CBSE Class 10 Science HOTs Our Environment Set 06

This page gives 21 higher order thinking (HOTs) questions with step-by-step answers for CBSE Class 10 Science Chapter 13, Our Environment. The questions ask you to predict what happens when a link of a food chain is removed, to apply the 10% law, to reason about biomagnification of pesticides, waste and plastics, and to explain why an aquarium, a cropland or a short food chain behaves as it does.

Very Short and Short Answer HOTS Questions

Question 1. Consider a food chain consisting of wheat, rat, snakes, peacock. What will happen if all snakes are killed?
Answer:
The food chain is Wheat → Rat → Snake → Peacock.
Snakes are the predators of rats. If all snakes are killed, nothing controls the rat population, so the number of rats increases and they eat much more wheat.
The peacock loses its prey in this chain, so its population decreases unless it finds other food.
So the balance of the whole food chain is disturbed.
In simple words: Removing a middle link makes the lower link multiply and the upper link starve.

Teacher's Note:
1. Always follow the effect both downwards (prey) and upwards (predator) from the removed link.
2. Write the food chain first; it makes the answer easy to follow.

Question 2. During heavy rain in a village the rain water carried excessive fertilizer to a pond. How will it affect the fish production in pond in the long run?
Answer:
The fertilizer contains nitrogen and phosphorus compounds which act as nutrients for algae.
The algae multiply very fast and cover the pond. When they die, decomposers use up the dissolved oxygen of the water while decomposing them.
Shortage of oxygen kills the fish, so the fish production decreases in the long run.
In simple words: Extra fertiliser feeds algae, the algae use up the oxygen, and the fish die.

Teacher's Note:
1. The key chain of ideas is: more nutrients, more algae, less oxygen, dead fish.
2. Fertilisers are non-biodegradable chemicals and reach water bodies with run-off.

Question 3. “Save the Tiger” campaign is being over emphasised these days by our government. What may be possible reason?
Answer:
The tiger is a top carnivore. It controls the number of herbivores such as deer.
If tigers disappear, herbivores increase in number and overgraze the vegetation, which upsets the whole food chain.
Saving the tiger therefore maintains the ecological balance and also protects the forest in which it lives.
In simple words: Saving the tiger saves the balance of the forest.

Teacher's Note:
1. Top carnivores are few in number because only a little energy reaches the top trophic level.
2. Link the answer to the 10% law and to ecological balance.

Question 4. Why is food chain having two trophic levels most advantageous in terms of energy?
Answer:
At every step of a food chain only about 10% of the energy passes to the next trophic level; the rest is lost as heat or used in life processes.
A chain with only two trophic levels has just one transfer, so the least energy is lost.
For example, if the producer has 1000 J, the herbivore in a two-level chain gets 100 J, but a carnivore in a three-level chain would get only 10 J.
In simple words: The shorter the chain, the less energy is wasted.

Teacher's Note:
1. Use a small numerical example to support the 10% law.
2. This is why vegetarian food habits give more usable energy.

Question 5. Why have been kulhads banned for serving tea on platforms ? What types of cups are used in trains and platforms ?
Answer:
Kulhads are made from the top fertile layer of soil. Making them on a very large scale would cause loss of this fertile top soil, which is needed for the growth of plants.
Hence kulhads have been banned for serving tea on platforms.
Nowadays disposable paper cups are used in trains and on platforms, as they are biodegradable and can be recycled.
In simple words: Clay cups eat up fertile soil, so paper cups are used instead.

Teacher's Note:
1. Both plastic cups (non-biodegradable) and kulhads (soil loss) have a drawback.
2. Paper cups are the better option because they are biodegradable.

Question 6. An artificial ecosystem having all the producer and no consumer was formed. What will happen to this ecosystem.
Answer:
Producers will make food by photosynthesis, but there are no consumers to eat them and pass energy on.
The producers will grow unchecked and compete for light, water and space, and the dead plant matter will pile up if there are no decomposers.
The flow of energy through trophic levels will not take place, so the ecosystem will be unbalanced and not self-sustaining.
In simple words: An ecosystem needs consumers and decomposers as well as producers to stay balanced.

Teacher's Note:
1. Mention at least one effect on energy flow and one on balance.
2. Check the final answer against the chapter.

