Our Environment — Class 10 Science practice questions

Question 1

Read the following passage and answer the questions that follow:

A farmer in a village noticed that the fish in his pond were dying. On investigation, he found that a pesticide called DDT was being used in large amounts on the surrounding farms. DDT washed into the pond with rainwater. The farmer also noticed that the fish-eating birds near the pond were becoming weak and unable to reproduce normally, even though the amount of DDT directly entering the pond seemed very small.

The food chain in the pond is:
Phytoplankton → Small fish → Large fish → Fish-eating birds

The food chain in the pond is:
Phytoplankton → Small fish → Large fish → Fish-eating birds

(a) Name the phenomenon that explains why the fish-eating birds were most affected, even though the initial concentration of DDT in the water was very low.
(b) At which trophic level would the concentration of DDT be the highest in this food chain?
(c) The farmer decides to stop using chemical pesticides altogether and instead uses biological methods to control pests. Give TWO advantages of this decision for the environment.

Accepted Answer

(a) The phenomenon is called Biological Magnification (Biomagnification).
DDT is a non-biodegradable chemical that is not broken down or excreted by organisms. As it passes from one trophic level to the next, it accumulates in the body tissues of organisms. Since a large amount of food (and thus DDT) from the lower trophic level is consumed to sustain each organism at the higher trophic level, the concentration of DDT increases progressively at each step of the food chain. This is why the fish-eating birds, being at the highest trophic level, accumulated the greatest concentration of DDT in their bodies.

(b) The concentration of DDT would be the highest at the 4th trophic level, i.e., in the Fish-eating birds, as they are at the top of this food chain and DDT accumulates with each trophic transfer.

(c) Two advantages of using biological methods instead of chemical pesticides:
(i) Biological control methods use natural predators, parasites, or microorganisms to control pests, so they do not introduce toxic, non-biodegradable chemicals into the environment. This prevents biomagnification of harmful substances in food chains and protects organisms at higher trophic levels.
(ii) Biological methods do not cause soil and water pollution. Chemical pesticides can persist in soil and wash into water bodies, harming aquatic life and contaminating drinking water sources. Avoiding them helps maintain the health of ecosystems and biodiversity.

Question 2

Read the following passage carefully and answer the questions that follow:

A group of environmental science students set up two model ecosystems in sealed glass tanks. Tank 1 contained grass → grasshoppers → frogs → snakes → hawks, with the grass receiving 10,000 J of solar energy. Tank 2 was identical but the students removed all the snakes from the food chain. After several weeks, the students observed dramatic differences between the two tanks. They also measured the amount of energy available at each trophic level in Tank 1 and found that at each step, only 10% of the energy from the previous level was transferred to the next. Meanwhile, they noticed that DDT sprayed on the grass in a nearby field had accumulated in very high concentrations in the body fat of the hawks at the top of the food chain in a neighbouring natural ecosystem.

(a) Calculate the amount of energy available to the frogs in Tank 1. Show your working.

(b) In Tank 2, after the snakes were removed, the frog population first increased and then crashed to near zero. Give one reason for this.

(c) Explain why DDT concentration was highest in the hawks and not in the grass, even though DDT was originally applied only to the grass. Also state the term used for this phenomenon.

Accepted Answer

(a) Energy available to the frogs:

The food chain in Tank 1 is:
Grass → Grasshoppers → Frogs → Snakes → Hawks

Solar energy received by grass (T1) = 10,000 J

Applying the 10% law at each trophic level:

Energy available to grasshoppers (T2) = 10% of 10,000 = 10,000 × (10/100) = 1,000 J

Energy available to frogs (T3) = 10% of 1,000 = 1,000 × (10/100) = 100 J

∴ The energy available to the frogs is 100 J.                                                    [1 mark]

(b) In Tank 2, after snakes were removed, the frog population initially increased because there was no predator to control it. However, as the frog population grew rapidly, the grasshopper population (their prey) was over-consumed and declined sharply. With insufficient food available, the frog population subsequently crashed to near zero due to starvation / lack of prey.                                                    [1 mark]

(c) DDT was originally sprayed on the grass at a low concentration. When grasshoppers ate large amounts of grass, DDT — being a non-biodegradable chemical — was not broken down and accumulated in their bodies. Frogs then ate many grasshoppers, accumulating even higher concentrations of DDT. Snakes ate many frogs, and hawks ate many snakes, so at each successive trophic level the concentration of DDT in body fat increased further. Since hawks are at the top of the food chain (highest trophic level), they consumed the most organisms and therefore accumulated the greatest concentration of DDT, far higher than was present in any individual organism at a lower level.

