Chapter 15 — Ecological Issues
Key Concepts Quick Recap
| Term | Definition |
|---|
| Greenhouse effect | Trapping of heat in atmosphere by greenhouse gases |
| Global warming | Rise in average Earth temperature due to enhanced greenhouse effect |
| Ozone depletion | Reduction of stratospheric ozone layer by CFCs and other compounds |
| Acid rain | Rain with pH < 5.6 due to SO₂ and NOₓ dissolving in atmospheric water |
| Eutrophication | Excess nutrients in water bodies → algal bloom → O₂ depletion → fish death |
| Biomagnification | Increase in concentration of toxic substances at successive trophic levels |
| Deforestation | Removal of forest cover for agriculture, timber, urbanisation |
| Afforestation | Planting trees in areas that were not previously forested |
| Reforestation | Replanting trees in deforested areas |
| BOD | Biological Oxygen Demand — O₂ needed to decompose organic matter in water; indicator of pollution |
I. Choose the Best Answer (MCQs)
1. Which gas is primarily responsible for the greenhouse effect? (a) Oxygen (b) Nitrogen (c) Carbon dioxide (d) Argon
Answer(c) Carbon dioxide CO₂ is the most significant greenhouse gas by contribution to climate change (60% of enhanced greenhouse effect). Others: CH₄ (methane), N₂O (nitrous oxide), CFCs, water vapour. CO₂ absorbs and re-emits infrared radiation → traps heat → greenhouse effect.
2. The Montreal Protocol (1987) deals with: (a) Reduction of CO₂ emissions (b) Protection of wetlands (c) Phase-out of ozone-depleting substances (CFCs) (d) Control of acid rain
Answer(c) Phase-out of ozone-depleting substances (CFCs) The Montreal Protocol is an international treaty to phase out substances that deplete the ozone layer — primarily CFCs (Freons) used in refrigerators, aerosols, and foam. It is considered one of the most successful international environmental agreements — the ozone hole over Antarctica has been slowly recovering.
3. Acid rain is caused by the emission of: (a) CO₂ and CH₄ (b) SO₂ and NOₓ (c) CFCs (d) CO only
Answer(b) SO₂ and NOₓ SO₂ (from burning coal and petrol) + water → H₂SO₄ (sulphuric acid). NOₓ (from vehicles and industry) + water → HNO₃ (nitric acid). Rain with pH < 5.6 = acid rain. Effects: damages monuments (Taj Mahal), corrodes metals, kills aquatic life, damages forests.
4. Biomagnification refers to: (a) Increase in population (b) Progressive increase in concentration of non-biodegradable pollutants at successive trophic levels (c) Magnification of microbes (d) Increase in biodiversity
Answer(b) Progressive increase in concentration of non-biodegradable pollutants at successive trophic levels Non-biodegradable toxins (DDT, heavy metals, PCBs) are not excreted — they accumulate in fat tissues. At each trophic level, organisms consume many of the level below → toxin concentrates. Highest concentration: top predators (eagles, dolphins, humans). Example: DDT concentration was 25 ppm in fish-eating birds vs. 0.003 ppb in water.
5. Eutrophication in water bodies is mainly caused by: (a) Excess CO₂ (b) Excess nitrogen and phosphorus (nutrients) (c) Heavy metal pollution (d) Acid rain
Answer(b) Excess nitrogen and phosphorus Eutrophication: excess nutrients (N, P from agricultural run-off, sewage) → algal bloom → algae die → decomposition by bacteria → O₂ consumed → dissolved O₂ drops (hypoxia) → fish and aquatic life die → "dead zones."
6. BOD (Biological Oxygen Demand) is used as an indicator of: (a) Air pollution (b) Soil pollution (c) Water pollution (d) Noise pollution
Answer(c) Water pollution BOD measures the amount of dissolved oxygen consumed by microorganisms to decompose organic matter in water over 5 days at 20°C. High BOD = high organic pollution = low dissolved O₂ = poor water quality. Clean water: BOD < 2 mg/L. Heavily polluted water: BOD > 10 mg/L.
7. Which of the following is an example of in situ ex situ conservation of forest genetic resources? (a) National park (b) Seed bank (c) Zoo (d) Botanical garden
Answer(a) National park National parks are in situ conservation — protection of species in natural habitats. Seed banks (ex situ), Zoos (ex situ), Botanical gardens (ex situ) — all conserve outside natural habitat.
II. Short Answer Questions
Q1. What is global warming? What are its consequences?
Answer
Global warming is the gradual increase in Earth's average surface temperature due to the enhanced greenhouse effect caused by human activities.
Causes:
- Burning fossil fuels → CO₂ and SO₂.
- Deforestation → reduced CO₂ absorption.
- Agriculture → CH₄ (cattle, rice paddies) and N₂O (fertilisers).
- Industry → CFCs, HFCs.
