1. Introduction to Our Environment and the Ecosystem
The environment is everything that surrounds and affects a living organism — the air we breathe, the water we drink, the soil we stand on, the sunlight and temperature we feel, and every other organism that shares that space. It is the sum total of all living and non-living things around us.
An ecosystem is a self-maintaining unit of living organisms (biotic components) interacting with one another and with the non-living (abiotic) components of their environment in a particular area. A pond, a forest, a grassland, a desert and a garden are all examples of ecosystems.
For example, think of a pond: the water, mud, dissolved oxygen, sunlight and temperature are its non-living parts; the fish, frogs, insects, algae, microscopic organisms and water plants are its living parts. Every organism both depends on the environment and changes it. The size of an ecosystem can be tiny (a drop of water in a puddle is teeming with a whole ecosystem) or as huge as a whole forest.
2. Biotic and Abiotic Components of an Ecosystem
The living members of an ecosystem — plants, animals, bacteria, fungi and other microorganisms. They are grouped as producers, consumers and decomposers.
The non-living physical and chemical factors of an ecosystem — air, water, soil, sunlight, temperature, humidity, atmospheric gases and mineral nutrients. They rarely need to be separated from living things because organisms depend on them continuously.
How biotic and abiotic components interact
- Plants use sunlight, water, carbon dioxide and soil minerals (abiotic) to make food; without these there would be no producers.
- Animals depend on producers (directly or indirectly) for food and on abiotic factors for respiration, water and shelter.
- Temperature, rain and soil type determine which ecosystems exist in a place — for example a desert supports different plants and animals than a tropical forest.
- Organisms also modify their environment: earthworms aerate the soil, plants release oxygen, and fallen leaves enrich the soil with humus.
A simple real-world example: in your school garden (an ecosystem), sunlight and rain (abiotic) help the grass and neem tree grow (producers); grasshoppers and beetles eat the grass (primary consumers); sparrows and frogs eat the insects (secondary consumers); and soil bacteria and fungi decompose fallen leaves and droppings (decomposers), returning nutrients to the soil.
Whenever you are asked to name components of an ecosystem, always give examples from both groups — at least two abiotic factors and the three kinds of biotic components with one example each.
3. Producers, Consumers and Decomposers
Organisms that make their own food from simple inorganic substances using energy (usually sunlight). All green plants and some algae and bacteria are producers. They form the first trophic level and are the entry point of energy into an ecosystem.
Organisms that cannot make their own food and obtain energy by eating producers or other consumers. They are also called heterotrophs. Examples: deer, goat, lion, birds, humans.
Primary consumers eat producers directly (herbivores like rabbit, goat, grasshopper).
Secondary consumers eat primary consumers (small carnivores or omnivores like frog, sparrow).
Tertiary consumers eat secondary consumers (large carnivores like snake, hawk, lion). Some food chains have quaternary (fourth-level) carnivores such as the eagle that hunts the snake.
Organisms such as bacteria and fungi that break down the dead bodies of plants and animals and the waste products of organisms into simpler substances. In doing so they release nutrients (like nitrogen and minerals) back into the soil and atmosphere, where producers can use them again.
Role of decomposers — why they matter
- They recycle nutrients: nitrogen, phosphorus, potassium and other minerals locked up in dead matter are returned to the soil for plants.
- They clean the environment: without decomposers, dead bodies and waste would pile up and never vanish.
- They keep the flow of matter cycling in an ecosystem (even though energy flow is one-way).
- Decomposers are not part of a food chain’s trophic levels in the same way as producers and consumers, but they form the important link between the dead matter of every level and the soil.
Are green plants the only producers? In most land ecosystems, yes — green plants. But in oceans and ponds, phytoplankton and algae are the main producers. Bacteria that use chemical energy (chemosynthetic bacteria) are producers too, though they are not mentioned as often in school answers.
4. Interaction Between Biotic and Abiotic Components
The living and non-living parts of an ecosystem are not separate lists — they work as one unit. The links are continuous:
| Abiotic factor | How living things depend on it | How living things affect it |
|---|---|---|
| Sunlight | Drives photosynthesis in producers | Trees create shade, changing the light that reaches the ground |
| Water | Needed by all organisms for life processes and by plants for making food | Plants release water vapour (transpiration); animals return water to the cycle |
| Oxygen | Used for respiration by plants, animals and most microorganisms | Green plants release oxygen during photosynthesis |
| Carbon dioxide | Used by plants for photosynthesis | Respiration and decomposition release it back into the air |
| Soil minerals | Absorbed by plant roots with water | Decomposers return minerals to the soil |
| Temperature | Controls growth, activity and survival of organisms | Large forests cool the surrounding air; cities warm it |
Key idea — the ecosystem works because biotic and abiotic components constantly exchange matter and energy.
5. Food Chain
A food chain is the sequence of organisms through whom food (and the energy stored in it) passes, each organism eating the one before it and being eaten by the one after it. It always begins with a producer (green plant) at the base.
The arrows in a food chain point in the direction of energy transfer — from the organism being eaten towards the organism that eats it:
Grass → Grasshopper → Frog → Snake → Eagle
Grass → Deer → Lion
Phytoplankton → Small fish → Large fish → Bird
Notice that each organism in the chain occupies a trophic (feeding) level. The grasshopper feeding on grass and the eagle feeding on the snake are at different levels, and energy moves only in one direction — from the producer upwards through the consumers.
6. Trophic Levels
A trophic level is the position that an organism occupies in a food chain, decided by how many energy-transfer steps the organism is away from the producer. The word comes from the Greek trophî, meaning food or nourishment.
| Trophic level | Who occupies it | Example in Grass → Grasshopper → Frog → Snake → Eagle |
|---|---|---|
| First (T-1) | Producers (autotrophs) | Grass |
| Second (T-2) | Primary consumers (herbivores) | Grasshopper |
| Third (T-3) | Secondary consumers | Frog |
| Fourth (T-4) | Tertiary consumers | Snake |
| Fifth (T-5) | Quaternary (top) carnivores | Eagle |
The number of trophic levels in a food chain is the number of organisms in the chain — but count the producer as level 1. In a grazing food chain there are usually not more than four or five trophic levels, because the energy left is too little to support another level.
7. Energy Flow in an Ecosystem and the Approximate 10% Rule
The sun is the ultimate source of energy for most ecosystems. Green plants (producers) capture solar energy and convert it into chemical energy stored in food (carbohydrates) by photosynthesis. In a few special habitats (deep-sea vents), some bacteria use chemical energy instead — but for school-level answers, remember the sun.
How energy moves through trophic levels
- Producers store the energy they capture in their bodies. When a primary consumer eats a producer, some of that energy is transferred.
- When that consumer is eaten by a secondary consumer, energy moves to the third trophic level, and so on.
- Energy flow is unidirectional (one-way): it passes from producers to consumers and is never passed back from consumers to producers.
