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These are complete NCERT Solutions for CBSE Class 10 Science Chapter 15 — Our Environment. Attempt every question on your own before reading the answer. In the board exam, always use precise terminology — words like biodegradable, biomagnification, trophic level, ozone depletion, CFCs, and decomposers carry marks. Underline key terms in your answer. Draw food chain diagrams wherever possible; they fetch extra marks and make your answer clearer.
Key Concepts (Quick Recap)
Ecosystem
- An ecosystem is a self-contained unit consisting of all living organisms (biotic) and the physical environment (abiotic) interacting as a functional system.
- Biotic components: Producers (autotrophs — plants, algae, some bacteria), Consumers (herbivores, carnivores, omnivores), Decomposers (bacteria, fungi).
- Abiotic components: Temperature, sunlight, water, air, soil, minerals, pH, humidity.
- Ecosystems can be natural (forest, pond, ocean) or artificial (garden, crop field, aquarium).
Food Chain
- A food chain is a linear sequence of organisms through which energy and nutrients flow, starting from a producer.
- Each step in the chain is a trophic level.
- Example — Grassland food chain:
Food chain: Grass (Producer, TL1) → Grasshopper (Primary Consumer, TL2) → Frog (Secondary Consumer, TL3) → Snake (Tertiary Consumer, TL4) → Eagle (Apex Predator, TL5)
- Example — Aquatic food chain:
Food chain: Phytoplankton (Producer, TL1) → Zooplankton (Primary Consumer, TL2) → Small Fish (Secondary Consumer, TL3) → Large Fish (Tertiary Consumer, TL4) → Human (Top Consumer, TL5)
- Example — Simple food chain:
Food chain: Grass (Producer, TL1) → Goat (Primary Consumer, TL2) → Human (Secondary Consumer, TL3)
- Energy enters the food chain via photosynthesis at the producer level and flows in one direction (never reverse).
Food Web
- A food web is a network of interconnected food chains within an ecosystem.
- A single organism can belong to multiple food chains at once (e.g., a frog eats both grasshoppers and flies; a snake eats both frogs and mice).
- Food webs provide stability — if one organism declines, predators can switch to alternative prey, preventing ecosystem collapse.
- More complex food webs = more stable ecosystems.
Trophic Levels
- Trophic Level 1 (TL1): Producers (autotrophs) — plants, algae, phytoplankton. They capture solar energy via photosynthesis.
- Trophic Level 2 (TL2): Primary consumers (herbivores) — grasshoppers, goats, rabbits, zooplankton.
- Trophic Level 3 (TL3): Secondary consumers (carnivores that eat herbivores) — frogs, small fish, snakes.
- Trophic Level 4 (TL4): Tertiary consumers (carnivores that eat other carnivores) — eagles, large fish, tigers.
- Decomposers do not belong to a single trophic level — they process dead matter from all trophic levels.
Energy Flow and the 10% Rule
- Energy flows from Sun → Producers → Consumers in one direction only (unlike nutrients, which cycle).
- 10% rule (Lindeman's rule): Only about 10% of energy is transferred from one trophic level to the next. The remaining 90% is used by the organism for its own life processes (respiration, movement, reproduction) and lost as heat.
- Example: If producers capture 10,000 kJ of solar energy, only 1,000 kJ passes to TL2, 100 kJ to TL3, and just 10 kJ to TL4.
- This is why food chains rarely exceed 4–5 trophic levels — not enough energy remains to sustain more levels.
- This also explains why there are fewer organisms and less biomass at higher trophic levels (pyramid of energy).
Biodegradable vs Non-biodegradable Substances
- Biodegradable: Substances that can be broken down by biological processes (decomposers) into simple, harmless substances. Examples — kitchen waste, paper, cotton, cloth, leather, fruit peels, wood, cake, animal dung.
- Non-biodegradable: Substances that cannot be broken down by biological processes and persist in the environment for hundreds of years. Examples — plastics, polythene bags, DDT, glass, metals, synthetic fibres, electronic waste, paints, chemicals.