Consequences: 1. Polar ice melting → sea level rise → flooding of coastal cities (Mumbai, Kolkata at risk). 2. Extreme weather events: More intense cyclones, droughts, floods. 3. Shift in seasons: Disrupted agriculture (crop failures). 4. Coral bleaching: Warm water kills coral symbiotic algae (zooxanthellae). 5. Species migration and extinction: Range shifts, some species unable to adapt. 6. Spread of tropical diseases: Malaria, dengue spreading to higher altitudes.
Q2. What is ozone depletion? How does it affect living organisms?
Answer
The ozone layer (stratosphere, 15-50 km altitude) absorbs 97-99% of the Sun's harmful UV-B and UV-C radiation.
Cause of depletion: CFCs (chlorofluorocarbons) → UV photolysis → Cl radicals:
CCl₃F → •CClF + Cl•
Cl• + O₃ → ClO• + O₂
ClO• + O → Cl• + O₂
One Cl atom destroys ~100,000 O₃ molecules before being inactivated.
Effects of UV-B increase: 1. On humans: Increased skin cancer (melanoma), cataracts, immune suppression. 2. On plants: Reduced photosynthesis, chlorophyll destruction, stunted growth. 3. On aquatic ecosystems: Damages phytoplankton, zooplankton → disrupts marine food chain. 4. Materials: Degradation of plastics, rubber, paints.
SolutionMontreal Protocol (1987) — phased out CFCs; replaced by HFCs (though these are greenhouse gases).
Q3. What are the types and effects of water pollution?
Answer
Water pollution is contamination of water bodies (rivers, lakes, groundwater, oceans) making them harmful to humans and aquatic life.
Types and sources:
| Type | Source | Effects |
|---|
| Sewage/domestic | Household waste, toilets | Pathogens, eutrophication, BOD increase |
| Industrial effluents | Factories, chemical plants | Heavy metals (Hg, Pb, Cd), toxic chemicals |
| Agricultural run-off | Pesticides, fertilisers | Eutrophication, pesticide accumulation |
| Oil spills | Tankers, oil wells | Kills marine life, coats birds/marine mammals |
| Thermal | Power plant cooling water | Lowers dissolved O₂, kills fish |
| Radioactive | Nuclear plants | Long-term genetic damage |
Key indicator: BOD (Biological Oxygen Demand) — higher BOD = more pollution.
Mercury poisoning example: Minamata disease (Japan, 1950s) — industrial mercury contaminated fish → people who ate fish developed neurological disorders.
III. Long Answer
Q1. Describe the causes and effects of air pollution and measures to control it.
Answer
Air pollution = presence of harmful substances in the atmosphere at concentrations that affect health and environment.
Sources:
- Vehicles (CO, NOₓ, hydrocarbons, particulates, lead from leaded petrol).
- Industries (SO₂, fly ash, heavy metals).
- Burning of biomass/fossil fuels (CO₂, CH₄, particulates).
- Construction (dust, PM10, PM2.5).
- Agriculture (NH₃ from fertilisers, pesticide vapours).
Effects:
- On human health: Respiratory diseases (asthma, bronchitis), lung cancer, cardiovascular disease. PM2.5 (fine particles) most dangerous — enters bloodstream.
- On plants: Leaf damage, reduced photosynthesis, premature leaf drop.
- Acid rain: pH < 5.6; damages monuments, aquatic ecosystems, forests.
- Global warming: CO₂, CH₄, N₂O trap heat.
- Smog: Photochemical smog (NOₓ + VOCs + sunlight → ozone + PAN) irritates eyes and lungs.
Control measures: 1. Catalytic converters in vehicles: convert CO, NOₓ → CO₂, N₂ (less harmful). 2. CNG and electric vehicles to replace petrol/diesel. 3. Industrial filters — electrostatic precipitators remove particulates from emissions. 4. Renewable energy — solar, wind replace coal. 5. Afforestation — trees absorb CO₂, filter particulates. 6. Legislation: Air (Prevention and Control of Pollution) Act, 1981; National Clean Air Programme (NCAP).
Common Mistakes
- Confusing ozone in the troposphere (pollutant) and stratosphere (beneficial UV shield).
- Saying CO₂ alone causes ozone depletion — CFCs cause ozone depletion; CO₂ causes global warming.
- Confusing BOD (water pollution) with biomagnification (food chain pollutant accumulation).
- Saying acid rain is caused by CO₂ — it is caused by SO₂ and NOₓ.
Exam Checklist
- Greenhouse gases: CO₂, CH₄, N₂O, CFCs.
- Montreal Protocol (1987): CFC phase-out for ozone layer protection.
- Acid rain: SO₂ + NOₓ + H₂O → H₂SO₄ + HNO₃; pH < 5.6.
- Eutrophication: excess N, P → algal bloom → O₂ depletion → fish death.
- Biomagnification: non-biodegradable toxins increase at each trophic level; top predators most affected.
- BOD: high BOD = high organic pollution = low O₂.
- Ozone depletion: Cl from CFCs → catalytic destruction of O₃.