Energy is lost at each step — the approximate 10% principle
At every trophic level, a large part of the food energy is lost, mostly as heat produced during respiration, in performing everyday activities (movement, digestion, growth), and through unconsumed or undigested matter. As a rough ecological guideline, only about 10% of the energy available at one trophic level is passed on to the next. This is known as the ten percent law. It is an approximate, general principle — it is not an exact mathematical law that holds identically in every ecosystem.
Suppose the producers (say, grass) contain approximately 10,000 units of energy:
Primary consumers (grasshopper) ≈ 1,000 units
Secondary consumers (frog) ≈ 100 units
Tertiary consumers (snake) ≈ 10 units
This is an illustrative application of the approximate 10% transfer principle — it shows how little energy reaches the top of a long food chain.
The 10% value is a broad ecological guideline used to show trends and comparisons, not a fixed constant. Do not write "exactly 10% always" or use it to calculate a precise prediction for a real ecosystem. In the exam, saying "about 10% on average" or "roughly 10%" is the correct wording.
Why food chains are usually short
Because so much energy is lost at every step, very little energy reaches the higher trophic levels. Beyond about four or five trophic levels, the remaining energy is too small to support living organisms, so food chains in nature remain short.
8. Food Web
A food web is a network of many interconnected food chains in an ecosystem. In nature an organism rarely feeds on only one kind of food or is eaten by only one predator, so food chains join together to form a web.
Importance of food webs
- They show the real feeding relationships in nature, where one organism can be part of several food chains at once.
- They make the ecosystem more stable: if one food source is unavailable, an organism can feed on another, so the web does not collapse easily.
- Removal of one species in a food chain breaks it, but a food web can often absorb the change through alternative paths.
A rat eats grass (primary consumer), but the rat itself is hunted by a snake, a hawk and a cat — so in different chains the rat is at the second trophic level while the snake, hawk and cat are at the third (or higher) levels. A single organism can thus occupy different feeding relationships in the same web.
9. Biological Magnification
Biological magnification is the increase in the concentration of a harmful (usually non-biodegradable) chemical at successive trophic levels of a food chain. Because the chemical is not broken down or removed by the body, its concentration keeps increasing as it moves up the chain.
How it happens
- A pesticide (e.g. DDT) or heavy metal enters a water body or soil from fields.
- Producer (plankton or grass) absorbs a small amount from its surroundings.
- A primary consumer eats many producers, building up the chemical faster than it can remove it.
- Each successive consumer eats many organisms from the level below, so the concentration becomes largest at the highest trophic levels.
Suppose pond water contains DDT at about 0.02 parts per million (ppm):
Water (0.02 ppm) → Zooplankton (approx. 0.5 ppm) → Small fish (approx. 2 ppm) → Large fish (approx. 25 ppm) → Fish-eating birds (approx. several hundred ppm)
The numbers are illustrative, but they show the key idea: concentration rises with each trophic level.
Effect on organisms at higher trophic levels
- Birds that eat fish and mammals that hunt fish are often the worst affected because they sit at the top of long food chains.
- High concentrations can harm reproduction (for example, thinning of eggshells in some birds) and damage the nervous system of predators.
- Biological magnification is one reason farmers are advised to use pesticides in careful, limited amounts and prefer biodegradable alternatives.
Biological magnification is about a chemical building up through food chains. It is not the same as the general pollution of an area — though pollution is often the starting point.
10. Biodegradable and Non-Biodegradable Substances
Substances that are broken down by microorganisms (decomposers) into simpler harmless substances in a reasonable time are called biodegradable. Examples: vegetable waste, fruit peels, food waste, paper, cotton cloth, cow dung, and fallen leaves.
Substances that cannot be broken down by microorganisms into simpler harmless forms are called non-biodegradable. They persist in the environment for a very long time. Examples: plastic, polythene, glass, DDT and many synthetic pesticides, and certain synthetic chemicals and synthetic fibres.
Decomposition — the clean-up process
Decomposition is the breaking down of dead and waste organic matter into simpler substances by bacteria and fungi (the decomposers). Biodegradable waste decomposes naturally over weeks to months, releasing nutrients back to the soil. Non-biodegradable waste does not decompose, so plastic thrown on the ground may remain for hundreds of years, choking soil, harming animals that swallow it, and blocking drains.
Comparison table
| Point of comparison | Biodegradable substances | Non-biodegradable substances |
|---|---|---|
| Broken down by microorganisms | Yes, naturally over time | No (or extremely slowly) |
| Effect on the environment | Returns nutrients to the soil; little waste stays behind | Accumulates; can pollute soil, water and air for years |
| Examples | Food waste, paper, cotton, leaves, dung | Plastic, glass, DDT, synthetic fibres |
| Handling | Compost, natural decay, biogas | Segregate and recycle; reduce its use |
A substance is not simply "biodegradable" or "non-biodegradable" forever — the word depends on the conditions and time. Most paper decomposes in weeks, but some glossy coated paper and some "bioplastics" take much longer. Do not claim every material in a broad category (all plastics, or all papers) behaves identically in every condition.
11. Waste Management
Waste management is the proper handling of waste — its collection, segregation, reuse, recycling, treatment and safe disposal — so that it harms the environment and people as little as possible.
Why responsible waste management matters
- Improperly dumped waste pollutes soil, water and air and spreads diseases.
- Non-biodegradable waste that reaches lakes, rivers and oceans harms aquatic life.
- Burning plastic and other waste releases harmful gases into the air.
- Reducing and recycling waste saves resources and energy that would be used to make new products.
Waste segregation: wet and dry
| Type | What it contains | What to do with it |
|---|---|---|
| Wet (biodegradable) waste | Vegetable and fruit peels, food scraps, tea leaves, eggshells | Compost it — the compost fertilises soil |
| Dry (non-biodegradable) waste | Plastic, glass, paper, metal, packaging | Segregate recyclables and send them for recycling |
Practical steps students and families can follow
- Use two bins — one for wet, one for dry waste — and label them clearly.
- Carry a cloth bag or carry-bag to market instead of accepting plastic bags every time.
- Carry a water bottle and lunch box to avoid buying single-use plastic bottles and packets.
- Reuse glass jars, paper (as rough work), and gift-wrapping.
- Compost kitchen waste in a small home compost pit or bin; the compost can be added to potted plants.
- Recycle paper, plastic, glass and metal by sending them to the appropriate collection points.
- Buy only what is needed — reducing consumption is the most effective step of all.
Recycling and reuse still use some energy and resources. Reducing the amount of waste we create is the most effective and the first of the 3Rs. Order of preference: Reduce → Reuse → Recycle.
12. Ozone Layer and Ozone Depletion
Ozone is a molecule made of three atoms of oxygen (O₃). It is a pale blue gas, different from the ordinary oxygen (O₂) that animals breathe.
The ozone layer is a concentration of ozone present high up in the stratosphere of the atmosphere, roughly 15–35 km above the Earth’s surface. It is significant because it absorbs most of the harmful ultraviolet (UV) radiation coming from the Sun.
Ozone depletion
Ozone depletion is the thinning of the ozone layer, caused mainly by certain man-made chemicals called chlorofluorocarbons (CFCs). CFCs were once widely used in refrigerators, air conditioners, aerosol sprays, and foam-making.
