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How Nature Works in Harmony: questions with answers

Class 8 Science, Chapter 12 (NCERT). 15 questions by concept, 1 with pictures. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Habitat and its biotic and abiotic components

Question 1

Insects, fungi and small creatures live in and on a rotting log. Is the log a habitat?

  1. (a)No, a habitat has to be as large as a pond or a forest.
  2. (b)No, because a log is dead.
  3. (c)Yes, since a habitat is wherever an organism lives, however small it is.
  4. (d)Yes, and every creature in it meets exactly the same conditions.
Show answer

(c) Yes, since a habitat is wherever an organism lives, however small it is.

A habitat is where an organism lives and finds suitable conditions. A rotting log can provide such conditions for insects and fungi. A habitat need not be as large as a pond or forest.

How organisms meet their biotic and abiotic needs

Question 2

A fish obtains gases through its gills while underwater. Does it need oxygen?

  1. (a)Yes. Its gills take up oxygen dissolved in the water.
  2. (b)No, because they live in water rather than in air.
  3. (c)Yes, but it obtains oxygen only by breaking water molecules apart.
  4. (d)Only when the water is warm.
Show answer

(a) Yes. Its gills take up oxygen dissolved in the water.

Fish need oxygen for respiration. Gills take up dissolved oxygen; they do not split water molecules. Dissolved oxygen can come from exchange with air and photosynthesis in the water.

Interactions between biotic and abiotic components

Question 3

A green plant is kept in complete darkness for several weeks, with adequate soil and water. Once its stored food runs low, what is likely to happen?

  1. (a)It grows well, if it is watered more often to make up for the dark.
  2. (b)It grows well, since soil and water are all a plant needs.
  3. (c)It weakens because it cannot replace its food through photosynthesis.
  4. (d)It weakens because darkness stops its roots absorbing any water.
Show answer

(c) It weakens because it cannot replace its food through photosynthesis.

A green plant needs light for photosynthesis. It can use stored food for a time, but adequate soil and water cannot replace the light needed to make more food.

Community

Question 4

A description covers the plants, animals and microorganisms of a valley, its river, soil and climate, and their interactions. Which level does it describe?

  1. (a)A population, because all the different species in one valley form one population.
  2. (b)An individual.
  3. (c)An ecosystem.
  4. (d)A community.
Show answer

(c) An ecosystem.

A community covers the living things only. Once the non-living surroundings are included as well, and the interactions between the two, what is being described is an ecosystem.

Competition keeps populations in balance

Question 5

Two rabbits in the same field need the same plants for food, and there is not enough for both. Can they compete with each other?

  1. (a)Only if the field is small.
  2. (b)Yes, but only because the two rabbits must belong to different species.
  3. (c)Yes, because individuals of the same species can compete for limited food.
  4. (d)No, because organisms compete only with those of other kinds.
Show answer

(c) Yes, because individuals of the same species can compete for limited food.

Competition occurs when organisms need a shared resource that is limited. It can occur within a species as well as between species. The two rabbits compete for the limited plant food.

Ecosystems and their types

Question 6

A large natural forest and a small farmed field are compared. Which of the two is an ecosystem?

  1. (a)The field only, since its boundaries are clear.
  2. (b)Both of them.
  3. (c)The forest only, since an ecosystem must be large.
  4. (d)The forest only, since a farm is made by people.
Show answer

(b) Both of them.

An ecosystem includes organisms interacting with each other and with non-living surroundings. Both a forest and a farmed field have these interactions. Neither small size nor human management excludes a place from being an ecosystem.

Population

Question 7

After a dry summer, food in a field becomes scarce. If other influences stay similar, what is likely to happen to its rabbit population?

  1. (a)It stays as it was, because a population is a fixed number.
  2. (b)It rises, because scarcity makes animals breed faster.
  3. (c)It is likely to decrease because the field can support fewer rabbits.
  4. (d)It stays as it was, but other kinds of animal join the same population.
Show answer

(c) It is likely to decrease because the field can support fewer rabbits.

A population can change through births, deaths, immigration and emigration. Food scarcity can lower survival or reproduction and cause animals to leave. It therefore tends to reduce the population, rather than guaranteeing an exact count.

The nested levels from individual to ecosystem

Question 8

Within one ecosystem, order these levels from an individual organism to the most inclusive level: community, ecosystem, individual, population.

same centre
  1. (a)Individual, community, population, ecosystem.
  2. (b)Individual, population, ecosystem, community.
  3. (c)Individual, population, community, ecosystem.
  4. (d)Population, individual, community, ecosystem.
Show answer

(c) Individual, population, community, ecosystem.

One organism is an individual. All the individuals of its kind in an area make a population. All the populations of the area together make a community. The community with its non-living surroundings makes an ecosystem.

Mutualism, commensalism and parasitism

Question 9

One organism lives on another and benefits from doing so. The host is neither helped nor harmed. What is this relationship called?

