 1,00,000 = 10,000 units
• Fox (3rd trophic level) = 10% of 10,000 = 1,000 units
• Lion (4th trophic level) = 10% of 1,000 = 100 units

∴ Energy available to the lion = 100 units

(ii) Since only 10% of energy is transferred at each trophic level, very little energy remains by the time it reaches the lion (4th trophic level). Less available energy can support fewer individuals, so the number of organisms decreases at higher trophic levels.

(iii) When foxes were wiped out, rabbit numbers exploded and grass was severely overgrazed. This shows that every organism in a food chain is interdependent — removal of one organism at any trophic level disturbs the balance of the entire ecosystem.

Source: Chapter 13, Section 13.1.1 — Food Chains and Webs

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• (i) Always show each step of the 10% law calculation clearly — examiners award marks for working, not just the final answer.
• (ii) Link the reason directly to energy loss: less energy = fewer individuals. This is the concept of the energy pyramid.
• (iii) The biologist's observation is a classic example of a trophic cascade — use it to explain interdependence and balance in an ecosystem. Stick to the passage; don't over-elaborate.