Question 7. Why are milk packet as a kitchen waste are more harmful than spoilt residual food.
Answer:
Milk packets are made of plastic, which is non-biodegradable. Microbes cannot break it down, so it stays in the environment for a very long time, chokes drains and harms animals that eat it.
Spoilt residual food is biodegradable. Bacteria and fungi decompose it into harmless substances and it can be turned into compost.
Hence milk packets are more harmful.
In simple words: Plastic packets never rot, but spoilt food does.

Teacher's Note:
1. Always name an example of each kind of waste.
2. Check the final answer against the chapter.

Short Answer HOTS Questions

Question 8. Your uncle has come from the village to renew the contract to supply frogs to the laboratories of the colleges of the town. While talking to you, he mentioned that cases of malaria have increased in his village. In addition population of grasshoppers has also increased that are damaging crops.
(i) What could be the reasons for such problems faced by villagers?
(ii) What suggestions will you give to your uncle?
Answer:
(i) Frogs feed on mosquitoes and grasshoppers. Because frogs are being caught and supplied to the laboratories, their number has fallen. With fewer frogs, mosquitoes and grasshoppers increase. Mosquitoes spread malaria and grasshoppers damage the crops.
(ii) He should stop supplying frogs from the village, because the reduced frog population has upset the food chain and the balance between frogs, grasshoppers and mosquitoes must be restored.
In simple words: Fewer frogs means more mosquitoes and grasshoppers.

Teacher's Note:
1. Frogs are secondary consumers that control insect populations.
2. Name the food chain: grass, grasshopper, frog.

Question 9. A huge water body was being used for fishing, but after the set of industries near this water body when people consume fishes they started falling ill.
(i) What might be the cause of their illness after setting up of industries?
(ii) What steps must be taken by authorities to overcome this problem?
(iii) Name the biological phenomenon involved in this case.
Answer:
(i) The effluents of the industries contain non-biodegradable toxic wastes. They mix with the water, get accumulated in the body of the fishes, and make the people ill when they eat these fishes.
(ii) The effluents should be properly treated before they are let into the water body.
(iii) The phenomenon is biomagnification (biological magnification).
In simple words: Toxic chemicals pile up in fish, and people who eat the fish get the highest dose.

Teacher's Note:
1. Humans are at the top of the food chain, so they receive the highest concentration.
2. Treatment of effluents is the correct preventive step.

Question 10. A team of Indian researchers went to Antarctica to study the ozone layer. They confirmed the presence of largest ozone hole over Antarctica and was just short of 27 million sq. km. After few days of their return, one of the scientists developed rashes, burning sensation and other skin problems which the doctors have confirmed as skin cancer.
(i) What may be the cause of cancer just after return from Antarctica?
(ii) What do we learn from this incident?
Answer:
(i) Over Antarctica the ozone layer has a big hole. The scientist was exposed to harmful ultraviolet (UV) radiation of the Sun, which passed through the hole and can cause skin cancer.
(ii) We learn that the ozone layer is essential for life on Earth because it absorbs the harmful UV radiation. Chemicals such as CFCs used in refrigerators, air conditioners and aerosols damage it, so their use must be controlled.
In simple words: Less ozone lets more UV reach us, and UV can cause skin cancer.

Teacher's Note:
1. Name the radiation (UV) and the harmful effect (skin cancer).
2. The lesson is about protecting the ozone layer by controlling CFCs.

Question 11. Why does vegetarian habit help us in getting more energy? In terms of energy who is at an advantageous position (vegetarian or a non-vegetarian) and Why?
Answer:
Vegetarians get food directly from plants (producers), while non-vegetarians get food from animals that feed on plants.
By the 10% law, if plants have 100 J, a herbivore gets only 10 J from them. A non-vegetarian who eats that animal gets only 1 J, but a vegetarian who eats the plants directly gets the full 100 J.
So the vegetarian is at an advantageous position in terms of energy.
In simple words: The fewer steps between the plant and you, the more energy you receive.

Teacher's Note:
1. Use the same starting energy for both diets so that the comparison is clear.
2. At each step 90% of the energy is lost.

Question 12. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?
Answer:
Yes, the impact is different at different trophic levels. If all the producers are removed, all the herbivores die of starvation and the carnivores that depend on them are also affected.
If all the organisms of a higher trophic level are removed, the number of organisms of the lower trophic level increases greatly and upsets the ecosystem.
No, the organisms of any trophic level cannot be removed without damaging the ecosystem, because every level depends on the others.
In simple words: Every trophic level is needed; removing one harms the rest.