  1. (a)Commensalism.
  2. (b)Mutualism.
  3. (c)Competition.
  4. (d)Parasitism, since one of them is living on the other.
Show answer

(a) Commensalism.

The three relationships are told apart by what happens to each partner. Both benefit in mutualism; one benefits and the other is unaffected in commensalism; one benefits and the other is harmed in parasitism.

Food chains and trophic levels

Question 10

A student wants to show a grazing food chain from its producer but writes only mouse → snake → eagle. The mouse eats grain. Which correction is needed?

  1. (a)Make no correction; the producer can be omitted even when showing the chain from its producer.
  2. (b)Put the grain-producing plant after the eagle.
  3. (c)Add the grain-producing plant before the mouse.
  4. (d)Reverse every arrow so each points from the eater to its food.
Show answer

(c) Add the grain-producing plant before the mouse.

The sequence shows part of a feeding chain. To show this grazing chain from its producer, include the plant supplying grain to the mouse. Arrows point from food to its consumer.

Producers, consumers and the feeding categories

Question 11

A mushroom grows on a rotting log, taking what it needs from the dead wood. Is it a producer?

  1. (a)No. This fungus obtains food by decomposing the dead wood.
  2. (b)No, it is a herbivore.
  3. (c)Yes, because it produces spores and so makes more of its own kind.
  4. (d)Yes, because it grows from the ground without moving, as a plant does.
Show answer

(a) No. This fungus obtains food by decomposing the dead wood.

The described fungus obtains food from dead wood rather than making it through photosynthesis. It breaks down that organic matter, so it acts as a decomposer. Fungi have varied feeding relationships; this example concerns a wood decomposer.

Every organism matters: direct and indirect effects

Question 12

In a pond study, fish affect dragonflies, which affect pollinating insects. Researchers measure a large resulting change in seed production by nearby plants. What does this show about indirect effects?

  1. (a)That the fish must be eating the flowers after all.
  2. (b)An indirect effect can be substantial even when the organisms do not interact directly.
  3. (c)That such an effect is always weak, and the flowers must differ for another reason.
  4. (d)That more of one animal always means less of every other.
Show answer

(b) An indirect effect can be substantial even when the organisms do not interact directly.

The effect passes from fish through dragonflies and pollinators to plants. The measured change shows that an indirect effect need not be weak. It does not establish that it equals a particular direct effect.

Food webs

Question 13

Grass feeds hares and deer. Disease reduces the hares, while deer numbers, plant growth conditions and other feeding pressures stay similar. What is the expected short-term effect on the grass?

  1. (a)More grass remains because fewer hares graze it.
  2. (b)There is less of it, since the hares kept the grass healthy.
  3. (c)It is unaffected, since the deer are in a different chain.
  4. (d)It is unaffected, since the change stays within the hares' own chain.
Show answer

(a) More grass remains because fewer hares graze it.

With other influences held similar, fewer hares remove less grass. This predicts more remaining grass in the short term. Longer-term outcomes can change as other populations respond.

How one change leads to another

Question 14

Indian bullfrogs eat crop pests. If harvesting sharply reduces the frogs and other influences stay similar, what is likely to happen to those pests?

  1. (a)Their numbers may rise because fewer frogs eat them.
  2. (b)Their numbers fall, since removing an animal reduces the numbers around it.
  3. (c)They move to a different area at once.
  4. (d)Their numbers are unchanged, since only natural events affect an ecosystem.
Show answer

(a) Their numbers may rise because fewer frogs eat them.

Removing many frogs reduces one source of predation on crop pests. This can allow pest numbers to rise. Other predators and environmental conditions also affect the outcome.

Decomposers and decomposition

Question 15

A proposed soil ecosystem contains green plants but no decomposers. Can photosynthesis alone recover the nutrients locked in its accumulating dead leaves?

  1. (a)No. Photosynthesis makes food but does not replace decomposition of the dead leaves.
  2. (b)Yes, because sunlight instantly converts dead leaves into soil minerals.
  3. (c)Yes, because producers make food and can therefore run the ecosystem without decomposers.
  4. (d)No, because nutrients can never be reused once plants have absorbed them.
Show answer

(a) No. Photosynthesis makes food but does not replace decomposition of the dead leaves.

Photosynthesis and decomposition have different roles. Decomposers break down dead organic matter and help return nutrients for plant uptake. Photosynthesis alone does not perform that recycling.

Common misconception in How Nature Works in Harmony

Open full misconception guide →

Do aquatic animals need oxygen?

Wrong

Fish have no need for oxygen because they live underwater.

Why it feels right. Oxygen seems like something available only in air.

Right

Fish typically take up dissolved oxygen through gills.

Worked example

Oxygen path
Dissolved oxygen in water→Across gills→Blood
Water passing over gills carries dissolved oxygen, which can enter the fish's blood.
  1. Water flows over gills.
  2. Dissolved oxygen moves into the blood.
  3. A drop in dissolved oxygen can make the water unsuitable for fish.