This progressive increase in the concentration of a harmful, non-biodegradable chemical at successive trophic levels of a food chain is called Biological Magnification (also known as Biomagnification).                                                    [2 marks]

Question 3

Read the following passage and answer the questions that follow:

A group of students conducted a field study in a freshwater lake ecosystem near their school. They identified the following organisms and recorded their feeding relationships:

• Phytoplankton (microscopic algae)
• Zooplankton (tiny aquatic animals)
• Small fish (feed on zooplankton)
• Large fish (feed on small fish)
• Fish-eating birds (osprey)

They also noticed that the water of the lake near a farming area had unusually high concentrations of a pesticide called DDT. Water samples showed 0.002 ppm DDT, while osprey eggs collected from nests showed 25 ppm DDT.

(a) Identify the trophic level of small fish in the food chain constructed from the organisms listed above.

(b) The students found that only 10% of the energy available at one trophic level is transferred to the next trophic level. If small fish at the third trophic level contain 500 J of energy, how much energy was originally available at the first trophic level (phytoplankton)?

(c) Explain why the concentration of DDT is much higher in osprey eggs (25 ppm) than in the lake water (0.002 ppm). What term is used for this phenomenon? State one consequence of this accumulation for the osprey population.

Accepted Answer

(a) Small fish occupy the Third Trophic Level (T-3) in the food chain. The complete food chain is: Phytoplankton → Zooplankton → Small fish → Large fish → Osprey. Phytoplankton are producers (T-1), zooplankton are primary consumers (T-2), and small fish are secondary consumers (T-3). [1 mark]

(b) According to the 10% law, only 10% of energy is transferred from one trophic level to the next.

Small fish are at the third trophic level (T-3) and contain 500 J.

Let energy at T-1 (phytoplankton) = x J.

Energy available at T-2 (zooplankton) = 10% of x = x/10

Energy available at T-3 (small fish) = 10% of x/10 = x/100

According to the question:
x/100 = 500 J

∴ x = 500 × 100 = 50,000 J

The energy originally available at the first trophic level (phytoplankton) was 50,000 J. [1 mark]

(c) The much higher concentration of DDT in osprey eggs compared to lake water is because DDT is a fat-soluble, non-biodegradable chemical. It cannot be broken down by the metabolic processes of organisms and is not excreted. Instead, it accumulates in the fatty tissues of each organism. As DDT passes from one trophic level to the next through feeding, its concentration progressively increases because each organism at a higher trophic level must consume a large number of organisms from the lower trophic level, taking in and retaining all the DDT stored in them. This progressive increase in concentration of harmful chemicals at each successive trophic level of a food chain is called Biological Magnification (or Biomagnification). Since the osprey is at the highest trophic level (T-5), it accumulates the maximum concentration of DDT. One consequence for the osprey population is that high DDT levels interfere with calcium metabolism, causing the birds to lay eggs with thin, fragile shells that break before hatching, leading to a sharp decline in the osprey population. [2 marks]

Question 4

Read the following passage and answer the questions that follow:

A school in a hilly town organised an 'Eco-Awareness Week'. During the week, students studied the food web of a nearby forest. They found that grass grows abundantly and is eaten by rabbits and deer. Foxes eat rabbits. A hawk preys on both rabbits and foxes. Students also noticed that pesticides sprayed on the grass by a nearby farm were accumulating in increasing concentrations as they moved up the food chain — a phenomenon their teacher called 'biological magnification'.

The students also collected litter samples from the forest floor. They found two types of waste: fallen leaves and fruit peels (Type A) and discarded plastic bags and thermocol cups (Type B).

(a) Write one food chain from the passage that has the maximum number of trophic levels.

(b) In the food chain you wrote in (a), which organism would show the highest concentration of pesticide due to biological magnification? Give one reason.

(c) The students found that Type A waste disappeared from the forest floor within weeks, but Type B waste remained unchanged for months.
(i) Name the category to which Type B waste belongs and define it.
(ii) State one harmful effect of Type B waste on the ecosystem.