Teacher's Note:
1. Give one example for removing producers and one for removing a higher level.
2. This is an NCERT question; the final sentence must be a clear No.

Question 13. (a) Why do we generate a lot of waste now-a-days? (b) Name two factors that have contributed to the deterioration of our environment.
Answer:
(a) We generate a lot of waste nowadays because of the use of more and more disposable substances, a change in our attitude, and improvements in our lifestyle due to advanced technology, along with changes in packaging that make much waste non-biodegradable.
(b) Two factors are the growth of the human population and the advancement of technology.
In simple words: More people, more technology and more disposable goods mean more waste.

Teacher's Note:
1. Connect lifestyle to the kind of waste produced.
2. Mention population growth and technology for part (b).

Long Answer and Case Based HOTS Questions

Question 14. Shown below are two food pyramids. In the land ecosystem the pyramid is traditional, with producers being greater in mass than primary consumers and so on. Sometimes, in aquatic ecosystems, an inverted pyramid exists. Here, the total mass of producers (phytoplankton) is much smaller than the top consumers (big fishes). The aquatic food chain is: Phytoplankton → Zooplankton → Small fishes → Big fishes.
(A) Which level is likely to have the most amount of energy in such an aquatic ecosystem? Give a reason to support your answer.
(B) Such aquatic ecosystems are considered to be unstable. Justify this statement.
Answer:
(A) The producers (phytoplankton) have the most energy. They capture the energy of sunlight, and the energy available decreases at every higher trophic level as only about 10% is passed on. The pyramid of energy is therefore always upright, even when the pyramid of mass is inverted.
(B) The mass of producers is small, so the food available to the higher trophic levels is small. The consumers then run short of food and die sooner than usual, so the system is unstable.
In simple words: The base always has the most energy, but a thin base cannot support a heavy top for long.

Teacher's Note:
1. Do not confuse the pyramid of biomass with the pyramid of energy.
2. Energy pyramids are always upright.

Question 15. (A) In a class of 40 students, a survey was conducted to find out the eating habits of the students. It was found that 27 students are vegetarians and 13 are non-vegetarians. In terms of energy, which students are at an advantageous position (vegetarian or non-vegetarian)?
(B) Categorise the following organisms into producers, herbivores, carnivores and decomposers. Create a three step food chain from the given organisms. Tree, Vulture, Grasshopper, Bacteria, Rabbit, Lion, Eagle, Deer, Frog, Grass, Fungi.
(C) What is the significance of a food chain?
Answer:
(A) The 27 vegetarian students are at an advantage. They eat plants directly, so if the producers hold 1000 J they receive 100 J, whereas non-vegetarians who eat herbivores receive only 10 J.
(B) Producers: tree, grass. Herbivores: grasshopper, rabbit, deer. Carnivores: vulture, lion, eagle, frog. Decomposers: bacteria, fungi.
A three step food chain: Grass → Grasshopper → Frog (or Grass → Rabbit → Lion).
(C) A food chain helps us understand the feeding relations between organisms, the flow of energy (it is unidirectional) and how toxic substances move through an ecosystem (biological magnification).
In simple words: Plant eaters get more energy; a food chain shows who eats whom and how energy moves.

Teacher's Note:
1. A three step chain has a producer, a herbivore and a carnivore.
2. Place decomposers outside the chain; they act at every level.

Question 16. Study the following food chain: Grass → Sheep/Goat → Human; Algae → Fish → Human.
(A) Who is the ultimate sufferer of using pesticides like DDT on land and in water? Give reason for your answer.
(B) If 1000 J of energy is available at the producer level in each of the above food chain, what amount of energy will be available to human?
(C) In view of the given food chain, comment whether shifting human’s position from trophic level 3 to trophic level 2 will cause any change in the amount of pesticide and energy that he receives in the food chain.
Answer:
(A) Human beings suffer most. Pesticides such as DDT are non-biodegradable and accumulate at every trophic level, so the top organism of both chains, the human, has the maximum concentration.
(B) By the 10% law: 1000 J → 100 J (sheep or fish) → 10 J (human).
(C) Yes. If the human moves to trophic level 2 (eats the grass or algae directly), he receives less pesticide and more energy (100 J instead of 10 J).
In simple words: Eating lower in the food chain gives more energy and fewer pesticides.

