Ecosystems and ecology
Domain 2 tests ecology as a quantitative and conceptual toolkit: energy flow, nutrient cycling, population dynamics, succession and limiting factors. Expect calculations of productivity and efficiency and interpretation of pyramids.
Core rules
- Energy flows one way; nutrients cycle. The first law says energy is conserved; the second says each transfer loses useful energy as heat.
- GPP is total photosynthesis; NPP = GPP minus respiration. NPP is the energy available to consumers and decomposers.
- Ecological efficiency is typically about 10% between trophic levels, so biomass and energy pyramids usually taper.
- Liebig's law: growth is limited by the scarcest resource. Shelford's law: too much or too little of a factor can limit growth.
- Succession changes species composition and usually increases biomass and soil development, but not always diversity.
Worked example
A grassland has GPP = 24,000 kJ m⁻² yr⁻¹ and plant respiration = 14,000. NPP = 10,000 kJ m⁻² yr⁻¹. Primary consumers assimilate 900 kJ m⁻² yr⁻¹. Efficiency = 900 ÷ 10,000 × 100 = 9%. Reasoning: use NPP, not GPP, as the denominator for plant-to-herbivore efficiency; the rest is lost as heat, unassimilated material and waste. If a pyramid of numbers shows many insects on one tree, it can invert, so use energy or biomass where appropriate.
Traps
- Using GPP when NPP is required.
- Claiming energy is recycled; only nutrients cycle.
- Assuming all pyramids must be upright.
- Confusing primary succession (no soil) with secondary succession (soil present).
- Ignoring decomposers, which return nutrients but do not form a trophic level in the same way.