Ecosystems ICSE Class 8: Forest Ecosystem, Food Chains, Food Webs, Pyramid of Numbers and Interdependence

What Ecosystems is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Biology: your school sets and marks the exam. Ecosystems sits in the Theme: Ecosystems theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

The forest ecosystem: biotic and abiotic

Biotic components: all living things in the ecosystem.

• Flora (plants) of an Indian forest: tall trees such as sal, teak, bamboo and banyan; shrubs, climbers, ferns, mosses, grasses.
• Fauna (animals): insects, earthworms, frogs, snakes, birds (parrots, owls, eagles), squirrels, monkeys, deer, wild boar, elephants, bears, leopards, tigers.

Abiotic components: the non-living physical and chemical factors: air, water, soil, minerals, sunlight, temperature, humidity and wind.

A forest has layers: the canopy of tall trees at the top, a lower layer of smaller trees, then shrubs, then herbs and grasses, and the forest floor with fallen leaves, fungi and soil animals. Less light reaches each lower layer.

Interaction works both ways:

• Abiotic → biotic: sunlight lets plants photosynthesise; rainfall decides whether a forest grows at all; temperature controls when trees flower.
• Biotic → abiotic: the tree canopy shades the floor and keeps it cool and humid; fallen leaves rot and enrich the soil; roots hold the soil and prevent erosion; plants add oxygen to the air and water vapour by transpiration.

Worked example

Classify as biotic or abiotic: humidity, fungi, soil minerals, deer, sunlight, bamboo.

Try it first, then show the solutionSolution

1. Living things are biotic: fungi, deer, bamboo.
2. Non-living factors are abiotic: humidity, soil minerals, sunlight.

Answer: Biotic: fungi, deer, bamboo. Abiotic: humidity, soil minerals, sunlight.

Producers, consumers and decomposers

Producers make their own food from simple substances by photosynthesis, using sunlight. All green plants are producers: trees, shrubs, grasses. They are the only way energy from the Sun enters the ecosystem.

Consumers cannot make food and depend directly or indirectly on producers.

• Primary consumers (herbivores) eat plants: deer, rabbit, elephant, grasshopper, caterpillar.
• Secondary consumers (carnivores) eat herbivores: frog, fox, small birds that eat insects.
• Tertiary consumers eat secondary consumers: a snake that eats frogs, an eagle that eats snakes. Animals at the top that are not eaten by others, such as the tiger and the eagle, are top carnivores; the tiger is a secondary consumer when it eats deer.
• Omnivores eat both plants and animals: bear, monkey, wild boar.

Decomposers are bacteria and fungi that feed on dead plants and animals and their wastes, breaking them down into simple substances. These minerals return to the soil and are used again by plants. Without decomposers, dead matter would pile up and minerals would be locked away.

Worked example

In a forest, what would happen in a few years if all the bacteria and fungi died?

Try it first, then show the solutionSolution

1. Dead leaves, wood and animal bodies would not rot and would pile up on the forest floor.
2. Minerals would not return to the soil, so plants would lack nutrients, and all food chains would suffer.

Answer: Dead matter would pile up and the soil would run out of minerals.

Food chain and food web

A food chain is a sequence of organisms in which each one eats the one before it, showing how food and energy pass from one organism to the next. The arrow points from the eaten to the eater.

Forest food chains:

• Grass → deer → tiger
• Leaves → caterpillar → small bird → snake → eagle
• Fruits → squirrel → snake → eagle

Each step is a trophic level: producers (level 1), herbivores (level 2), carnivores (level 3 and above).

Energy is lost at each step (used for movement, body heat and life processes), so a chain rarely has more than four or five links. As a rough rule, only about one-tenth of the energy at one level is passed on to the next.

A food web is a network of interconnected food chains. In a real forest, most animals eat more than one kind of food and are eaten by more than one kind of animal: a snake eats frogs, rats and birds; a hawk eats snakes and rats. Food webs make an ecosystem stable: if one food is scarce, animals can eat another.

Worked example

In the chain leaves → caterpillar → small bird → snake, the leaves hold 10 000 J of energy. If about one-tenth passes on at each step, how much reaches the snake?