CFCs released from old appliances and sprays drift up to the stratosphere. In the strong sunlight there, a CFC molecule releases chlorine atoms. A single chlorine atom can break down thousands of ozone (O₃) molecules over time, converting them back to ordinary oxygen and thereby thinning the ozone layer.
Environmental consequences
- More harmful UV radiation reaches the Earth’s surface.
- It increases skin cancers and cataracts and can weaken the human immune system.
- It can damage crops and forests and harm the tiny organisms (phytoplankton) at the base of ocean food chains.
Protecting the ozone layer
- The international Montreal Protocol (1987) is the landmark agreement under which nations agreed to phase out the production and use of CFCs and other ozone-depleting substances.
- Modern refrigerators and ACs use CFC-free alternatives.
- Proper disposal and recycling of old cooling appliances prevents the release of their CFCs.
- The ozone layer is now slowly recovering, which shows that international cooperation can protect the environment.
Ozone in the upper atmosphere (stratosphere) protects us by absorbing UV and is good. Ozone near the ground (troposphere, from vehicle and industrial pollution) is a harmful pollutant in polluted cities. Questions in this chapter are about the protective stratospheric ozone layer.
13. Important Definitions
| Term | Definition |
|---|---|
| Ecosystem | A self-maintaining unit in which living organisms interact with each other and with the non-living components of their environment in an area. |
| Biotic component | The living members of an ecosystem — producers, consumers and decomposers. |
| Abiotic component | The non-living physical and chemical factors of an ecosystem — air, water, soil, sunlight, temperature and minerals. |
| Producer | An organism (such as a green plant) that makes its own food from simple inorganic substances using sunlight, forming the first trophic level. |
| Consumer | An organism that cannot make its own food and feeds on producers or other consumers. |
| Decomposer | An organism (bacterium or fungus) that breaks down dead organisms and waste into simpler substances, returning nutrients to the soil. |
| Food chain | The sequence of organisms through which food energy passes, each organism eating the one before it, always starting with a producer. |
| Trophic level | The position of an organism in a food chain, decided by the number of energy-transfer steps from the producer. |
| Food web | A network of many interconnected food chains in an ecosystem. |
| Energy flow | The unidirectional transfer of energy from producers to consumers through successive trophic levels of a food chain. |
| Biological magnification | The increase in the concentration of a non-biodegradable chemical at successive trophic levels of a food chain. |
| Biodegradable substance | A substance that can be broken down by microorganisms into simpler harmless substances. |
| Non-biodegradable substance | A substance that cannot be broken down by microorganisms into simpler harmless substances and therefore persists in the environment. |
| Ozone | A molecule of three atoms of oxygen (O₃) that absorbs harmful UV radiation; found mainly in the stratosphere. |
| Ozone depletion | The thinning of the ozone layer, caused mainly by CFCs releasing chlorine that destroys ozone molecules. |
| Waste management | The proper collection, segregation, reuse, recycling, treatment and safe disposal of waste to minimise harm to the environment. |
14. Important Facts to Remember
| Fact | Why it matters |
|---|---|
| Producers form the first trophic level. | Every food chain starts at these autotrophs, which capture solar energy. |
| Energy flow in an ecosystem is unidirectional. | Energy passes from producers to consumers but never flows back to producers. |
| Energy decreases at successive trophic levels. | Most food energy is lost as heat at each step, so less remains for the next level. |
| Approximately 10% of energy is transferred to the next trophic level. | This approximate rule explains why food chains are short. |
| Decomposers recycle nutrients by breaking down dead organic matter. | Minerals locked in dead bodies are returned to the soil for producers. |
| Food webs consist of interconnected food chains. | Real ecosystems are webs, which are more stable than single chains. |
| Non-biodegradable substances persist for long periods. | They accumulate in the environment and can cause biological magnification. |
| Biological magnification can cause higher concentrations of pollutants at higher trophic levels. | Top carnivores are most affected by persistent non-biodegradable chemicals. |
| Ozone absorbs harmful UV radiation. | The stratospheric ozone layer shields life from UV that causes skin cancer and eye damage. |
| CFCs are associated with ozone depletion. | CFCs release chlorine that destroys ozone molecules, thinning the layer. |
| Waste reduction, reuse and recycling reduce environmental impact. | The 3R approach lowers pollution and saves resources. |
Sun → energy source; plants → producers, first trophic level; ≈10% energy transferred per level; decomposers → recycle nutrients; DDT → biological magnification example; plastic → non-biodegradable; CFCs → ozone depletion; Montreal Protocol → protection step.
15. Important Diagrams at a Glance
| Fig. | Diagram | What to include |
|---|---|---|
| Fig. 1 | Components of an ecosystem | Two labelled groups: biotic (producers, consumers, decomposers) and abiotic (air, water, soil, sunlight, temperature); two-way interaction arrows |
| Fig. 2 | Simple food chain | At least four organisms joined by arrows; arrows point from the eaten to the eater (direction of energy flow) |
| Fig. 3 | Trophic levels | Producer at the base (level 1), then herbivores, small carnivores, large carnivores; label T-1 to T-5 |
| Fig. 4 | Energy flow (10% rule) | Each level with its approximate energy; arrow from one level to the next labelled about 10%; heat-loss arrows |
| Fig. 5 | Food web | Several overlapping chains with shared organisms; label the different trophic levels |
| Fig. 6 | Biological magnification | Pond, producers → fish → fish-eating birds; dots of increasing size showing rising chemical concentration |
| Fig. 7 | Biodegradable vs non-biodegradable | Two columns with examples; decomposer acting on biodegradable waste; "persists" for non-biodegradable |
| Fig. 8 | 3R and waste segregation | Reduce, Reuse, Recycle icons; a green (wet) and a blue (dry) bin |
| Fig. 9 | Ozone layer and UV protection | Sun and UV rays, ozone layer as a band in the stratosphere, absorbed UV arrow and forbidden UV reaching Earth |
In food-chain and food-web diagrams, the arrowheads must point towards the eater. Misplaced arrows are the most common single mistake in this chapter and cost easy marks.
16. Solved Examples
Work each one on your own first, then compare. Always write the concept tested and give the reason — that is how the examination expects you to answer.
Basic Concept Questions
Define an ecosystem and give two examples from your surroundings.
Concept tested: definition of ecosystem.
Solution: An ecosystem is a self-maintaining unit in which living (biotic) components interact among themselves and with the non-living (abiotic) components of the environment in an area.
Answer: examples: a pond, a garden, a grass lawn, a forest.
Name the two broad components of an ecosystem with two examples each.
Concept tested: biotic and abiotic components.
Solution: Biotic (living): plants, animals, bacteria. Abiotic (non-living): air, water, soil, sunlight, temperature.
Answer: biotic — producers, consumers, decomposers; abiotic — physical and chemical factors like air, water, soil and sunlight.
What is a producer? Why must every food chain start with a producer?
Concept tested: autotrophs as the energy entry point.
