This MCQ module is based on: Ecosystems and Food Chains
TOPIC 43 OF 50
Ecosystems and Food Chains
🎓 Class 8
Science
CBSE
Theory
Ch 12 — Sound
⏱ ~28 min
🌐 Language: [gtranslate]
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Myaischool Lt Science Assessment
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[myaischool_lt_science_assessment grade_level="class_8" science_domain="physics" difficulty="basic"]
Probe and Ponder — An Elephant's Highway
In the lush forests that stretch across southern Karnataka and northern Tamil Nadu, a herd of elephants sets out on its age-old walk. The matriarch leads her family along a narrow strip of forest that connects two larger reserves — this strip is called an elephant corridor. But this year, a new highway slices straight across their path. Farms have replaced some of the trees. Where will the herd drink water? Where will the young calves rest? When one species' road is broken, a whole tapestry of life trembles.
- Why does the disappearance of a forest patch matter to elephants kilometres away?
- What feeds the trees that feed the elephants?
- When a tiger eats a deer, where does the energy ultimately come from?
- If all decomposers vanished overnight, why would forests be buried in dead leaves?
In this chapter we explore how every living thing — from a tiny soil fungus to a tusker — is knotted together into a living system that works, quite astonishingly, in harmony. We call such a system an ecosystem.
12.1 What Is an Ecosystem?
An ecosystem is not just a place — it is a working partnership. A pond, a paddy field, the Western Ghats rainforest, even the small patch of soil under a banyan tree are all ecosystems. Each one is a busy community where plants, animals, microbes, air, water and sunshine interact every moment of every day.
Definition: An ecosystem is a natural unit where living organisms (biotic components) and their non-living environment (abiotic components) interact with each other to exchange energy and matter.
Two Sides of Every Ecosystem
Every ecosystem has two kinds of components that are always woven together:
Biotic Components
All living things — plants, animals, fungi, bacteria and other microbes. They are born, grow, reproduce and die.
Abiotic Components
Non-living things — air, water, soil, sunlight, temperature and minerals. They set the stage for life to thrive.
Types of Ecosystems Around Us
Ecosystems can be as huge as the Sundarbans mangrove forest or as tiny as a rock-pool near a temple. Broadly they fall into two families — terrestrial (on land — forests, grasslands, deserts, mountains) and aquatic (in water — ponds, rivers, lakes, seas, estuaries). A paddy field in West Bengal and a kitchen-garden in Kerala are both small human-made ecosystems.
12.2 The Roles Living Things Play — Producers, Consumers, Decomposers
In every ecosystem, each organism has a job to do. These jobs keep energy and nutrients flowing. Three main roles are played out again and again, in every forest and every pond.
1. Producers — The Green Chefs
Producers are the green plants and algae that capture sunlight and cook food through photosynthesis. They convert carbon dioxide and water into sugars, releasing oxygen as a by-product. Because they feed themselves, they are also called autotrophs. Without producers, no ecosystem could exist.
2. Consumers — The Eaters
Consumers cannot make food. They must eat other organisms. We call them heterotrophs. Consumers come in three flavours:
- Herbivores — plant-eaters such as deer, cow, grasshopper, elephant.
- Carnivores — meat-eaters such as tiger, snake, eagle.
- Omnivores — both plant and meat eaters, such as humans, bear, crow.
3. Decomposers — The Clean-Up Crew
Decomposers — mainly bacteria and fungi, with helpers like earthworms and termites — break down dead bodies, fallen leaves and animal droppings. They recycle the locked-up nutrients back into the soil and air, so producers can use them once more. They are the ecosystem's unpaid, invisible sanitation workers.
Remember: Producers trap energy. Consumers pass it on. Decomposers return nutrients to the Earth. Remove any one group and the whole ecosystem collapses.
12.3 Food Chains — Who Eats Whom?
A food chain is a simple story of eating and being eaten. Each arrow in a food chain points from the organism being eaten to the one doing the eating, showing the direction in which energy flows.
Trophic Levels — Floors of a Food Chain
Each step in a food chain is called a trophic level. Producers form the first level, herbivores the second, primary carnivores the third, and so on. At every step, only about 10% of the energy is passed upward; the rest escapes as heat or is used up in life processes. That is why there are many more grasses than grasshoppers, many more grasshoppers than frogs — and only a few eagles on top.
12.4 Food Webs — Nature's Tangled Net
In real life, most animals eat more than one type of food. A frog may eat grasshoppers today and earthworms tomorrow. A snake can be eaten by an eagle or by a mongoose. When several food chains interlink, they form a food web.