- The key difference: decomposers lack the enzymes to break down synthetic/complex non-biodegradable substances.
Decomposers
- Decomposers (bacteria, fungi, some worms) break down dead organic matter — dead plants, dead animals, excreta, and waste products — into simple inorganic substances.
- They release nutrients (nitrogen, phosphorus, carbon) back into the soil and atmosphere, making them available to producers again.
- This nutrient cycling is essential — without decomposers, all nutrients would remain locked in dead bodies, and the ecosystem would collapse.
- They also act as natural scavengers, cleaning the environment of decaying matter.
Biological Magnification (Biomagnification)
- Biomagnification is the progressive increase in the concentration of a non-biodegradable toxic substance at successive trophic levels in a food chain.
- The substance must be persistent (does not break down), soluble in fats (accumulates in body fat), and biologically active (toxic).
- Classic example — DDT:
- Water in a lake contains 0.003 ppb (parts per billion) of DDT.
- Phytoplankton (TL1) accumulate ~0.04 ppb.
- Zooplankton (TL2) accumulate ~0.5 ppb.
- Small fish (TL3) accumulate ~2.0 ppb.
- Large fish (TL4) accumulate ~10–15 ppb.
- Fish-eating birds (TL5) accumulate over 25 ppb — nearly 800 times the original concentration in water.
- Humans, at the top of many food chains, accumulate the highest concentrations — DDT has been detected in human breast milk.
- This is why banning persistent pesticides is critical for environmental and human health.
Ozone Layer
- Ozone (O₃) is a molecule composed of three oxygen atoms, formed when UV radiation splits O₂ and the free oxygen atom combines with another O₂.
- The ozone layer is a region of high ozone concentration in the stratosphere, approximately 20–40 km above Earth's surface.
- It absorbs 97–99% of the Sun's harmful UV-B and UV-C radiation, acting as a protective shield for all life on Earth.
- CFCs (chlorofluorocarbons), used in refrigerators, air conditioners, aerosol sprays, and fire extinguishers, are the main cause of ozone depletion. UV radiation breaks down CFCs in the stratosphere, releasing chlorine atoms that catalyse the destruction of ozone molecules (one chlorine atom can destroy up to 100,000 ozone molecules).
- Consequences of ozone depletion: increased UV reaching Earth → skin cancer, cataracts, weakened immune systems in humans; DNA mutations; reduced phytoplankton (base of marine food chains); crop damage; harm to amphibians and other UV-sensitive organisms.
- Note: Ozone in the stratosphere is protective, but ozone at ground level (troposphere) is a harmful pollutant causing respiratory problems.
Montreal Protocol (1987)
- An international treaty signed in 1987 to phase out the production and consumption of ozone-depleting substances, particularly CFCs.
- Signed by 197 countries — the first treaty to achieve universal ratification.
- CFCs are being replaced by HCFCs (less harmful to ozone) and HFCs (ozone-safe).
- As a result, the ozone layer is showing early signs of recovery and is expected to return to pre-1980 levels by around 2050–2070.
Waste Management: The 3Rs
- Reduce: Minimise waste generation — use fewer disposables, avoid over-packaging, buy only what is needed.
- Reuse: Use items more than once — carry cloth bags instead of plastic bags, reuse containers, donate usable items.
- Recycle: Process waste materials into new products — recycle paper, glass, metals, and certain plastics.
- Always separate biodegradable and non-biodegradable waste at source to enable proper disposal and recycling.
- Composting: Biodegradable waste (kitchen scraps, garden waste) can be converted into nutrient-rich compost for soil.
- Biogas generation: Organic waste and animal dung can be used in biogas plants to produce methane for cooking and lighting, with the slurry serving as fertiliser.
- Incineration: Burning waste at high temperatures to reduce volume — but releases toxic gases and ash; not suitable for plastics or wet waste.
- Eutrophication: Excess nutrients (from fertiliser run-off or decomposing organic waste) enter water bodies → algal blooms → oxygen depletion → death of fish and other aquatic life.
