Our Environment
What the exam tests
Our Environment tests ecosystem structure, food chains and webs, trophic levels, energy flow, biomagnification, ozone depletion and waste management. The 10% law and the effect of removing one trophic level are frequently asked.
Core rules
- An ecosystem has biotic components (producers, consumers, decomposers) and abiotic components (light, temperature, water, soil).
- Producers make food by photosynthesis; consumers eat other organisms; decomposers break down dead matter and return nutrients.
- A food chain shows one path of energy; a food web shows interconnected chains.
- Trophic levels are feeding positions. Energy decreases at each step; about 10% is transferred, the rest is lost as heat and used in life processes.
- Biomagnification is the increase in concentration of non-biodegradable pollutants at higher trophic levels.
- Ozone in the stratosphere absorbs ultraviolet radiation. CFCs break ozone down; the Montreal Protocol restricts them.
- Waste management follows reduce, reuse and recycle. Biodegradable waste can be composted; non-biodegradable waste needs careful disposal.
Worked example
Grass → grasshopper → frog → snake. If the grass contains 10,000 J, the grasshopper receives about 1,000 J, the frog about 100 J and the snake about 10 J. This follows the 10% law. It explains why food chains rarely have more than four or five trophic levels: too little energy remains to support another level.
Traps that cost marks
- Adding energy at each trophic level instead of reducing it.
- Confusing biomagnification with bioaccumulation; biomagnification is the increase along the chain.
- Saying decomposers are a trophic level of consumers; they are separate.
- Blaming CO2 for ozone depletion; CFCs are the main cause.