Accepted Answer

(a) Grass → Rabbit → Fox → Hawk
(This chain has four trophic levels — the maximum from the given passage.)
[1 mark]

(b) The hawk would show the highest concentration of pesticide.
Reason: The hawk is at the highest trophic level (fourth) in this food chain. Pesticides are non-biodegradable and cannot be broken down by biological processes, so they accumulate in the body of each organism. As they pass from one trophic level to the next, their concentration keeps increasing. Since the hawk eats organisms that have already accumulated pesticide from lower levels, the highest amount gets concentrated in its body.
[1 mark]

(c)
(i) Type B waste (plastic bags and thermocol cups) belongs to the category of non-biodegradable waste.
Non-biodegradable materials are substances that cannot be broken down by biological processes (by decomposers/microorganisms) and persist in the environment for a very long time.
[1 mark]

(ii) One harmful effect: Non-biodegradable waste such as plastic bags can be accidentally ingested by animals (such as cows, deer, or aquatic animals), blocking their digestive tracts and causing injury or death. / Non-biodegradable substances enter food chains and undergo biological magnification, increasing in concentration at higher trophic levels and harming organisms at the top of the food chain.
(Any one valid harmful effect accepted.)
[1 mark]

Question 5

Read the following passage carefully and answer the questions that follow:

A village near a forest has a small pond. The pond supports a community of producers (aquatic plants), herbivores (small fish), carnivores (large fish), and top carnivores (herons — a type of bird). The villagers noticed that after a nearby factory began releasing chemical waste into the pond, the herons started dying in large numbers, even though the chemical concentration in the pond water was very low and seemed harmless. A scientist visited and explained that the chemical accumulates progressively in the bodies of organisms as it passes through the food chain. She also pointed out that the pond's food chain is part of a larger ecosystem that depends on the flow of energy and cycling of materials.

(a) Name the phenomenon by which the concentration of harmful chemicals increases at each successive trophic level in a food chain.

(b) Which organisms in the pond food chain would have the HIGHEST concentration of the harmful chemical in their bodies? Give one reason.

(c) The aquatic plants in the pond trap 10,000 J of energy from sunlight. Using the 10% energy flow law, calculate how much energy would be available to the large fish (carnivores) in this pond food chain. Show your working.

OR

(c) The scientist explained that unlike energy, certain materials are returned to the environment and reused. (i) Name the process responsible for returning carbon from dead organisms back to the atmosphere. (ii) State one reason why decomposers are essential for the sustainability of any ecosystem.

Accepted Answer

(a) The phenomenon is called Biological Magnification (also accepted: Biomagnification). [1 mark]

(b) The herons (top carnivores / organisms at the highest trophic level) would have the highest concentration of the harmful chemical in their bodies. [½ mark]
Reason: As the chemical passes from one trophic level to the next, it is not broken down or excreted. It accumulates in the body of each organism. Since herons are at the top of the food chain, they receive and accumulate the chemical from all the trophic levels below them, resulting in the highest concentration. [½ mark]

(c) The pond food chain is:
Aquatic plants → Small fish → Large fish → Herons

Energy available at each trophic level (applying the 10% law):

Trophic Level 1 — Aquatic plants (Producers): 10,000 J

Trophic Level 2 — Small fish (Herbivores):
= 10% of 10,000 J
= 10/100 × 10,000
= 1,000 J

Trophic Level 3 — Large fish (Carnivores):
= 10% of 1,000 J
= 10/100 × 1,000
= 100 J

∴ Energy available to the large fish (carnivores) = 100 J [2 marks]

(Award 1 mark for correct application of 10% law with working; 1 mark for correct final answer of 100 J.)

OR

(c) (i) The process responsible for returning carbon from dead organisms back to the atmosphere is decomposition (carried out by decomposers / microorganisms such as bacteria and fungi), which releases CO₂ into the atmosphere through respiration. [1 mark]
(Accept: microbial decomposition / decay / breakdown of organic matter by decomposers.)

(ii) Decomposers are essential for the sustainability of any ecosystem because they break down the complex organic matter of dead organisms into simple inorganic substances (nutrients) and return them to the soil and atmosphere. This replenishes the nutrient pool, making materials available again for producers (plants). Without decomposers, dead organic matter would keep accumulating and essential nutrients would not be recycled, causing the ecosystem to collapse. [1 mark]
(Award 1 mark for any one correct point: nutrient recycling / breaking down dead matter / returning minerals to soil / enabling producers to reuse nutrients.)