Intext Questions
Page 257
Q1. What are trophic levels? Give an example of a food chain and state the different trophic levels in it.
Answer
Trophic levels are the specific positions that organisms occupy in a food chain based on their feeding relationship. Each step or level in a food chain is called a trophic level. Organisms occupying the same trophic level share the same source of nutrition.
Example — A grassland food chain:
Food chain: Grass (Producer, TL1) → Grasshopper (Primary Consumer, TL2) → Frog (Secondary Consumer, TL3) → Snake (Tertiary Consumer, TL4) → Eagle (Apex Predator, TL5)
| Trophic Level | Organism | Role |
|---|
| TL1 | Grass | Producer (autotroph) — makes food by photosynthesis |
| TL2 | Grasshopper | Primary consumer (herbivore) — eats the producer |
| TL3 | Frog | Secondary consumer (carnivore) — eats the herbivore |
| TL4 | Snake | Tertiary consumer (carnivore) — eats the secondary consumer |
| TL5 | Eagle | Apex predator (top carnivore) — eats the tertiary consumer |
Q2. What is the role of decomposers in the ecosystem?
Answer
Decomposers (bacteria and fungi) play an indispensable role in the ecosystem:
1. Nutrient cycling: They break down dead organic matter (dead plants, animals, excreta) into simple inorganic substances (CO₂, water, nitrogen, phosphorus, minerals) and return them to the soil and atmosphere. These nutrients are then absorbed by producers, completing the nutrient cycle. 2. Cleaning the environment: They act as natural scavengers, removing dead and decaying matter. Without them, dead bodies and waste would accumulate endlessly. 3. Maintaining ecosystem balance: By recycling materials, they ensure a continuous supply of raw materials for producers, keeping the ecosystem functioning.
Without decomposers, nutrients would remain locked in dead organic matter forever, producers would have no raw materials, and the entire ecosystem would collapse.
Page 261
Q1. What is ozone and how does it affect any ecosystem?
Answer
Ozone (O₃) is a molecule composed of three oxygen atoms. It is present mainly in the stratosphere (20–40 km above Earth), forming the ozone layer.
Effect on ecosystems:
- The ozone layer absorbs 97–99% of the Sun's harmful ultraviolet (UV-B and UV-C) radiation, acting as a protective shield for all life on Earth.
- Without the ozone layer, intense UV radiation would:
- Cause DNA mutations in all living organisms.
- Increase skin cancer and cataracts in humans.
- Destroy phytoplankton — the base of marine food chains — leading to collapse of aquatic ecosystems.
- Damage crops and forest cover, reducing agricultural productivity.
- Harm amphibian eggs and larvae, threatening amphibian populations.
- Depletion of the ozone layer (caused by CFCs) allows more UV to penetrate, threatening all ecosystems.
- At ground level (troposphere), ozone acts as a pollutant — it damages plant tissues and causes respiratory problems in humans and animals.
Q2. What is the better way to manage garbage — burning or dumping in open places? What are the better alternatives?
Answer
Neither burning nor open dumping is an acceptable method of waste disposal.
- Burning releases toxic gases (dioxins, furans, CO₂, particulate matter) causing air pollution and health problems (asthma, respiratory diseases).
- Open dumping leads to soil and water contamination, breeds disease vectors (flies, mosquitoes, rats), produces foul odours, and causes groundwater pollution through leachate.
Better alternatives (the 3Rs):
1. Reduce: Generate less waste — avoid disposable items, minimise packaging, use cloth bags instead of plastic. 2. Reuse: Use items repeatedly — refill containers, donate usable goods, repurpose old items. 3. Recycle: Process waste materials into new products — recycle paper, glass, metals, and certain plastics. 4. Segregate waste at source: Separate biodegradable and non-biodegradable waste. 5. Composting: Convert biodegradable waste into nutrient-rich compost for gardens and agriculture. 6. Biogas plants: Use organic waste and animal dung to produce biogas (methane) for cooking and fertiliser (slurry). 7. Sanitary landfills: For non-recyclable, non-biodegradable waste — lined and covered to prevent leachate contamination.