Teacher's Note:
1. Apply 10% twice for a three level chain.
2. The phenomenon in part (A) is biomagnification.

Question 17. What would happen if all the decomposers were eliminated from the earth? Explain.
Answer:
The dead bodies of plants and animals, excreta and litter would pile up, leaving no space for new generations of organisms.
The nutrients locked in dead matter would not return to the soil, so the cycling of nutrients would stop. Plants could not get minerals, no new organic matter would be made, and all life would finally be destroyed.
So decomposers are essential for recycling of matter and for cleaning the environment.
In simple words: Without decomposers dead matter piles up and nutrients are never reused.

Teacher's Note:
1. Mention both the pile-up of wastes and the stopping of nutrient recycling.
2. Decomposers are bacteria and fungi.

Question 18. (i) How do food chains get shortened? How does the shortening of food chain affect the biosphere?
(ii) How will you justify that vegetarian food habits give us more calories?
Answer:
(i) Undesirable activities of man, such as hunting tigers for their skins, eliminate the organisms of one or more trophic levels, so the food chain gets shortened.
It causes an imbalance in the functioning of the ecosystem. If one trophic level is removed, the organisms below it increase in number and those above it decrease sharply.
(ii) A vegetarian food chain is shorter. Only 10% of energy passes to each next level, so a person who eats plants directly gets the maximum amount of energy.
In simple words: Removing a level breaks the balance; eating plants gives the most energy.

Teacher's Note:
1. Give one human activity as an example in part (i).
2. Link part (ii) to the 10% law.

Question 19. Your mother always thought that fruit juices are very healthy for everyone. One day, she read in the newspaper that some brands of fruit juices in the market have been found to contain certain level of pesticides in them. She got worried as pesticides are injurious to health.
(a) How would you explain to your mother about fruit juices getting contaminated with pesticides?
(b) It is said that the harmful pesticides enter our body as well as the bodies of other organisms, they get accumulated and beyond a limit causes harm and damage to our organs. Name the phenomenon and write about it.
Answer:
(a) Farmers spray pesticides on crops and fruit trees. The pesticides are absorbed by the plants or stay on the fruits and enter the juice, and some reach the fruit through the soil and water.
(b) The phenomenon is biological magnification (biomagnification). Pesticides are non-biodegradable, so they are not broken down in the body and their concentration increases at every trophic level of the food chain. Human beings are at the top, so they receive the largest amount, which damages organs.
In simple words: Pesticides stay in the food we eat and build up in our body.

Teacher's Note:
1. Explain in simple language, as if talking to your mother.
2. Name the phenomenon in the answer.

Question 20. (A) Construct a terrestrial food chain comprising four trophic levels. (B) What will happen if we kill all the organisms in one trophic level? (C) Calculate the amount of energy available to the organisms at the fourth trophic level if the energy available to the organisms at the second trophic level is 2000 J.
Answer:
(A) Grass → Grasshopper → Frog → Snake.
(B) The transfer of energy to the next level stops. The organisms of the lower trophic level increase and those of the higher level starve, so the ecosystem is disturbed.
(C) Second trophic level (grasshopper) = 2000 J. Third level (frog) = 10% of 2000 = 200 J. Fourth level (snake) = 10% of 200 = 20 J.
In simple words: Each step passes on one tenth of the energy: 2000 J, 200 J, 20 J.

Teacher's Note:
1. Count the steps carefully: from the second to the fourth level is two transfers.
2. Some books give 2 J for this question, which is a miscount; the correct value is 20 J.

Question 21. A modern insecticide has been introduced with certain new properties like, accumulation in the bodies of predators, broken down by soil bacteria, easily washed into lakes and rivers and taken up by plant roots. Among all these properties, which one will help in reducing or keeping the level of environment pollution to lowest?
Answer:
The helpful property is that it is broken down by soil bacteria, which means the insecticide is biodegradable.
It will therefore be changed into harmless substances and will not persist in the soil or water.
The other properties (accumulation in predators, washing into water bodies, uptake by roots) all spread the chemical through the food chain and increase pollution.
In simple words: A biodegradable insecticide disappears instead of building up.

Teacher's Note:
1. Non-biodegradable chemicals are the ones that undergo biomagnification.
2. Pick the one property that helps and justify why the others do not.