Solution: A producer is an organism that makes its own food from simple inorganic substances using light energy, e.g. green plants. Only producers can capture energy from the sun and store it in food; no consumer can add new energy to an ecosystem.
Answer: producers capture solar energy, so the food chain must begin at them.
Classify rabbit, frog, lion and grass as producer, primary, secondary or tertiary consumer.
Concept tested: trophic classification.
Solution: grass — producer; rabbit eats grass — primary consumer; frog eats insects — secondary consumer; lion eats herbivores — tertiary (top) consumer.
Answer: grass = producer; rabbit = primary consumer; frog = secondary consumer; lion = tertiary consumer.
Why are decomposers important even though producers can make food?
Concept tested: role and necessity of decomposers.
Solution: Dead bodies and wastes would accumulate and minerals would stay locked inside them. Decomposers (bacteria and fungi) break down dead matter and return nutrients to the soil, which producers then use.
Answer: they recycle nutrients and keep the ecosystem clean.
Food Chain and Trophic-Level Questions
Define a food chain. Write the food chain Grass → Grasshopper → Frog → Snake → Eagle and name the trophic level of each.
Concept tested: food chain and trophic levels.
Solution: Grass = first trophic level (producer); Grasshopper = second (primary consumer); Frog = third (secondary consumer); Snake = fourth (tertiary consumer); Eagle = fifth (top carnivore/quaternary consumer).
Answer: levels 1 to 5 from grass to eagle respectively.
In the chain Plants → Deer → Tiger, which is the first trophic level? What type of consumer is the deer?
Concept tested: identification of trophic levels.
Solution: Plants are the first trophic level (producers). The deer eats plants, so it is a primary consumer (herbivore) at the second trophic level. The tiger is a tertiary consumer (third level).
Answer: first trophic level = plants; deer = primary consumer at the second trophic level.
Is it necessary to start all food chains from plants? Explain with one example.
Concept tested: producers as the base of energy capture.
Solution: Every food chain must begin with a producer. In most land ecosystems the producer is a green plant, but in water bodies it may be phytoplankton or algae. Example: Phytoplankton → Small fish → Large fish → Fish-eating bird.
Answer: the producer (plant or phytoplankton) is always the first trophic level.
How many trophic levels can a food chain normally have? Justify.
Concept tested: reason for short food chains.
Solution: A grazing food chain usually has three to five trophic levels. Since only about 10% of the energy passes to the next level, the energy left after four or five steps is too little to support another level.
Answer: normally 3–5 levels because energy becomes insufficient.
A student draws the food chain: Snake → Frog → Grasshopper → Grass. Point out the mistake and correct it.
Concept tested: correct direction of food chains.
Solution: A food chain must start with the producer and arrows must point from the eaten towards the eater. The correct chain is Grass → Grasshopper → Frog → Snake.
Answer: the order was reversed; the correct chain starts with grass.
Energy Flow and the 10% Rule
What is the ultimate source of energy for most ecosystems?
Concept tested: source of ecosystem energy.
Solution: The sun. Green plants capture sunlight and convert it into chemical energy during photosynthesis; this energy then passes through the food chain.
Answer: the sun is the ultimate source of energy for most ecosystems.
Why is energy flow through an ecosystem unidirectional?
Concept tested: direction of energy flow.
Solution: Energy is absorbed by producers from sunlight, transferred to consumers through food, and finally lost as heat. This energy cannot be returned from consumers back to producers, so the flow is one-way only.
Answer: energy moves only from producers to consumers (never back), so the flow is unidirectional.
If producers contain about 10,000 units of energy, estimate the energy available at the next three trophic levels using the approximate 10% rule.
Concept tested: numerical application of the 10% principle.
Solution: Primary consumers ≈ 10% of 10,000 = 1,000 units. Secondary consumers ≈ 10% of 1,000 = 100 units. Tertiary consumers ≈ 10% of 100 = 10 units.
Answer: 1,000 → 100 → 10 units. (Illustrative, using the approximate rule.)
Where does the "lost" energy go at each trophic level?
Concept tested: reasons for energy loss.
Solution: Most food energy is used or lost as heat during respiration, in everyday activities (digestion, movement, maintaining body temperature) and through undigested material. Only a small amount is stored as new body tissue for the next level.
Answer: lost largely as heat and used in life processes, not stored for the next level.
Food Web Questions
Define a food web and explain how it differs from a food chain.
Concept tested: food web vs food chain.
Solution: A food web is a network of many interconnected food chains in an ecosystem. A food chain is a single linear sequence; a food web is many such chains joined by shared organisms.
Answer: food web = many interlinked food chains; more complex and realistic than a single chain.
In a grassland food web, a snake can eat a frog or a rat, and an eagle can eat the snake or the rabbit. Write two food chains found in this web.
Concept tested: extracting individual chains from a web.
Solution: Chain 1: Grass → Grasshopper → Frog → Snake → Eagle. Chain 2: Grass → Rat → Snake → Eagle. (Also possible: Grass → Rabbit → Eagle.)
Answer: any two correct chains starting from the producer shown above.
Why is a food web more stable than a single food chain?
Concept tested: importance of food webs.
Solution: In a single chain, if one organism disappears the whole chain breaks. In a web, an organism can feed on an alternative food source, so the loss of one species is absorbed and the ecosystem continues to function.
Answer: alternative feeding relationships prevent a total collapse.
Biological Magnification
Define biological magnification with an example.
Concept tested: definition and example.
Solution: Biological magnification is the increase in the concentration of a non-biodegradable chemical (like a pesticide) at successive trophic levels. Example: DDT entering a pond is found in increasing concentration from water and producers up to fish and fish-eating birds.
Answer: concentration of the chemical rises up the food chain.
Why does the concentration of DDT increase at higher trophic levels even though DDT is sprayed in small amounts?
Concept tested: reason for biomagnification.
Solution: DDT is non-biodegradable, so it is not broken down by organisms and stays in their bodies. Each consumer eats many organisms from the level below, taking in the DDT stored in all of them. So the chemical accumulates and its concentration rises with each trophic level.
Answer: non-biodegradable DDT accumulates; consumers eat many prey, concentrating the chemical.
Give one effect of biological magnification on organisms at the highest trophic levels.
Concept tested: consequences of biomagnification.
Solution: Top carnivores (such as fish-eating birds) accumulate the highest concentration of the chemical, which can harm their health and reproduction — for example, thinning of eggshells in some birds and nervous-system damage.
Answer: top predators suffer the highest concentrations and the most harm.
Biodegradable and Non-Biodegradable
Classify vegetable waste, plastic bag, cotton cloth, glass bottle, paper and a metal tin as biodegradable or non-biodegradable.
Concept tested: classification with generalisations avoided.
Solution: Biodegradable: vegetable waste, cotton cloth, paper (decomposed by microorganisms). Non-biodegradable: plastic bag, glass bottle, metal tin (not decomposed by microorganisms; need recycling).
Answer: biodegradable — vegetable waste, cotton, paper; non-biodegradable — plastic, glass, metal.
