Ecosystem and pollution questions are among the most predictable in general science, because they turn on a small set of definitions and abbreviations. What is a trophic level, what BOD measures, which gas causes which problem, which pollutant causes which disease. None of it changes. This post covers the ecosystem structure, the energy rules, the pollutants and their effects, the major environmental terms, and 15 previous year questions.
Key Points (Quick Revision)
- 10 per cent law — only about a tenth of energy passes to the next trophic level
- BOD measures organic pollution in water; high BOD means more polluted
- Chlorofluorocarbons deplete the ozone layer; carbon dioxide and methane cause global warming
- Minamata disease — mercury · Itai-Itai disease — cadmium · Blue baby syndrome — nitrates
- Ozone layer is in the stratosphere; ozone at ground level is a pollutant
1. Structure of an ecosystem
An ecosystem is a community of living organisms together with the non-living environment they interact with. Its components are asked directly.
| Component | What it includes |
|---|---|
| Abiotic | Non-living factors — light, temperature, water, soil, air, minerals |
| Producers or autotrophs | Green plants and algae, which make their own food by photosynthesis |
| Consumers or heterotrophs | Herbivores, carnivores and omnivores |
| Decomposers or saprotrophs | Bacteria and fungi, which break down dead matter |
The trophic levels follow from that. Producers occupy the first level, herbivores or primary consumers the second, small carnivores the third, and top carnivores the fourth. A food chain is one such sequence; a food web is many interconnected chains, which is what actually exists in nature.
The 10 per cent law, associated with Lindeman, is the most asked fact in this section: only about ten per cent of the energy at one trophic level is transferred to the next, the rest being lost mainly as heat. That is why food chains are short and why there are far fewer top carnivores than plants.
The consequence is the ecological pyramid. The pyramid of energy is always upright, which is a favourite question, because energy always decreases upwards. The pyramid of numbers and the pyramid of biomass can be inverted — a single large tree supporting many insects inverts the pyramid of numbers.
2. Air pollution and the gases involved
| Pollutant | Main source | Chief effect |
|---|---|---|
| Carbon monoxide | Incomplete combustion, vehicles | Binds to haemoglobin, reducing oxygen transport |
| Carbon dioxide | Burning of fossil fuels | Principal greenhouse gas |
| Sulphur dioxide | Coal burning, smelting | Acid rain, respiratory illness |
| Oxides of nitrogen | Vehicles, power plants | Acid rain, photochemical smog |
| Chlorofluorocarbons | Refrigerants, aerosols | Ozone layer depletion |
| Methane | Paddy fields, livestock, landfills | Greenhouse gas, far more potent than carbon dioxide per unit |
| Particulate matter, PM2.5 and PM10 | Vehicles, construction, burning | Respiratory and cardiovascular disease |
| Ground-level ozone | Formed by reaction of nitrogen oxides and hydrocarbons in sunlight | Component of photochemical smog |
The ozone distinction is the most common trap in this topic. Ozone in the stratosphere is protective, absorbing ultraviolet radiation, and its depletion by chlorofluorocarbons is the problem. Ozone at ground level is itself a pollutant and a component of smog. A question asking whether ozone is good or bad is testing whether you noticed which layer it named.
Keep ozone depletion and global warming apart, since candidates routinely merge them. Chlorofluorocarbons deplete ozone; carbon dioxide, methane, nitrous oxide and water vapour drive the greenhouse effect. The Montreal Protocol deals with ozone-depleting substances and the Kyoto Protocol and Paris Agreement deal with greenhouse gases.
3. Water pollution and its measures
| Term | What it means |
|---|---|
| BOD, Biochemical Oxygen Demand | Oxygen needed by microorganisms to break down organic matter. Higher BOD means more polluted water |
| COD, Chemical Oxygen Demand | Oxygen needed to oxidise both organic and inorganic matter; always higher than BOD for the same sample |
| Dissolved Oxygen | Oxygen available to aquatic life. Lower DO means more polluted water |
| Eutrophication | Nutrient enrichment, usually by nitrates and phosphates, causing algal bloom and oxygen depletion |
| Biomagnification | Increasing concentration of a persistent pollutant at successive trophic levels — DDT is the standard example |
BOD and DO move in opposite directions, and that inverse relationship is the exact question. High BOD with low dissolved oxygen indicates heavily polluted water; clean water has low BOD and high DO.
Biomagnification explains why top predators carry the highest pesticide loads even when the water contains very little — each trophic level concentrates what the level below it accumulated. DDT and mercury are the examples asked.
4. Pollutants and the diseases they cause
| Disease | Caused by |
|---|---|
| Minamata disease | Mercury poisoning — affects the nervous system |
| Itai-Itai disease | Cadmium poisoning — affects bones and kidneys |
| Blue baby syndrome, methaemoglobinaemia | Nitrates in drinking water |
| Fluorosis | Excess fluoride in drinking water — affects teeth and bones |
| Black foot disease | Arsenic in drinking water |
| Silicosis | Silica dust, an occupational disease |
| Asbestosis | Asbestos fibres |
| Byssinosis | Cotton dust |
Minamata with mercury and Itai-Itai with cadmium are the two asked most, and they are placed as options against each other constantly. The way to hold them apart is by what they attack — mercury attacks the nervous system, cadmium attacks the bones. The Minamata Convention deals with mercury, which is a linked current-affairs question.
