Ecosystems
Core framework
This domain tests ecosystem structure and function: trophic organisation, productivity, energy flow, nutrient cycles, succession and ecosystem services.
Core concepts
- Structure: abiotic components plus producers, consumers (primary, secondary, tertiary) and decomposers.
- Productivity: GPP is the total organic matter fixed by producers; NPP = GPP − respiration by producers. NPP is the energy available to consumers.
- Energy flow is one-way; pyramids of energy are always upright. Pyramids of number and biomass can be inverted, for example many insects on one tree, or phytoplankton biomass less than zooplankton biomass at a moment in time.
- Nutrient cycles are gaseous (carbon, nitrogen) or sedimentary (phosphorus, sulphur). Phosphorus has no significant gaseous phase.
- Succession is primary or secondary; hydrarch runs from wet to dry, xerarch from dry to wet; pioneer → seral → climax.
- Ecosystem services: provisioning, regulating, supporting and cultural.
Worked example
In a pond, GPP is 5,000 kcal m⁻² yr⁻¹ and plant respiration is 1,500. NPP = 5,000 − 1,500 = 3,500 kcal m⁻² yr⁻¹. If consumers use 10% of NPP, energy at the primary consumer level is about 350 kcal. NPP, not GPP, is the base available to the next trophic level.
Traps
- Using GPP instead of NPP in food-chain calculations.
- Assuming all ecological pyramids are upright.
- Saying phosphorus cycles through the atmosphere.
- Confusing primary succession on bare rock with secondary succession after a disturbance that leaves soil.