Try it first, then show the solutionSolution

1. Leaves → caterpillar: 10 000 ÷ 10 = 1000 J; → small bird: 100 J.
2. → snake: 100 ÷ 10 = 10 J.

Answer: About 10 J

Pyramid of numbers

A pyramid of numbers shows the number of organisms at each trophic level, drawn as stacked bars with producers at the base.

Upright pyramid (the usual shape): many producers, fewer herbivores, still fewer carnivores, and very few top carnivores.

• Example: lots of grass plants → fewer grasshoppers → fewer frogs → fewer snakes → one or two eagles.
• It is upright because energy decreases at each level, so each level can support fewer individuals.

Forest (spindle-shaped) pyramid: in a forest, a few large trees feed a very large number of herbivores (insects, birds, squirrels), which feed fewer carnivores. So the base is narrower than the second level and the pyramid looks like a spindle or diamond.

Inverted pyramid (parasite chain): one tree → many birds → very many lice on the birds. The numbers increase upwards.

Worked example

Why is the pyramid of numbers in a forest not a perfect upright pyramid?

Try it first, then show the solutionSolution

1. The producers are a few very large trees, each feeding thousands of insects and many birds.
2. So the herbivore bar is wider than the producer bar, giving a spindle shape.

Answer: A few large trees support many herbivores, so the base is narrow (spindle shape).

Interdependence: symbiosis, parasitism, predation

No organism lives alone; organisms depend on one another.

Symbiosis (mutualism): a close association of two different organisms in which both benefit.

• Lichen: an alga and a fungus living together on tree bark and rocks. The alga makes food by photosynthesis; the fungus gives shelter, water and minerals.
• Root nodules: Rhizobium bacteria live in the roots of leguminous plants (pea, gram); the bacteria fix nitrogen for the plant and get food and shelter.
• Bees and flowers: bees get nectar, flowers get pollinated.

Parasitism: one organism, the parasite, lives on or in another, the host, and takes food from it; the parasite benefits and the host is harmed.

• Cuscuta (dodder, amarbel), a yellow, leafless climber, sends suckers into a host plant to absorb food.
• Ticks and leeches on forest animals; tapeworms and roundworms inside animals.

Predation: one animal, the predator, kills and eats another, the prey.

• Tiger and deer; owl and mouse; frog and insect; snake and frog.
• Predators keep prey numbers in check. If tigers are removed, deer multiply, overgraze and damage the forest.

	Symbiosis	Parasitism	Predation
Effect on first	benefits	parasite benefits	predator benefits
Effect on second	benefits	host harmed (usually not killed at once)	prey killed

Worked example

Classify each relationship: (i) Cuscuta on a tree (ii) lichen (iii) leopard and monkey.

Try it first, then show the solutionSolution

1. (i) Cuscuta takes food from the tree, which is harmed: parasitism.
2. (ii) Alga and fungus both benefit: symbiosis. (iii) Leopard kills and eats the monkey: predation.

Answer: (i) Parasitism (ii) Symbiosis (iii) Predation

Abiotic factors in brief

Factor	Role in the forest
Air	supplies oxygen for respiration and carbon dioxide for photosynthesis
Soil	anchors plants, holds water and minerals; home of earthworms, insects and decomposers; forest soil is rich in humus from rotting leaves
Water	needed by all life; heavy rainfall gives dense evergreen forests, less rain gives deciduous forests
Sunlight	source of energy for photosynthesis; controls flowering and the opening of stomata; tall trees compete for light
Temperature	affects the rate of life processes; decides which plants and animals can live there; many animals hibernate or migrate in cold seasons
Humidity	the amount of water vapour in air; high under the canopy, so ferns, mosses and fungi thrive; affects transpiration
Wind	spreads pollen and seeds; increases transpiration; strong winds can uproot trees

These factors decide which organisms can live in a place and how many.

Worked example

Why are mosses and ferns common on the floor of a dense forest but rare in an open field nearby?

Try it first, then show the solutionSolution

1. The canopy keeps the forest floor shady and humid, and the soil moist.
2. Mosses and ferns need moist, shady conditions; the open field is sunny, dry and windy.

Answer: The forest floor is shady and humid, which mosses and ferns need.