Why does a plastic packet thrown on the ground remain for years while a banana peel disappears? Explain.
Concept tested: why non-biodegradable materials persist.
Solution: The banana peel is biodegradable — bacteria and fungi decompose it into simpler substances. Plastic is non-biodegradable — no decomposer breaks it down, so it persists for a very long time and pollutes the environment.
Answer: banana peel is decomposed by microorganisms; plastic is not.
Application and Environmental Reasoning
If all the frogs in the chain Grass → Grasshopper → Frog → Snake → Eagle were removed, what would happen?
Concept tested: effect of removing one organism.
Solution: The grasshopper population would grow rapidly (no frog to eat it) and overgraze the grass; the snake would lose its food and the snake and eagle populations would fall. The chain would be disturbed or broken.
Answer: grasshoppers increase, grass decreases, and snake and eagle populations decline.
A family separates its kitchen waste and sends it to a compost bin. Explain how this helps the environment.
Concept tested: application of waste management.
Solution: Kitchen waste is biodegradable. Composting lets bacteria decompose it into manure, which can be added to soil. This reduces the waste sent to landfills, returns nutrients to the soil, and avoids the pollution caused by dumping or burning.
Answer: composting recycles nutrients, reduces waste, prevents pollution.
Why should CFC-based refrigerators be disposed of carefully?
Concept tested: CFC and ozone depletion.
Solution: If old CFC-containing refrigerant escapes from a discarded refrigerator, the CFCs rise to the stratosphere where they release chlorine and destroy ozone molecules, contributing to ozone depletion and letting more harmful UV reach Earth. Proper disposal prevents this release.
Answer: escaping CFCs deplete the protective ozone layer.
A field was sprayed with a pesticide. After a few weeks, a hawk living near the field was found to have a much higher level of the pesticide in its body than the insects that were sprayed. Explain.
Concept tested: application of biological magnification.
Solution: The pesticide is non-biodegradable and accumulates in organisms. The sprayed insects (or grain) contain a little; the birds feeding on them store more; the hawk, at the top of the chain, eats many such organisms and therefore has the highest concentration — this is biological magnification.
Answer: biological magnification concentrates the pesticide at the highest trophic level (hawk).
17. Application-Based Examples
These questions connect the ideas of the chapter to everyday and environmental situations. Apply the concepts rather than recalling definitions.
Original — CBSE Exam-Style
In a grassland, people kill all the snakes to protect themselves. What changes might occur in the grasshopper and grass populations afterwards? Explain briefly.
Answer:
Snakes eat frogs, which eat grasshoppers. With snakes gone, frogs increase and eat more grasshoppers, so grasshopper numbers may fall; alternatively in a simpler chain (grass → grasshopper → frog → snake), removing the snake changes predator numbers and disturbs the whole chain — populations no longer stay balanced because the food chain has been broken.
Original — CBSE Exam-Style
A pond food chain is: Phytoplankton → Small fish → Large fish → Kingfisher. If the producers store about 100,000 units of energy, estimate the energy available to the kingfisher using the approximate 10% rule.
Answer:
Phytoplankton 100,000 → small fish ≈10,000 → large fish ≈1,000 → kingfisher ≈100 units. (Illustrative application of the approximate 10% transfer rule.)
Original — CBSE Exam-Style
At a school, wet kitchen waste and dry plastic waste are thrown together into one bin. Suggest two problems this creates and one correct practice.
Answer:
Mixing makes recycling of dry waste difficult, produces foul smell and attracts flies/rodents, and wet-dry mix cannot be easily composted or recycled. Correct practice: use two bins — compost the wet waste and send the dry waste for recycling.
Original — CBSE Exam-Style
A beach near a city is covered with plastic bottles and carry-bags. Explain how this harms marine life and suggest two steps to reduce it.
Answer:
Plastic is non-biodegradable water animals mistake it for food, swallow it and choke or starve; plastic can also entangle animals. Steps: reduce single-use plastic by carrying reusable bottles/bags, dispose of dry waste correctly and recycle; organise beach clean-ups and spread awareness.
Original — CBSE Exam-Style
A farmer sprays DDT on crops. Which organism in the chain Grass → Goat → Man is likely to accumulate the most DDT, and why?
Answer:
Man (at the top of this chain) accumulates the most by biological magnification, because DDT is non-biodegradable, builds up in each organism, and man eats the goat which has absorbed DDT from the grass.
Original — CBSE Exam-Style
Old air-conditioners and refrigerators that used CFCs were common in many homes. Why is their disposal a matter of ozone-layer concern?
Answer:
If incompletely removed, CFCs escape into the air, rise to the stratosphere and release chlorine which destroys ozone. This thins the ozone layer, allowing more harmful UV to reach the Earth. Correct disposal and recycling of old cooling appliances prevents this.
Original — CBSE Exam-Style
Your house generates food scraps, old notebooks, glass jars and plastic bottles. Suggest the best handling for each, using the 3R ideas.
Answer:
Food scraps → compost (recycle nutrients). Old notebooks → reuse rough pages or recycle paper. Glass jars → reuse (storage) or recycle. Plastic bottles → reduce use, then reuse, then recycle; avoid single-use plastic.
Original — CBSE Exam-Style
A new housing society wants to manage its waste responsibly with minimum cost. Suggest a three-step plan based on waste segregation and the 3Rs.
Answer:
Step 1: segregate wet and dry waste at source with two bins per household. Step 2: compost the wet waste in a common pit; use the compost in the garden. Step 3: send dry waste to recyclers; reuse what can be reused and reduce single-use items. Avoid burning or dumping waste.
18. Competency and Reasoning Questions
These test your ability to interpret food chains, reason about energy, and take environmental decisions. Read the whole question before answering.
Original — CBSE Exam-Style
Assertion (A): Energy flow through an ecosystem is unidirectional.
Reason (R): Energy absorbed by producers is returned to the producers again by decomposers.
(a) Both A and R are true and R is the correct explanation of A (b) Both A and R are true but R is not the correct explanation (c) A is true but R is false (d) A is false but R is true
Answer:
(c) A is true but R is false. Energy flows one way from producers to consumers, and is lost as heat; decomposers recycle matter (nutrients), not energy, so R is wrong.
Original — CBSE Exam-Style
In a forest food web, the population of deer increases sharply for a few seasons. How could this affect the producers, the predators, and other herbivores? Give a reasoned answer.
Answer:
More deer graze the plants, so the producer population (and food for other herbivores) may decrease, causing competition among herbivores. In turn, the predators (tigers) get more food, so their population may rise, eventually bringing the deer back down. The whole web readjusts as populations are linked.
Original — CBSE Exam-Style
Using the food web Grass → Grasshopper/Rat/Rabbit, then from them to Frog, Snake and Eagle, identify (i) one herbivore, (ii) one secondary consumer, (iii) the top carnivore.
Answer:
(i) Herbivore: grasshopper / rat / rabbit. (ii) Secondary consumer: frog or snake (eats herbivores). (iii) Top carnivore: eagle (eats snake and rabbit, at the highest level).