Chapter-End Exercise
Q1. Which of the following groups contain only biodegradable items?
Answer
(a) Grass, flowers and leather (b) Grass, wood and plastic (c) Fruit peels, cake and lime juice (d) Cake, wood and grass
Answer(c) Fruit peels, cake and lime juice
All three items in option (c) — fruit peels, cake, and lime juice — are biodegradable. They can be broken down by decomposers (bacteria, fungi) into simple substances.
Let us evaluate each option:
- (a) Grass, flowers and leather: All biodegradable. Grass and flowers are plant material; leather is animal skin that decomposes. This option is also correct but the NCERT expected answer is (c).
- (b) Grass, wood and plastic: Plastic is non-biodegradable — this group is eliminated.
- (c) Fruit peels, cake and lime juice: All biodegradable — correct answer.
- (d) Cake, wood and grass: All biodegradable, but the NCERT expected answer is (c).
Q2. Which of the following constitute a food chain?
Answer
(a) Grass, wheat and mango (b) Grass, goat and human (c) Goat, cow and elephant (d) Grass, fish and goat
Answer(b) Grass, goat and human
A food chain must show a linear sequence of who eats whom, starting from a producer and moving through consumers.
- (a) Grass, wheat and mango — All three are producers (plants). No consumer feeding relationship exists. Not a food chain.
- (b) Grass, goat and human — Grass (producer, TL1) → Goat (primary consumer/herbivore, TL2) → Human (secondary consumer, TL3). This is a valid food chain.
- (c) Goat, cow and elephant — All three are herbivores at the same trophic level. No one eats the other. Not a food chain.
- (d) Grass, fish and goat — Grass is a terrestrial producer; fish do not eat grass; goats do not eat fish. No valid feeding sequence. Not a food chain.
Q3. Which of the following are environment-friendly practices?
Answer
(a) Carrying cloth bags to put purchases in when going to the market (b) Switching off unnecessary lights and fans (c) Walking to school instead of getting your mother to drop you on her scooter (d) All of the above
Answer(d) All of the above
All three practices are environment-friendly:
- (a) Cloth bags: Reduce plastic bag usage — plastic bags are non-biodegradable and cause pollution. Cloth bags are reusable and biodegradable.
- (b) Switching off lights and fans: Conserves electricity — reducing energy consumption means less burning of fossil fuels in power plants, reducing CO₂ emissions and air pollution.
- (c) Walking to school: Reduces vehicular fuel consumption — less air pollution, less noise pollution, and conservation of fossil fuels.
Q4. What will happen if we kill all the organisms in one trophic level?
Answer
If all organisms in one trophic level are eliminated, it causes severe disruption to the entire ecosystem through cascading effects:
Impact on the trophic level below (their prey):
- Predators that kept the prey population in check are gone → prey population explodes (overpopulation).
- Overpopulation leads to overgrazing/overconsumption of the trophic level below the prey.
- This depletes resources (plants, forage), eventually causing the prey population itself to crash due to starvation and resource exhaustion.
- Example: If all frogs (TL3) are killed, insect populations (TL2) multiply unchecked, destroying grass (TL1), then insects themselves starve.
Impact on the trophic level above (their predators):
- Predators lose their food source → population declines sharply due to starvation.
- Predators may migrate, switch to alternative prey (disrupting other food chains), or go locally extinct.
- Example: If all frogs are killed, snakes (TL4) that depend on frogs decline or switch to eating other prey, causing imbalance in those food chains too.
Overall impact on the ecosystem:
- The food web is destabilised — interconnected food chains are disrupted.
- Biodiversity decreases as some species die out and others overpopulate.
- The ecosystem may shift to an entirely different, often degraded, state.
- Decomposers may be affected indirectly if the flow of dead organic matter changes drastically.
Q5. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the effect of removing producers be different from that of removing apex consumers?
Answer
Yes, the impact is very different depending on which trophic level is removed.
Removing producers (TL1):
- This is the most catastrophic disruption possible.
- Producers are the foundation of the entire ecosystem — all energy enters through them via photosynthesis.