Ecosystems in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Biotic and Abiotic Components of a Forest Ecosystem
• Food Chains, Food Webs, Pyramid of Numbers and Interdependence

Ecosystems: key facts

Ecosystem

$\text{ecosystem}=\text{biotic components}+\text{abiotic components}$

Interacting as one unit.

Food chain

$\text{producer}\to \text{herbivore}\to \text{carnivore}\to \text{top carnivore}$

Arrow from the eaten to the eater.

Forest food chain

$\text{grass}\to \text{deer}\to \text{tiger}$

Three trophic levels.

Energy rule of thumb

$\text{energy at next level}\approx \frac{1}{10}\times \text{energy at this level}$

Why chains are short and pyramids narrow upward.

Interdependence

$\text{symbiosis }(+,+);\text{parasitism }(+,-);\text{predation }(+,-)$

+ benefits, − harmed; in predation the prey is killed.

Ecosystems: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Ecosystems

The question patterns to expect, and the method that works for each.

Tricks to remember Ecosystems

Short memory aids. Say each one aloud once, then use it on a question the same day.

• PCD: Plants Cook, Creatures Dine, Decomposers Dispose

  Producers make food, consumers eat it, decomposers recycle what is left.

  Use it when: The three biotic groups.

• The arrow means 'is eaten by'

  Grass → deer reads 'grass is eaten by deer'.

  Use it when: Drawing arrows the right way.

• Ten per cent tax

  Every step up the chain pays a 90% energy tax; only a tenth goes on.

  Use it when: Energy sums and why chains are short.

• Plus-plus, plus-minus, plus-dead

  Symbiosis (+,+), parasitism (+,−), predation (+, prey dead).

  Use it when: Classifying relationships.

• Forest pyramid is a kite

  Few big trees at the bottom, lots of insects in the middle, few tigers at the top: a kite (spindle) shape.

  Use it when: Pyramid of numbers in a forest.

Ecosystems: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 6-mark question split into 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Ecosystems

The slips examiners see most often, and what to write instead.

• ✗Mistake: Drawing food-chain arrows backwards

  The arrow shows the direction of energy flow, from the eaten to the eater.

  Wrong working

  tiger → deer → grass

  Right working

  grass → deer → tiger

  ✓Do this: Read the arrow as 'is eaten by'.

  Usually costs: 1 mark for the whole chain.

• ✗Mistake: Starting a chain with an animal

  Every food chain must begin with a producer (green plant).

  ✓Do this: Check that the first organism makes its own food.

  Usually costs: 1 mark.

• ✗Mistake: Calling all close relationships symbiosis

  In parasitism the host is harmed; in symbiosis both benefit.

  ✓Do this: Ask: does the second partner gain or lose?

  Usually costs: 1 mark.

• ✗Mistake: Using non-forest examples

  The curriculum asks for the forest ecosystem only; pond or desert examples may not be credited.

  ✓Do this: Use sal, teak, deer, tiger, eagle, mushrooms and similar.

  Usually costs: Up to 1 mark per example.

• ✗Mistake: Saying a forest pyramid is always upright

  A few large trees feed many herbivores, so the base is narrower: a spindle shape.

  ✓Do this: Think about the size of the producers.

  Usually costs: 1-2 marks.

Practice questions on Ecosystems

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   In the food chain grass → deer → tiger, the deer is a:
   (a) producer
   (b) primary consumer
   (c) secondary consumer
   (d) decomposer

   Hint

   It eats plants.

   Answer

   (b)

2. Q2 · 1 mark · Very short answerExam level

   Name the relationship between an alga and a fungus in a lichen.

   Hint

   Both benefit.

   Answer

   Symbiosis (mutualism).

3. Q3 · 2 marks · Short answerExam level

   Give two examples each of forest flora and forest fauna.

   Hint

   Plants; animals.

   Answer

   Flora: sal, teak (or bamboo, ferns). Fauna: deer, tiger (or monkey, eagle, elephant).

4. Q4 · 2 marks · Short answerExam level

   What are omnivores? Give two forest examples.

   Hint

   Eat both.

   Answer

   Animals that eat both plants and animals; bear and monkey (or wild boar).

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Food webs make an ecosystem more stable than a single food chain.
   Reason (R): In a food web, animals have more than one source of food.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Alternatives.