Original — CBSE Exam-Style
Two food chains are given: (i) Grass → Deer → Lion and (ii) Phytoplankton → Fish → Bird → Humans. Which energy source and which energy-loss idea are common to both? Why is chain (ii) longer?
Answer:
Both start from producers that capture solar energy, and both lose about 90% of energy at each step. Chain (ii) has more steps; since only about 10% passes on each time, a longer chain leaves less energy at the top, so humans at the end of a long chain receive far less energy than the lion in the short chain.
Original — CBSE Exam-Style
A gardener wants to dispose of dry leaves and old plastic pots. She is given two choices: burn them together, or segregate and compost the leaves and recycle/avoid the plastic. Which choice do you support and why?
Answer:
Support the second choice. Burning dry leaves produces smoke and air pollution and burning plastic releases harmful gases. Composting the leaves returns nutrients to the soil, and recycling or avoiding plastic prevents persistent non-biodegradable waste from polluting the environment.
Original — CBSE Exam-Style
A monitoring study of a lake reported the following average concentration (in ppm) of a pesticide: water 0.02, zooplankton 0.5, small fish 2, large fish 25, fish-eating birds 300. Identify the pattern and explain the biological principle behind it.
Answer:
The concentration rises steadily from water to birds — this is biological magnification. The pesticide is non-biodegradable, so it accumulates in organisms; each consumer eats many prey from the level below, so the chemical builds up to the highest level in fish-eating birds at the top of the chain.
19. Case-Based Learning
Read each passage carefully, then answer the questions that follow. Marks shown are typical board weightage.
Original — CBSE Exam-Style
A farmer’s field has a food chain: Grass → Grasshopper → Frog → Snake → Eagle. During the harvest, many grasshoppers are observed feeding on the crop.
Q1. Identify the producers and primary consumers in this chain. (1 mark)
Answer:
Producers: grass. Primary consumers: grasshopper.
Q2. Name the trophic level of the snake. (1 mark)
Answer:
Fourth trophic level (tertiary consumer).
Q3. If all the frogs were removed, predict what would happen to the grasshopper and grass populations. (2 marks)
Answer:
Grasshoppers, no longer eaten by frogs, would increase and overgraze the grass, reducing the producer population; the snake would lose food and its (and the eagle’s) population could also decline.
Q4. Where does the energy of this chain finally come from, and in which direction does it flow? (1 mark)
Answer:
From the sun, captured by the grass; it flows one way from producers to the higher trophic levels.
Original — CBSE Exam-Style
In a lake, the producers (phytoplankton) contain approximately 100,000 units of energy. A food chain exists: Phytoplankton → Zooplankton → Small fish → Large fish.
Q1. Using the approximate 10% transfer rule, estimate the energy at each consumer level. (2 marks)
Answer:
Zooplankton ≈10,000; small fish ≈1,000; large fish ≈100 units. (Approximate values.)
Q2. Explain why the energy at the large-fish level is so much less than at the producer level. (2 marks)
Answer:
At each level most energy is lost as heat during respiration and used for life activities, so only about 10% is passed on. Over three transfers the energy is therefore reduced greatly.
Q3. Why would adding two more trophic levels to this chain make survival at the top difficult? (1 mark)
Answer:
After each level only about 10% of energy remains, so beyond a few levels the energy left is too little to support another organism.
Q4. State how the direction of this energy flow differs from the cycling of matter. (1 mark)
Answer:
Energy flows one way (and is lost as heat); matter such as nutrients is recycled by decomposers.
Original — CBSE Exam-Style
A lake beside crop fields contains a small amount of a non-biodegradable pesticide. The food chain is: Phytoplankton → Small fish → Large fish → Fish-eating birds. Scientists measure the pesticide in each organism.
Q1. Name the process that increases the pesticide concentration at higher trophic levels. (1 mark)
Answer:
Biological magnification (biomagnification).
Q2. Which organism in the chain is likely to have the highest concentration, and why? (2 marks)
Answer:
The fish-eating birds at the top of the chain, because the non-biodegradable chemical accumulates in each organism and top consumers eat many prey, concentrating the chemical.
Q3. Give one harmful effect that this can have on the top-level organisms. (1 mark)
Answer:
Harm to health and reproduction; for example, thinning of eggshells in birds or damage to the nervous system.
Q4. Suggest one way to reduce this problem. (1 mark)
Answer:
Use biodegradable, safer alternatives to persistent pesticides and use pesticides in careful, limited amounts, or use integrated pest management that avoids such chemicals.
Original — CBSE Exam-Style
A city housing society sets up two bins for every home: one green bin for wet kitchen waste and one blue bin for dry waste. The wet waste is composted in a society garden and the dry waste is sent to a recycling centre.
Q1. Which substances are biodegradable in this arrangement? (1 mark)
Answer:
Wet kitchen waste such as food scraps, vegetable peels and tea leaves.
Q2. Why is it important to keep dry waste separate from wet waste? (2 marks)
Answer:
Mixed waste cannot be composted or recycled easily, produces foul smells and attracts pests, and the non-biodegradable dry waste spoils the compost. Segregation allows each to be handled properly.
Q3. Give one example each of reduce, reuse and recycle from a household. (2 marks)
Answer:
Reduce — avoid single-use plastic bottles; Reuse — use glass jars for storage; Recycle — send paper, plastic and metal for recycling.
Q4. How does composting help the environment? (1 mark)
Answer:
It converts biodegradable waste into manure through microorganism action, returning nutrients to soil and reducing the waste sent to landfills.
Original — CBSE Exam-Style
Scientists noticed that the ozone layer was getting thinner over parts of the Earth. They found that chemicals like CFCs, once released into the atmosphere, were damaging ozone high above the ground. Nations agreed under the Montreal Protocol to phase out CFCs.
Q1. What is ozone, and where is the protective ozone layer found? (1 mark)
Answer:
Ozone is a molecule of three oxygen atoms (O₃); the protective layer is in the stratosphere.
Q2. How do CFCs damage the ozone layer? (2 marks)
Answer:
CFCs released into the atmosphere rise to the stratosphere, where sunlight releases chlorine from them. Chlorine atoms break down ozone (O₃) molecules, thinning the ozone layer.
Q3. What are two harmful effects of increased UV radiation reaching the Earth? (2 marks)
Answer:
Increased risk of skin cancer and cataracts, weakening of the immune system, and damage to crops and phytoplankton (any two).
Q4. How can international cooperation like the Montreal Protocol help the environment? (1 mark)
Answer:
It phases out ozone-depleting substances like CFCs, allowing the ozone layer to slowly recover, showing that countries working together can protect the environment.