Question 6

Read the following passage and answer the questions that follow:

A team of environmental scientists studied two aquatic ecosystems — a natural lake (Lake A) and a fish farm (Lake B) constructed nearby. In Lake A, they found a food chain: Phytoplankton → Zooplankton → Small fish → Large fish → Osprey (a fish-eating bird). Water samples from Lake A showed traces of DDT (a pesticide used in nearby agricultural fields). The scientists measured DDT concentration at each trophic level and found it increased dramatically from phytoplankton to osprey. In Lake B (the fish farm), the owners regularly added fish feed, removed weeds, and stocked only one species of fish. After several years, Lake A continued to thrive, but Lake B showed declining fish health whenever the owners stopped maintenance.

(a) Name the phenomenon by which DDT concentration increases at each successive trophic level in Lake A.

(b) In the food chain of Lake A, identify the trophic level at which DDT concentration would be highest and give one reason.

(c) The scientists calculated that the phytoplankton in Lake A fixed 10,000 J of energy. Using the 10% law, calculate the energy available to the osprey. Also explain why Lake B cannot survive without human intervention, unlike Lake A.

Accepted Answer

(a) The phenomenon is called **Biological Magnification** (also accepted: Biomagnification). [1 mark]

(b) The **Osprey** (fifth trophic level / T5) would have the highest concentration of DDT. [½ mark]
Reason: DDT is a non-biodegradable chemical. It is not broken down or excreted efficiently by organisms. At each successive trophic level, the DDT ingested by consuming many organisms of the lower trophic level accumulates in the body. Since the osprey is at the highest trophic level and consumes large quantities of large fish over its lifetime, DDT accumulates to the greatest concentration in its body. [½ mark]

(c) **Energy calculation using the 10% Law:** [1 mark]

Energy fixed by Phytoplankton (T1) = 10,000 J

Applying the 10% law: only 10% of energy transfers to the next trophic level.

Energy available at T2 (Zooplankton) = 10% of 10,000 = **1,000 J**

Energy available at T3 (Small fish) = 10% of 1,000 = **100 J**

Energy available at T4 (Large fish) = 10% of 100 = **10 J**

Energy available at T5 (Osprey) = 10% of 10 = **1 J**

∴ The energy available to the osprey = **1 J**

**Why Lake B cannot survive without human intervention:** [1 mark]

Lake B is a human-made ecosystem (fish farm). It lacks biodiversity — only one species of fish is stocked, and natural producers, decomposers, and other consumers are absent or removed. It does not have a complete, balanced food web or nutrient cycling. All energy input (fish feed) and waste removal depend entirely on human management. Without these inputs, producers cannot sustain the fish population, decomposers cannot recycle nutrients, and the ecosystem collapses. Lake A, being a natural ecosystem, has a complete food web, diverse producers, consumers and decomposers, and functions through self-sustaining nutrient cycles without any external input.

Question 7

Read the following passage and answer the questions that follow:

A village near an industrial town has a small lake that was once rich in fish, frogs, and aquatic plants. Five years ago, a pesticide manufacturing unit was set up upstream. The factory discharges its treated effluents into the river that feeds the lake. Villagers noticed that fish populations declined first, then the frogs disappeared, and finally large water birds such as herons, which fed on these fish and frogs, were found dead near the lake shore. Environmental scientists collected samples and found that the concentration of a persistent organochlorine pesticide in the lake water was only 0.003 ppm — well below the legal safety limit. Yet the pesticide concentration measured in the liver tissue of the dead herons was 16.5 ppm.

(a) Name the phenomenon that explains why the pesticide concentration in the herons is thousands of times higher than in the lake water, even though the water appears 'safe'.

(b) At which trophic level do the herons feed in this ecosystem? Justify your answer using the sequence of organisms mentioned.

(c) The scientists found that only 10% of energy was transferred from one trophic level to the next, yet the pesticide concentration multiplied at each step instead of decreasing. Explain why energy and this pesticide behave oppositely as they move up the food chain. Also suggest ONE change in agricultural practice that could prevent such a situation from recurring in this region.

Accepted Answer

(a) The phenomenon is called biological magnification (biomagnification). It refers to the progressive increase in the concentration of harmful, non-biodegradable chemicals such as pesticides at successive trophic levels of a food chain. Because the pesticide is not broken down or excreted by organisms, it accumulates in their body tissues; each organism at a higher trophic level consumes many organisms from the level below, thereby concentrating the chemical further. [1 mark]

(b) The herons feed at the fourth trophic level (Tertiary consumers). The food chain described in the passage is:
Aquatic plants (Producers, T₁) → Fish (Primary consumers, T₂) → Frogs (Secondary consumers, T₃) → Herons (Tertiary consumers, T₄).
Since herons feed on frogs (which themselves feed on fish, which feed on aquatic plants), they occupy the fourth trophic level. [1 mark]

(c) Energy and the pesticide behave oppositely because of the fundamental difference in how they are processed by living organisms.