- Without producers, all consumers at every trophic level lose their energy source — herbivores starve first, then carnivores, then top predators.
- Even decomposers are affected — without plant and animal matter eventually, their food supply dwindles.
- The ecosystem collapses entirely — it cannot sustain itself without producers.
- Example: If all grass and plants in a grassland are removed, every organism from grasshoppers to eagles perishes.
Removing apex consumers (top trophic level):
- Less immediately catastrophic but still damaging.
- The prey of the apex consumers (TL below) will have no predator → their population increases.
- This leads to overconsumption of the next lower trophic level, causing a trophic cascade — a ripple effect down through the food chain.
- Example: Removing tigers from a forest → deer population explodes → deer overgraze → grass and shrubs are depleted → soil erosion → other herbivores that depend on the same plants suffer.
- However, the ecosystem may persist in an altered, often less diverse state (as seen in areas where large predators have been eliminated).
Removing intermediate trophic levels (TL2 or TL3):
- Disrupts the chain both above and below simultaneously.
- Organisms above lose food → population declines. Organisms below lose predators → population increases.
- This can cause dramatic restructuring of the ecosystem.
Conclusion: Removing producers causes total ecosystem collapse. Removing apex consumers causes trophic cascades but the ecosystem may survive in a degraded form. Removing intermediate levels causes bidirectional disruption. The lower the trophic level removed, the more severe the impact.
Q6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?
Answer
Biological magnification (biomagnification) is the progressive increase in the concentration of a persistent, non-biodegradable toxic substance as it passes from one trophic level to the next in a food chain.
Yes, the levels of magnification are different at different trophic levels — the concentration increases at each successive level.
Why this happens:
- The toxic substance (e.g., DDT, mercury, PCBs) is non-biodegradable — it does not break down in the body.
- It is fat-soluble — it accumulates in the fatty tissues of organisms and is not excreted.
- Organisms at each trophic level consume many organisms from the level below, absorbing all the toxin stored in each one.
- Since only ~10% of energy transfers to the next level, organisms at higher levels must eat far more biomass, accumulating proportionally more toxin.
DDT biomagnification example with specific numbers:
| Trophic Level | Organism | DDT Concentration |
|---|
| Water (environment) | Lake water | 0.003 ppb |
| TL1 | Phytoplankton | 0.04 ppb (~13x water) |
| TL2 | Zooplankton | 0.5 ppb (~167x water) |
| TL3 | Small fish | 2.0 ppb (~667x water) |
| TL4 | Large fish | 10–15 ppb (~3,300–5,000x water) |
| TL5 | Fish-eating birds | 25+ ppb (~8,300x water) |
- A fish-eating bird at TL5 has DDT levels over 800 times the original concentration in water.
- This high DDT concentration causes eggshell thinning in birds — shells break during incubation, leading to population crashes (as observed in bald eagles and pelicans).
- Humans at the top of food chains (eating fish, meat, dairy) accumulate the highest concentrations of these toxins.
Q7. What are the problems caused by the non-biodegradable wastes that we generate?
Answer
Non-biodegradable wastes cause serious environmental problems:
1. Soil pollution: Plastics, glass, metals, and chemicals accumulate in soil, reducing its fertility, blocking water infiltration, and making land unfit for agriculture. 2. Water pollution: Non-biodegradable waste in rivers, lakes, and oceans harms aquatic life. Plastics break into microplastics that enter the food chain. Industrial chemicals (heavy metals, dyes) contaminate drinking water. 3. Biomagnification: Persistent toxins like DDT, mercury, and PCBs accumulate up the food chain, reaching dangerous concentrations in top consumers including humans. 4. Harm to wildlife: Animals ingest plastic bags and mistaking them for food, causing intestinal blockage, starvation, and death. Marine animals get entangled in plastic debris (six-pack rings, fishing nets). 5. Extremely long persistence: Plastics take hundreds to thousands of years to degrade. A plastic bottle takes ~450 years; a plastic bag takes ~20 years; glass takes millions of years. They remain in the environment across generations. 6. Air pollution: Burning plastics and other non-biodegradable waste releases highly toxic gases — dioxins, furans, carbon monoxide, and heavy metal vapours — causing respiratory diseases and cancer. 7. Drainage blockage and flooding: Plastic bags and waste clog drains and sewers, causing waterlogging and urban flooding during rains. 8. Resource depletion: Materials like metals, glass, and some plastics that could be recycled are lost when dumped, wasting finite natural resources and the energy used to manufacture them. 9. Aesthetic degradation: Littered non-biodegradable waste makes surroundings ugly and unhygienic, affecting tourism and quality of life. 10. Groundwater contamination: Toxic chemicals from dumped waste (batteries, e-waste, paints) leach into groundwater, making it unsafe for drinking and irrigation.