   Answer

   (c)

6. Q6 · 3 marks · Short answerHarder exam level

   Draw a forest food web with at least four food chains, using deer, rabbit, grass, tree fruits, squirrel, fox, snake, tiger and eagle.

   Hint

   Arrows to the eater.

   Answer

   Example chains: grass → deer → tiger; grass → rabbit → fox; grass → rabbit → eagle; fruits → squirrel → snake → eagle; drawn as one connected web with grass and fruits at the bottom.

7. Q7 · 2 marks · Short answerHarder exam level

   If producers in a forest food chain trap 100 000 J, how much energy reaches an eagle at the fifth level, taking about one-tenth per step?

   Hint

   Four arrows.

   Answer

   Four steps: 100 000 → 10 000 → 1000 → 100 → 10 J.

8. Q8 · 2 marks · Short answerHarder exam level

   What is an inverted pyramid of numbers? Give an example.

   Hint

   Parasites.

   Answer

   A pyramid in which numbers increase towards the top. Example: one tree → many birds → very many lice on the birds.

9. Q9 · 2 marks · Short answerHarder exam level

   State the role of (i) humidity (ii) wind in a forest ecosystem.

   Hint

   Moisture-lovers; dispersal.

   Answer

   (i) High humidity under the canopy allows ferns, mosses and fungi to grow and reduces transpiration. (ii) Wind spreads pollen and seeds and increases transpiration; strong winds may uproot trees.

10. Q10 · 3 marks · Short answerHarder exam level

    Explain parasitism with Cuscuta as an example. Why is Cuscuta yellow?

    Hint

    No need for chlorophyll.

    Answer

    Cuscuta (the parasite) twines around a host plant and pushes suckers into it, absorbing ready-made food; the host is weakened. It has no (or very little) chlorophyll because it does not make its own food, so it looks yellow.

11. Q11 · 2 marks · Short answerHarder exam level

    How do biotic components change the abiotic environment of a forest? Give two ways.

    Hint

    Shade, humus, roots.

    Answer

    Any two: the canopy shades the floor and keeps it cool and humid; fallen leaves decompose into humus that enriches the soil; roots hold soil and prevent erosion; plants add oxygen and water vapour to the air.

12. Q12 · 3 marks · Short answerHOTS / competency

    A forest department releases many deer into a small forest with no predators. Predict the changes over a few years in deer, grass and soil, with reasons.

    Hint

    Overgrazing.

    Answer

    Deer increase at first because there are no predators. They overgraze, so grass and young plants decline. With less plant cover, the soil is exposed and eroded by rain and wind; finally food runs short and the deer population falls sharply.

13. Q13 · 3 marks · Short answerHOTS / competency

    A student says: 'Decomposers are not needed in a food chain because nobody eats them.' Explain why the student is wrong.

    Hint

    Recycling minerals.

    Answer

    Decomposers act at every level, breaking down dead producers and consumers and their wastes. They return minerals to the soil, which producers need to grow. Without them, nutrients would stay locked in dead matter, producers would fail, and all food chains would collapse.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define an ecosystem.Show answer

   A unit in which living things interact with each other and their non-living surroundings.

2. 2.Two examples of forest producers?Show answer

   Sal and teak (or bamboo, grass).

3. 3.What is a primary consumer?Show answer

   A herbivore, which eats plants.

4. 4.What is a food web?Show answer

   A network of interconnected food chains.

5. 5.Shape of the forest pyramid of numbers?Show answer

   Spindle-shaped.

6. 6.Partners in a lichen?Show answer

   An alga and a fungus.

7. 7.Name a parasitic plant.Show answer

   Cuscuta (dodder).

8. 8.Name four climatic factors.Show answer

   Sunlight, temperature, humidity, wind.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you list five biotic and five abiotic components of a forest?
• ☐Can you write three forest food chains with correct arrows?
• ☐Can you work out energy at the top of a chain using one-tenth?
• ☐Can you draw upright and spindle pyramids of numbers?
• ☐Can you give an example each of symbiosis, parasitism and predation?
• ☐Can you explain what happens if a predator or decomposers are removed?

Ecosystems: frequently asked questions