Energy decreases at each trophic level because a large part of the energy consumed at any level is used up by that organism for its own life processes — respiration, movement, maintaining body temperature, and other metabolic activities. As a result, only about 10% of the energy available at one trophic level is transferred to the next level (the Ten Per Cent Law). Energy is lost as heat and cannot be passed on.

The pesticide, by contrast, is a non-biodegradable chemical — it cannot be broken down by metabolic processes inside the organism's body, nor can it be excreted efficiently. It dissolves in fat and accumulates in the fatty tissues of each organism. When a heron eats many frogs, it retains the pesticide from all of them in its own body, so the concentration keeps multiplying as we move up the food chain. Unlike energy, the pesticide is not 'spent' at each level — it is stored and passed on in increasing amounts. [1.5 marks]

Suggested change in agricultural practice: The farmers and industries in this region should replace persistent organochlorine pesticides with biodegradable biopesticides (such as neem-based pesticides or microbial agents like Bacillus thuringiensis). Biodegradable chemicals are broken down naturally in the environment and do not accumulate in food chains, eliminating the risk of biological magnification. [0.5 mark]

Question 8

Read the following passage and answer the questions that follow:

A group of Class 10 students visited a pond ecosystem near their school. They observed the following organisms: phytoplankton, zooplankton, small fish, large fish, and a fish-eating bird. The teacher explained that only 10% of the energy available at one trophic level is transferred to the next trophic level. The students also noticed that the pond water near a factory outlet looked unusual — the fish population had declined, but the bird population was showing signs of illness even though the birds were not directly exposed to the factory chemicals.

(a) Name the phenomenon that explains why the birds became ill even though they were not directly exposed to the factory chemicals. [1]

(b) Arrange the organisms observed by the students into a food chain, starting from the producer. [1]

(c) If the phytoplankton in the pond contain 10,000 J of energy, calculate the energy available to the large fish. Also state one reason why so much energy is lost between trophic levels. [2]

OR

(c) The students noted that a grass → grasshopper → frog → snake → eagle food chain is operating in the field nearby. If grass contains 1,00,000 J of energy, how much energy is available to the snake? Also explain why it is advised to have food chains with fewer trophic levels for efficient energy use. [2]

Accepted Answer

(a) The phenomenon is biological magnification (biomagnification).
Harmful chemicals (like pesticides or heavy metals) from the factory enter the food chain at lower trophic levels. Since these chemicals are not broken down or excreted, they accumulate and become more concentrated at each successive trophic level. The fish-eating birds, being at the highest trophic level, accumulate the greatest concentration of these chemicals in their bodies and therefore show signs of illness. [1 mark]

(b) Food chain:
Phytoplankton → Zooplankton → Small fish → Large fish → Fish-eating bird [1 mark]
(Any correct linear arrangement starting with phytoplankton as producer earns full mark.)

(c) Energy available at each trophic level (using the 10% law):

Phytoplankton (T1) = 10,000 J
Zooplankton (T2) = 10% of 10,000 = 1,000 J
Small fish (T3) = 10% of 1,000 = 100 J
Large fish (T4) = 10% of 100 = 10 J

∴ Energy available to the large fish = 10 J [1 mark]

Reason for energy loss: A large amount of energy is used up by organisms at each trophic level for their own life processes such as respiration, movement, and maintaining body temperature. This energy is lost as heat and is not available to the next trophic level. [1 mark]

OR

(c) Energy available at each trophic level:

Grass (T1) = 1,00,000 J
Grasshopper (T2) = 10% of 1,00,000 = 10,000 J
Frog (T3) = 10% of 10,000 = 1,000 J
Snake (T4) = 10% of 1,000 = 100 J

∴ Energy available to the snake = 100 J [1 mark]

Food chains with fewer trophic levels are more energy-efficient because energy is lost at every trophic level (approximately 90% is lost as heat through respiration and other metabolic activities). The shorter the food chain, the less total energy is lost, and more energy is available to the organisms at higher levels. Therefore, humans obtain more energy by eating plants directly (shorter food chain) than by eating animals that have fed on plants (longer food chain). [1 mark]

Question 9

A municipality collects mixed waste from households containing vegetable peels, polythene bags, old newspapers, and rubber tyres. Which combination consists entirely of non-biodegradable waste?

