Ecology
What this domain tests
This section checks levels of ecological organisation, ecosystem components, food chains and webs, energy flow and the ten-percent law, nutrient cycles, ecological pyramids and species interactions.
Core rules
- Levels: organism → population → community → ecosystem → biome → biosphere.
- Ecosystem = biotic community + abiotic factors (light, temperature, water, soil, pH).
- Producers fix energy; consumers are primary, secondary or tertiary; decomposers break down dead matter and recycle nutrients.
- Energy flow is unidirectional and lost as heat at each transfer; only about 10% passes to the next trophic level (ten-percent law).
- Pyramids of number and biomass can be inverted (a single tree supports many insects); the pyramid of energy is always upright.
- Nutrients cycle: carbon via photosynthesis, respiration, combustion and decomposition; nitrogen via Rhizobium, Nitrosomonas, Nitrobacter and denitrifying Pseudomonas.
- Productivity: gross primary productivity minus respiration equals net primary productivity.
- Interactions: mutualism, commensalism, parasitism, predation, competition. Ecological succession moves a community towards climax.
Worked example
A grassland producer level holds 10,000 kJ. Applying the ten-percent law: primary consumers receive 1,000 kJ, secondary 100 kJ, tertiary 10 kJ. Reasoning: at each step roughly 90% is lost as heat in respiration and as undigested waste, so top predators need very large areas and cannot support big populations - the reason food chains rarely exceed four or five links.
Traps that cost marks
Saying energy cycles (it flows one way; nutrients cycle); assuming every pyramid can be inverted; counting decomposers as consumers; forgetting that respiration is subtracted when calculating net productivity.