Q8. If all the waste we generate is biodegradable, will this have no impact on the environment?
Answer
No — even if all waste is biodegradable, it can still have a significant impact on the environment if not managed properly.
1. Methane production: When large quantities of biodegradable waste decompose in landfills (anaerobic conditions), they produce methane (CH₄) — a greenhouse gas 25 times more potent than CO₂. This contributes to global warming and climate change. 2. Eutrophication: When biodegradable waste (food waste, agricultural run-off, sewage) enters water bodies, it provides excess nutrients → algal blooms → oxygen depletion → death of fish and other aquatic organisms. This process is called eutrophication. 3. Leachate contamination: Decomposing organic waste produces a liquid called leachate that can seep into groundwater and contaminate drinking water sources with harmful bacteria and chemicals. 4. Overwhelming decomposers: If waste generation exceeds the capacity of decomposers to process it, organic matter accumulates, causing foul odours, attracting pests, and creating unhygienic conditions. 5. Disease vectors: Rotting biodegradable waste attracts flies, mosquitoes, rats, and other disease-carrying organisms, increasing the risk of diseases like malaria, dengue, typhoid, and cholera. 6. Land use: Large amounts of waste require vast land for disposal — even biodegradable waste occupies space and degrades land quality. 7. Resource waste: Biodegradable waste contains nutrients that could be recovered through composting or biogas generation. Dumping them wastes these resources.
Conclusion: Biodegradable waste must also be managed properly — through composting, biogas generation, vermicomposting, and sanitary disposal — to minimise its environmental impact.
Q9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?
Answer
Why ozone layer damage is a concern:
1. The ozone layer absorbs 97–99% of the Sun's harmful UV-B and UV-C radiation, shielding all life on Earth. 2. Depletion of the ozone layer allows more UV radiation to reach Earth's surface, causing:
3. CFCs (chlorofluorocarbons) — used in refrigerators, air conditioners, aerosol sprays, and fire extinguishers — are the primary cause of ozone depletion. When CFCs reach the stratosphere, UV radiation breaks them down, releasing chlorine atoms. Each chlorine atom can destroy up to 100,000 ozone molecules, causing massive thinning of the ozone layer. 4. The ozone hole over Antarctica was discovered in the 1980s, showing severe depletion and raising global alarm.
- Skin cancer (melanoma and non-melanoma) in humans.
- Cataracts (clouding of the eye lens) leading to blindness.
- Weakened immune systems, making humans more susceptible to infections.
- DNA mutations in all living organisms.
- Damage to phytoplankton — the base of marine food chains — causing collapse of ocean ecosystems and reducing carbon dioxide absorption.
- Reduced crop yields — UV damages plant DNA and reduces photosynthesis.
- Harm to amphibians — UV damages their eggs and larvae, contributing to global amphibian decline.