Accepted Answer

Option (A) is correct.

Explanation: Biodegradable waste is broken down by microorganisms through natural decomposition; non-biodegradable waste cannot be decomposed by biological processes and persists in the environment. Vegetable peels and old newspapers are organic and cellulose-based materials respectively, so both are biodegradable. Polythene bags are made of synthetic polymer (polyethylene) and rubber tyres are made of vulcanised synthetic rubber — neither can be broken down by microorganisms. Therefore, the combination consisting entirely of non-biodegradable waste is polythene bags and rubber tyres.

Question 10

In a pond ecosystem, the following food web is observed: Phytoplankton is eaten by Water flea and Copepod. Water flea is eaten by Small fish and Tadpole. Copepod is eaten by Small fish. Small fish is eaten by Kingfisher. Tadpole is eaten by Heron. Which organism acts as a PRODUCER and is shared by the MAXIMUM number of food chains in this food web?

Accepted Answer

Option (A) is correct.

Explanation: In a food web, producers are autotrophic organisms that occupy the first trophic level. Phytoplankton is the only producer in this pond ecosystem, as it is the sole organism that is not consumed by anything else before it. Tracing all possible food chains from the food web yields three distinct chains: Phytoplankton → Water flea → Small fish → Kingfisher; Phytoplankton → Water flea → Tadpole → Heron; and Phytoplankton → Copepod → Small fish → Kingfisher. Phytoplankton appears as the starting organism in all three chains, making it shared by the maximum number (3) of food chains, while Water flea and Small fish each appear in only 2 chains and Copepod in only 1.

Question 11

A municipal park generates the following waste materials during routine maintenance: dried grass clippings, discarded plastic flower pots, fallen tree branches, and fruit peels from the canteen. Which of the following groups contains ONLY non-biodegradable waste?

Accepted Answer

Option (D) is correct.

Explanation: Non-biodegradable waste consists of materials that cannot be decomposed by microorganisms. Dried grass clippings, fallen tree branches, and fruit peels are all organic substances that decompose readily through microbial action and are therefore biodegradable. Discarded plastic flower pots and used synthetic rope are both made of synthetic polymers, which microorganisms cannot break down. Options (A) and (C) contain fruit peels, which are biodegradable; Option (B) includes fallen tree branches, which are also biodegradable. Only Option (D) contains exclusively non-biodegradable materials.

Question 12

Assertion (A) : The number of organisms decreases as we move to higher trophic levels in a food chain.
Reason (R) : Only about 10% of the energy is transferred from one trophic level to the next.

Accepted Answer

Option (A) is correct. The Assertion is true: as we move up the trophic levels, the number of organisms decreases because less energy is available to support a large population at each successive level. The Reason is also true: according to the 10% law, only about 10% of the energy received at one trophic level is transferred to the next, and the remaining 90% is used up in respiration and other life processes. R is the correct explanation of A because it is precisely this large loss of energy at each step that limits the number of organisms that can be sustained at higher trophic levels.

Question 13

A non-biodegradable insecticide (chlordane) is introduced into a river ecosystem through industrial effluents. It enters the following food chain: Water Hyacinth → Mayfly Larva → Catfish → Otter → Bald Eagle. If the concentration of chlordane in Water Hyacinth is 0.003 ppm, and biological magnification occurs at each trophic level, at which trophic level will the concentration of chlordane be the HIGHEST?

Accepted Answer

Option (D) is correct.

Explanation: Biological magnification is the phenomenon by which the concentration of a non-biodegradable substance progressively increases at each successive trophic level in a food chain. Since chlordane cannot be broken down by biological processes, it accumulates in the fatty tissues of organisms and is not excreted. Each organism at a higher trophic level consumes a large number of organisms from the level below, thereby accumulating a proportionally greater concentration of chlordane in its body. The Bald Eagle occupies the 5th and highest trophic level in the food chain Water Hyacinth → Mayfly Larva → Catfish → Otter → Bald Eagle, and therefore accumulates the highest concentration of chlordane through successive magnification across all four transfers.

Question 14

Which of the following correctly represents a food chain found in a pond ecosystem?