A food web gives an ecosystem stability. If one species declines — say, rabbits fall ill and vanish — the hawk can still eat mice or small snakes. A single chain breaks easily; a web holds firm.
12.5 The Sun — The Ultimate Source of Energy
Trace any food chain back to its beginning and you will always meet the Sun. Tiger energy came from deer energy; deer energy came from grass energy; grass energy came from sunlight. Green plants absorb less than 1% of the sunlight that falls on them, yet from that tiny trickle, the entire web of life on Earth is woven.
Grand Idea: Every sandwich you eat, every paper you write on, every cotton kurta you wear — all are solar energy dressed in different costumes. The Sun is the unseen fuel-tank of the living world.
🔍 Activity 12.1 — Build a Food Web of Your Neighbourhood
You will need: a sheet of chart paper, coloured pens, a notebook.
- Walk around your garden, park or field for 15 minutes. List every living thing you can see — plants, birds, insects, stray dogs, squirrels, worms.
- Classify each as Producer, Consumer (herbivore/carnivore/omnivore) or Decomposer.
- On the chart, draw each organism as a labelled circle.
- Draw arrows from the eaten to the eater to show who feeds whom.
- Count the number of arrows and compare with a partner's chart.
🔍 Predict: Will your web have more producers or more top consumers? What happens to your web if one kind of plant disappears?
You will almost certainly find far more producers (grass, flowers, trees, weeds) than top predators (cats, hawks). Removing one plant type does not usually collapse the whole web, because most consumers have alternative food sources — the web's many arrows share the load.
This is exactly why wildlife biologists worry when forests are fragmented: fewer species means fewer arrows, and a thinner web is easier to tear.
🎯 Competency-Based Questions
Anaya visits a paddy field near her grandmother's village in Odisha. She notices rice plants, a few grasshoppers, small frogs, a pond heron standing patiently, and earthworms churning the mud. Overhead, a kite circles. She starts making a list of who eats whom.
Q1. L1 Remember Name the three main roles that living organisms play in any ecosystem.
Answer: The three main roles are (i) Producers (green plants that make food), (ii) Consumers (animals that eat other organisms) and (iii) Decomposers (bacteria and fungi that break down dead matter).
Q2. L2 Understand Construct one food chain using the organisms Anaya observed in the paddy field.
Answer: Rice plant → Grasshopper → Frog → Pond heron → Kite. The arrows show the direction of energy flow from the producer (rice plant) through successive consumers.
Q3. L3 Apply The earthworms she sees are not a consumer in the usual sense. What role do they play and why is it important for the paddy field?
Answer: Earthworms act as decomposers (and soil-helpers). They eat dead leaves and organic matter, and their droppings enrich the soil with nutrients. They also aerate the soil by burrowing, which helps the rice roots breathe. Without them, the soil would slowly lose fertility.
Q4. L4 Analyse A pesticide wipes out the grasshoppers in Anaya's paddy field. Analyse the likely effects on the food web.
Answer: With grasshoppers gone, (i) frogs that ate them must switch to other insects or face starvation, (ii) pond herons may find fewer frogs and move away, (iii) rice plants may grow more vigorously briefly because one herbivore is removed, but (iv) other pest insects may increase, since grasshoppers also competed with them. The food web becomes simpler and less stable.
Q5. L5 Evaluate A classmate argues that the kite at the top of the chain is the "most important" organism. Evaluate this claim.
Answer: The claim is weak. The kite depends entirely on organisms below it. If rice plants (producers) disappeared, the entire chain would collapse and the kite would starve. Producers and decomposers are actually the most fundamental because they trap energy and recycle nutrients. Every trophic level is essential — none is more "important" than another in a balanced ecosystem.
🔗 Assertion–Reason Questions
Assertion (A): Green plants are called producers in an ecosystem.
Reason (R): They prepare their own food through photosynthesis using sunlight, water and carbon dioxide.
Answer: A. Because plants make their own food, they are the starting point of every food chain — hence the label "producers".
Assertion (A): A food web is more stable than a single food chain.
Reason (R): In a food web, consumers have alternative food sources if one species declines.
Answer: A. Multiple feeding links mean that the loss of one species does not automatically starve the others — exactly why webs are more resilient than chains.
Assertion (A): Food chains in nature rarely have more than four or five links.
Reason (R): Only about 10% of energy passes from one trophic level to the next, so very little is left for a sixth or seventh level.
Answer: A. The 10% energy-transfer rule limits how many levels a food chain can sustain before the available energy becomes too small to support another predator.
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