20. Important Comparison Tables
Quick side-by-side revision of the most frequently confused pairs.
| Point of comparison | Biotic components | Abiotic components |
|---|---|---|
| Nature | Living organisms | Non-living factors |
| Examples | Plants, animals, bacteria, fungi | Air, water, soil, sunlight, temperature |
| Roles | Produce, consume, decompose matter | Supply materials and conditions for life |
| Dependence | Depend on abiotic factors to survive | Exist whether or not organisms are present |
| Point of comparison | Producers | Consumers |
|---|---|---|
| Make their own food | Yes (autotrophs, e.g. green plants) | No (heterotrophs) |
| Capture energy from | Sunlight (through photosynthesis) | Other organisms (food) |
| Trophic level | First trophic level | Second and higher trophic levels |
| Example | Grass, neem tree, phytoplankton | Goat, frog, eagle |
| Point of comparison | Consumers | Decomposers |
|---|---|---|
| How they get food | Eat living organisms | Break down dead organisms and waste |
| Examples | Animals, birds, insects | Bacteria, fungi |
| Effect on matter | Transfer energy; do not recycle nutrients | Recycle nutrients back to the soil |
| Part of food-chain levels | Yes (trophic levels) | Act on all levels from outside the chain |
| Point of comparison | Food chain | Food web |
|---|---|---|
| Structure | Single linear sequence | Network of many interlinked chains |
| Organisms in feeding relations | Each organism in one chain | An organism can be in several chains |
| Stability | Less stable; breaks if one link is removed | More stable due to alternative food paths |
| Represents | One feeding path | Real feeding relationships in an ecosystem |
| Point of comparison | Biodegradable substances | Non-biodegradable substances |
|---|---|---|
| Decomposed by microorganisms | Yes | No (or extremely slowly) |
| Persist in the environment | No — broken down over time | Yes — remain for very long periods |
| Examples | Vegetable waste, paper, cotton | Plastic, glass, DDT |
| Handling | Compost and let it decompose | Segregate, reduce, reuse, recycle |
| Point of comparison | Reduce | Reuse | Recycle |
|---|---|---|---|
| Main idea | Create less waste | Use the item again | Convert waste into new products |
| Effort / energy | Best — least effort and resources | Less than recycling | Uses some energy and processing |
| Example | Buy only what is needed; refuse plastic bags | Refill a bottle; reuse jars and envelopes | Send paper, glass, metal and plastic for recycling |
| Priority | First | Second | Third |
21. Common Mistakes to Avoid
| Mistake | Why it happens | Correction |
|---|---|---|
| Reversing the direction of arrows in a food chain | Thinking the arrow "points at what is eaten" | Arrows point from the eaten towards the eater (direction of energy transfer) |
| Confusing trophic level with organism population | Mixing up the level with numbers of a species | A trophic level is a feeding step (e.g. herbivores at level 2), not how many organisms exist |
| Treating the 10% rule as an exact universal value | Remembering "10%" as a fixed law | Write "about 10%" or "approximately 10%" — it is a general ecological guideline, not an exact figure |
| Confusing food chain and food web | Using the terms loosely | A chain is one sequence; a web is many interlinked chains |
| Confusing biodegradable with recyclable | Thinking both mean "can be dealt with easily" | Biodegradable means broken down by microorganisms; recyclable means processed into new products |
| Ignoring decomposers | Focusing only on food chains | Decomposers recycle nutrients; mention them when writing about ecosystem balance |
| Confusing ozone with ordinary atmospheric oxygen | Both contain oxygen atoms | Ordinary oxygen is O₂ (breathed by animals); ozone is O₃, which absorbs UV |
| Writing inaccurate statements about CFCs and ozone depletion | Guessing the mechanism | CFCs release chlorine in the stratosphere which breaks down ozone molecules, thinning the layer |
| Confusing accumulation with biological magnification | Using the words interchangeably | Accumulation is a chemical building up in one organism; magnification is the increase along trophic levels of a food chain |
22. Board Exam Tips
- How to draw food chains: always begin with a producer, write arrows in the direction “eaten by”, and number the trophic levels or name them beside each organism.
- How to identify trophic levels: count from the producer as level 1. The organism that eats the producer is level 2, and so on. Top carnivores sit at the last level.
- How to solve 10% energy-transfer questions: start from the given producer energy, then multiply (or divide) by 10% (×0.1) at each step. Write "about/approximately" in the answer and show each step.
- How to answer biological magnification questions: name the process, state that the chemical is non-biodegradable and accumulates, and note that top consumers are most affected. Give the DDT-birds example if asked.
- How to differentiate biodegradable/non-biodegradable: base the answer on decomposition by microorganisms, and always add a couple of examples from each group.
- How to write ozone-related answers: define ozone (O₃), state it is found in the stratosphere, absorbs UV, and is depleted by CFCs (chlorine atoms) — then give the consequences and the Montreal Protocol as protection.
- How to approach case-based questions: read the passage twice, underline the food chain or data, then answer each sub-part using the given passage plus one chapter concept. Show your estimation steps in energy questions.
- How to present diagrams and labels: draw neatly with a pencil, label trophic levels or components, and check every arrowhead points the right way. Title the figure.
- Important keywords to use: biotic, abiotic, producer, consumer, decomposer, trophic level, unidirectional, biomagnification, biodegradable, non-biodegradable, stratosphere, UV radiation, CFCs, segregation, 3R (Reduce, Reuse, Recycle).
23. Quick Revision (5–10 Minutes)
| Concept | Key point | Remember |
|---|---|---|
| Ecosystem | Living + non-living components interacting in an area | Pond, forest, garden are examples |
| Components | Biotic (producers, consumers, decomposers); abiotic (air, water, soil, sunlight, temperature) | Biotic = living; abiotic = non-living |
| Food chain | Producer → primary → secondary → tertiary consumer | Arrows show energy transfer direction |
| Trophic levels | Producer = level 1; each feeding step adds one level | Count from the producer |
| Energy flow | Unidirectional; from sun to producers to consumers | Never flows back |
| 10% transfer | About 10% of energy passes to the next level | Approximate rule; food chains stay short |
| Food web | Many interlinked food chains | More stable than a single chain |
| Biological magnification | Non-biodegradable chemicals concentrate at higher levels | DDT in fish-eating birds |
| Biodegradable / non-biodegradable | Decomposed by microorganisms / not decomposed | Paper vs plastic (and glass) |
| 3R | Reduce, Reuse, Recycle | Reduce first, then reuse, then recycle |
| Ozone layer | Stratospheric O₃ layer absorbs UV | Depleted by CFCs; Montreal Protocol protects it |
One-line pointers
- Energy enters ecosystems through photosynthesis by producers.
- Decomposers recycle nutrients; they are not part of the trophic chain counting.
- A food chain normally has 3–5 trophic levels.
- Biological magnification hits top carnivores hardest.
- Non-biodegradable waste should be reduced, reused and recycled — never burnt or dumped.
- Ozone = O₃ (three oxygen atoms); ordinary oxygen = O₂.
24. Self-Practice Questions
MCQ
The ultimate source of energy for most ecosystems is the (a) Moon (b) Sun (c) wind (d) soil.
MCQ
Which of the following is a producer? (a) Deer (b) Frog (c) Grass (d) Snake.
MCQ
In the chain Grass → Grasshopper → Frog, the frog occupies the (a) first trophic level (b) second trophic level (c) third trophic level (d) fourth trophic level.