Blue baby syndrome with nitrates and fluorosis with fluoride are the two drinking-water answers, and both are asked in the water-quality context rather than as medical questions.
5. Environmental terms and bodies
| Term or body | What it is |
|---|---|
| Greenhouse effect | Trapping of infrared radiation by atmospheric gases, warming the surface |
| Acid rain | Rain made acidic by sulphur dioxide and nitrogen oxides, damaging monuments, soil and water bodies |
| Photochemical smog | Smog formed by sunlight acting on nitrogen oxides and hydrocarbons |
| Carbon footprint | Total greenhouse gases attributable to an activity or entity |
| Ecological succession | Gradual change in a community over time until a stable climax stage |
| Keystone species | A species whose removal disproportionately changes the ecosystem |
| Central Pollution Control Board | Statutory body under the Water Act; monitors and regulates pollution |
| National Green Tribunal | Set up in 2010 for effective disposal of environmental cases |
| Air Quality Index | Composite indicator of ambient air quality across defined categories |
The Taj Mahal and acid rain pairing is asked as its own question — sulphur dioxide from nearby sources reacting with marble, which is why the Taj Trapezium Zone was created. And the National Green Tribunal was established in 2010, which is the fact usually tested rather than its powers.
Air quality standards, index categories and pollution norms are revised periodically, so treat any numerical threshold as something to verify against a current source rather than as a fixed fact.
6. Previous year questions
- What percentage of energy passes to the next trophic level? — About 10 per cent
- Which ecological pyramid is always upright? — The pyramid of energy
- Green plants in an ecosystem are called what? — Producers or autotrophs
- Bacteria and fungi that break down dead matter are called what? — Decomposers
- What does BOD stand for and what does high BOD indicate? — Biochemical Oxygen Demand; more polluted water
- Which gas is chiefly responsible for ozone layer depletion? — Chlorofluorocarbons
- In which layer of the atmosphere is the ozone layer found? — Stratosphere
- Which protocol deals with ozone-depleting substances? — The Montreal Protocol
- Minamata disease is caused by which metal? — Mercury
- Itai-Itai disease is caused by which metal? — Cadmium
- Blue baby syndrome is caused by which pollutant in water? — Nitrates
- Excess fluoride in drinking water causes what? — Fluorosis
- Which gas binds with haemoglobin and reduces oxygen transport? — Carbon monoxide
- Nutrient enrichment causing algal bloom is called what? — Eutrophication
- In which year was the National Green Tribunal established? — 2010
7. How to revise this topic
Learn this topic as four short cards rather than as a chapter. Card one is the ecosystem structure and the 10 per cent law. Card two is the gas-to-effect table, with the ozone and greenhouse distinction kept explicit. Card three is BOD, COD, DO and eutrophication. Card four is the pollutant-to-disease list.
Then fix the four traps that account for most of the lost marks. Stratospheric ozone protects while ground-level ozone pollutes. Ozone depletion and global warming are different problems with different gases and different treaties. High BOD and low dissolved oxygen both mean polluted water. And mercury attacks nerves while cadmium attacks bones.
Everything in the first three cards is permanent. The only parts that move are air quality standards, pollution norms and any numerical threshold, so check those against a current source rather than memorising a figure from an old book.
8. Frequently Asked Questions
What is the 10 per cent law in an ecosystem?
Only about ten per cent of the energy available at one trophic level is transferred to the next, the rest being lost mainly as heat. It is why food chains are short and why top carnivores are far fewer than plants. The law is associated with Lindeman.
What is the difference between BOD and dissolved oxygen?
BOD is the oxygen microorganisms need to break down organic matter, so a high BOD means heavy organic pollution. Dissolved oxygen is the oxygen actually available to aquatic life, so a low value means polluted water. The two move in opposite directions.
Is ozone good or bad?
Both, depending on where it is. In the stratosphere it forms the protective layer that absorbs ultraviolet radiation, and its depletion by chlorofluorocarbons is a problem. At ground level it is itself a pollutant and part of photochemical smog.
Which diseases are caused by mercury and cadmium?
Mercury causes Minamata disease, which affects the nervous system, and cadmium causes Itai-Itai disease, which affects the bones and kidneys. They are placed as options against each other, so remember mercury for nerves and cadmium for bones.
What is biomagnification?
The increasing concentration of a persistent pollutant at each successive trophic level, so that top predators carry the highest load even when the surrounding water contains very little. DDT and mercury are the standard examples.
For more General Science topics in simple language, see our Atmosphere: Layers and Composition and Classification of Living Organisms posts, or browse the General Science section. Preparing for a specific exam? Start with the SSC CGL guide or join the 100 Hour GS Course.