(i) Phytoplankton → Zooplankton → Small fish → Large fish
(ii) Large fish → Small fish → Zooplankton → Phytoplankton
(iii) Zooplankton → Phytoplankton → Small fish → Large fish
(iv) Small fish → Large fish → Zooplankton → Phytoplankton

Accepted Answer

Option (i) is correct.

Explanation: In any food chain, energy flows from producers to consumers. Phytoplankton are the producers (autotrophs) that fix solar energy through photosynthesis, so they must always be at the first trophic level. Zooplankton are primary consumers that feed on phytoplankton, small fish are secondary consumers, and large fish are tertiary consumers. Options (ii), (iii), and (iv) place consumers before producers or reverse the direction of energy flow, which violates the fundamental rule that a food chain always begins with a producer.

Question 15

In a mountain forest ecosystem, a non-biodegradable pesticide (lindane) enters the following food chain through agricultural run-off: Pine Tree → Aphid → Blue Tit → Sparrowhawk → Golden Eagle. Which organism in this food chain will have the HIGHEST concentration of lindane accumulated in its body tissues?

Accepted Answer

Option (A) is correct.

Explanation: Non-biodegradable pesticides such as lindane are not broken down or excreted by organisms; instead, they accumulate in fatty tissues. At each successive trophic level, a consumer must eat a large number of organisms from the level below, so the pesticide concentration multiplies with each transfer — a process called biological magnification (biomagnification). In the given food chain, Pine Tree → Aphid → Blue Tit → Sparrowhawk → Golden Eagle, the Golden Eagle occupies the fifth and highest trophic level. Having accumulated lindane from every level below it, the Golden Eagle will have the highest concentration of lindane in its body tissues.

Question 16

Which of the following correctly represents the flow of energy in an ecosystem?

Accepted Answer

Option (B) is correct.

Explanation: In an ecosystem, energy flows in one direction — from Producers (plants) to Consumers (herbivores, then carnivores) and finally to Decomposers (bacteria and fungi), which break down dead organic matter. This is the universally accepted direction of energy flow and forms the basis of a food chain.

Question 17

Name the process by which green plants produce their own food using sunlight. At which trophic level do they occupy in a food chain?

Accepted Answer

(a) Green plants produce their own food by the process of Photosynthesis.
(b) They occupy the First trophic level (Producers) in a food chain.

Question 18

In a pond ecosystem, the following food chain exists: Phytoplankton → Zooplankton → Small Fish → Large Fish. If 0.5 J of energy is available to the Large Fish at the 4th trophic level, how much energy (in Joules) was available to the Phytoplankton at the producer level?

Accepted Answer

Option (C) is correct.

Explanation: According to the 10% law of energy transfer (Lindemann's law), only 10% of the energy available at one trophic level is transferred to the next. Let the energy at the producer level (Phytoplankton) = E. Applying the rule successively: energy at Zooplankton (2nd trophic level) = 0.1E; energy at Small Fish (3rd trophic level) = 0.01E; energy at Large Fish (4th trophic level) = 0.001E. Since energy available to Large Fish = 0.5 J, we get 0.001E = 0.5, ∴ E = 0.5 ÷ 0.001 = 500 J.

Question 19

Which of the following correctly represents a food chain?

Accepted Answer

Option (B) is correct.
Explanation: In a food chain, energy flows from producers to consumers. Grass is the producer (autotroph) that captures solar energy. Grasshopper (primary consumer) feeds on grass, frog (secondary consumer) feeds on grasshopper, snake (tertiary consumer) feeds on frog, and eagle (apex consumer) feeds on snake. Each arrow represents the direction of energy flow from prey to predator. All other options incorrectly place a consumer before the producer.

Question 20

In a savanna ecosystem, the following food chain exists: Shrubs → Zebra → Cheetah → Lion. If 50 J of energy is available to the Cheetah (3rd trophic level), how much energy (in Joules) was available to the Shrubs (producer level)?

Accepted Answer

Option (B) is correct.

Explanation: According to the 10% law of energy transfer, only 10% of the energy available at one trophic level is transferred to the next higher trophic level. In the given food chain, Shrubs form the 1st trophic level, Zebra the 2nd, and Cheetah the 3rd. Since the Cheetah (3rd trophic level) has 50 J, the energy available at the Zebra level (2nd trophic level) = 50 ÷ 0.10 = 500 J, and the energy available at the Shrubs level (1st trophic level) = 500 ÷ 0.10 = 5000 J.

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