Steps being taken to limit the damage:
1. Montreal Protocol (1987): An international treaty signed by 197 countries (the first to achieve universal ratification) committing to phase out the production and consumption of ozone-depleting substances, especially CFCs. This is the most important step taken. 2. Replacement with safer alternatives: CFCs are being replaced with HCFCs (hydrochlorofluorocarbons — less harmful to ozone) and HFCs (hydrofluorocarbons — ozone-safe, though some are potent greenhouse gases). 3. Ban on CFC-based products: Many countries have banned CFCs in aerosol sprays, refrigerants, and foam-blowing agents. 4. Kigali Amendment (2016): An extension of the Montreal Protocol to phase down HFCs (which, while ozone-safe, are potent greenhouse gases). 5. Research and monitoring: Continuous satellite monitoring of ozone levels globally (NASA, WMO) to track recovery and detect new threats. 6. Public awareness: Campaigns to educate people about avoiding products containing ozone-depleting substances and promoting eco-friendly alternatives. 7. Results: The ozone layer is showing early signs of recovery. It is projected to return to pre-1980 levels by approximately 2050–2070 if current measures continue.
Common Mistakes to Avoid
1. Confusing food chain with food web: A food chain is a single linear sequence; a food web is a network of many interconnected food chains. Do not use them interchangeably. An ecosystem always has a food web, not just one food chain.
2. Placing decomposers at a specific trophic level: Decomposers process dead matter from ALL trophic levels — they do not sit at one fixed level. They are not TL1, TL2, or any other single level.
3. Forgetting the 10% energy transfer rule: Only 10% of energy transfers between trophic levels. Many students write that "energy is transferred" without specifying how much — the 10% figure is important for board exams. Also remember this explains why food chains are short (4–5 levels max).
4. Confusing biodegradable with compostable: All compostable items are biodegradable, but not all biodegradable items are easily compostable. Composting requires specific conditions (moisture, aeration, temperature). "Biodegradable" just means it can eventually be broken down by microbes.
5. Wrong direction for biomagnification: Biomagnification increases UP the food chain (from producers to top consumers), not down. Some students write that toxins "spread through the ecosystem" without specifying that concentration is highest at the top. Always state: concentration increases at each successive trophic level.
6. Confusing ozone layer depletion with global warming: These are separate environmental problems. Ozone depletion is caused by CFCs and increases UV radiation. Global warming is caused by greenhouse gases (CO₂, methane) and traps heat. CFCs do contribute to both, but the mechanisms are different. Do not mix them up in your answers.
7. Attributing ozone depletion to CO₂: CO₂ causes global warming, NOT ozone depletion. Ozone depletion is caused by CFCs (chlorofluorocarbons). This is a very common error in board exams.
8. Thinking biodegradable waste has zero environmental impact: Biodegradable waste still causes problems — methane production (global warming), eutrophication (water pollution), disease vectors, and leachate contamination. It must be managed through composting, biogas, and proper disposal.
9. Writing "all organisms in one trophic level die" without explaining the cascade: When asked what happens if a trophic level is removed, you must explain BOTH the effect on the level above (starvation/decline) AND the level below (overpopulation/overconsumption). One-sided answers lose marks.
10. Not giving specific examples in food chain questions: Always provide a named example food chain (e.g., Grass → Grasshopper → Frog → Snake → Eagle) when asked about trophic levels. Abstract answers without examples score lower.
Exam Checklist
Before writing your exam answers, verify these points:
- [ ] Can I define ecosystem, biotic, and abiotic components with examples?
- [ ] Can I draw a food chain with at least 4 trophic levels, labelling each TL and the organism's role?
- [ ] Can I explain the 10% energy rule and why food chains are short?
- [ ] Can I distinguish between food chain (linear) and food web (network)?
- [ ] Can I list at least 4 examples each of biodegradable and non-biodegradable substances?
- [ ] Can I explain biomagnification with a numerical example (DDT)?
- [ ] Can I explain the role of decomposers in nutrient cycling?
- [ ] Can I describe how ozone is formed, why it is important, and what depletes it (CFCs)?
- [ ] Can I name the Montreal Protocol (1987) and its purpose?
- [ ] Can I explain why even biodegradable waste causes environmental problems?
Internal Links
- [Chapter 15 Guide — Our Environment](/school/cbse/class/10/science/cbse-our-environment)
- [All Class 10 Science Solutions](/school/cbse/class/10/science/solutions)
- [Class 10 Science Subject Guide](/school/cbse/class/10/science)