MCQ
Bacteria and fungi that break down dead organisms are called (a) producers (b) decomposers (c) primary consumers (d) herbivores.
MCQ
Which of these is biodegradable? (a) Plastic bag (b) Glass bottle (c) Vegetable waste (d) Metal tin.
MCQ
Biological magnification is the increase in concentration of a (a) nutrient (b) non-biodegradable chemical (c) gas (d) vitamin along trophic levels.
MCQ
The approximate amount of energy transferred to the next trophic level is (a) 90% (b) 50% (c) 10% (d) 100%.
MCQ
Which gas absorbs harmful UV radiation from the Sun in the stratosphere? (a) Oxygen (b) Nitrogen (c) Ozone (d) Carbon dioxide.
MCQ
Ozone depletion is mainly caused by (a) carbon dioxide (b) chlorofluorocarbons (c) oxygen (d) nitrogen dioxide.
MCQ
A pond, a patch of grass and a forest are all examples of (a) biomes (b) ecosystems (c) food chains (d) trophic levels.
VSA
Define ecosystem in one sentence.
VSA
Name the three kinds of organisms that make up the biotic components of an ecosystem, with one example each.
VSA
Which trophic level is always occupied by producers?
VSA
Write the chemical formula of ozone and state where the ozone layer is located.
VSA
Give one example each of a biodegradable and a non-biodegradable waste from a kitchen.
VSA
State the 3R principle of waste management.
SA I
Why is energy flow in an ecosystem unidirectional? Give a two-point explanation.
SA I
Write any two differences between a food chain and a food web.
SA I
In the chain Grass → Deer → Tiger, name the trophic level of each organism.
SA I
How do decomposers help in maintaining the soil fertility? Explain briefly.
SA I
If producers contain 40,000 units of energy, estimate the energy at the tertiary consumer level using the approximate 10% rule.
SA II
Explain how biological magnification happens with a DDT example, and state which organisms are affected most.
SA II
Why do most food chains have only three to five trophic levels? Answer with the energy principle.
SA II
Describe how CFCs cause ozone depletion and give two harmful effects of increased UV radiation reaching the Earth.
SA II
Suggest a three-action waste-management plan for a school canteen using segregation and the 3R ideas.
Case-based
A garden food web has grass, a rabbit, grasshoppers, a frog and a snake. The rabbit and grasshopper eat grass; the frog eats grasshoppers; the snake eats the frog and the rabbit.
(i) Which organisms are primary consumers?
(ii) Write one food chain of four organisms from this web.
(iii) If the snake is removed, what may happen to the rabbit and frog populations? Give a reason.
Case-based
A pond is found to contain a pesticide at 0.02 ppm. The concentration in zooplankton is 0.5 ppm, in small fish 2 ppm, in large fish 25 ppm and in fish-eating birds 300 ppm.
(i) Name the phenomenon shown by this data.
(ii) Which organism has the highest concentration and why?
(iii) Conservationists ask farmers to use less harmful chemicals. Why?
Case-based
A class kept a compost pit for vegetable peels and dry leaves, and a separate bin for plastic and glass.
(i) Into which two categories is this waste being divided?
(ii) What happens to the compost after a few weeks, and what is it used for?
(iii) Why should plastic and glass not be put into the compost pit?
HOTS
In a pond, the large fish are removed by fishermen. Using the chain Phytoplankton → Small fish → Large fish, predict two changes in the pond and explain your reasoning.
HOTS
"Energy flows one way through an ecosystem, but matter is recycled." Using producers, consumers and decomposers, explain why both statements are correct.
25. Answer Key and Hints
| Q | Answer / Hint |
|---|---|
| 1 | (b) Sun |
| 2 | (c) Grass |
| 3 | (c) third trophic level (grass 1, grasshopper 2, frog 3) |
| 4 | (b) decomposers |
| 5 | (c) Vegetable waste |
| 6 | (b) non-biodegradable chemical |
| 7 | (c) about 10% |
| 8 | (c) Ozone |
| 9 | (b) chlorofluorocarbons |
| 10 | (b) ecosystems |
| 11 | An ecosystem is a self-maintaining unit in which living organisms interact with each other and with the non-living components of their environment in an area. |
| 12 | Producers (green plants, e.g. grass); consumers (animals, e.g. deer); decomposers (bacteria and fungi, e.g. soil bacteria). |
| 13 | First trophic level (T-1). |
| 14 | Ozone = O₃; the ozone layer is in the stratosphere. |
| 15 | Biodegradable: vegetable peels. Non-biodegradable: a plastic wrapper (or glass bottle). |
| 16 | Reduce, Reuse, Recycle. |
| 17 | Energy enters from the sun through producers and passes to consumers; it is lost as heat and cannot be returned to producers, so the flow is one-way. |
| 18 | Chain = single sequence; web = network of chains. A chain is less stable; a web offers alternative feeding paths. |
| 19 | Grass = first (producer); deer = second (primary consumer); tiger = third (tertiary consumer). |
| 20 | Decomposers (bacteria and fungi) break down dead bodies and waste into simpler substances, returning nutrients (minerals) to the soil, which plants use, keeping the soil fertile. |
| 21 | Primary ≈4,000; secondary ≈400; tertiary ≈40 units (illustrative, using the approximate 10% rule). |
| 22 | DDT is non-biodegradable and accumulates in organisms; each consumer eats many prey, so concentration rises up the chain; the highest levels (top carnivores, fish-eating birds) are affected most. |
| 23 | Only about 10% of energy passes to the next level; after three to five levels not enough energy remains to support another organism, so chains are short. |
| 24 | CFCs rise to the stratosphere; sunlight releases chlorine atoms which destroy ozone (O₃) molecules, thinning the layer. Effects: more skin cancers and cataracts, weaker immune system, damage to crops and phytoplankton (any two). |
| 25 | Use two bins (wet/dry); compost wet waste or send to a composting unit; reduce single-use plastic, reuse containers, and send dry recyclables for recycling. |
| 26 | (i) Rabbit and grasshopper. (ii) e.g. Grass → Grasshopper → Frog → Snake. (iii) Frog population may rise (no snake to hunt it) and the rabbit population may also increase since the snake is no longer a predator. |
| 27 | (i) Biological magnification. (ii) Fish-eating birds — highest trophic level; the non-biodegradable chemical accumulates in each consumer. (iii) To reduce biomagnification and harm to top predators. |
| 28 | (i) Wet (biodegradable) and dry (non-biodegradable). (ii) It decomposes into compost/humus used as natural manure for soil. (iii) They are non-biodegradable and would not decompose, spoiling the compost. |
| 29 | Small fish (no longer eaten by large fish) increase in number and overfeed on phytoplankton, reducing the producer population; the pond may also lose the top-level predator, so the chain is disturbed. |
| 30 | Energy enters via producers (sunlight), passes one way to consumers and is lost as heat, so it is not returned. Matter (minerals, carbon, water) passes through producers and consumers and is returned to soil/air by decomposers, so it cycles. |