Systems and Cycles in the Environment
Systems and Cycles in the Environment
This domain examines ecosystems as interacting systems: populations, communities, habitats, food webs and the cycles that move matter and energy. Questions often require you to trace energy through trophic levels, interpret a pyramid, or explain a human impact on a cycle.
Core rules and concepts
- Energy enters most ecosystems as sunlight captured by producers and flows one way. At each transfer only about 10% is passed on as biomass; the rest is lost as heat through respiration, plus uneaten parts and excretion. Matter, unlike energy, is recycled.
- Food-chain arrows show the direction of energy flow: grass → grasshopper → lizard → hawk. Decomposers break down dead matter and return nutrients such as nitrates and phosphates to the soil.
- The carbon cycle involves photosynthesis, respiration, combustion, decomposition and fossilisation. The nitrogen cycle depends on nitrogen-fixing bacteria (including Rhizobium in root nodules), nitrifying bacteria, denitrifying bacteria and decomposers.
- Populations are limited by food, space, disease, predation and abiotic factors. Human activities - deforestation, galamsey siltation, bush burning, overfishing - alter these limits and can reduce biodiversity.
Worked example
A grassland receives 50,000 kJ m⁻² yr⁻¹ of usable energy in producers. Using a 10% transfer rule, calculate the energy reaching a tertiary consumer and explain the loss. Producers pass 5,000 kJ to primary consumers; 500 kJ to secondary consumers; 50 kJ to tertiary consumers. Reasoning: at each step 90% is lost, mainly as heat from respiration and in uneaten material, so long food chains support few top predators. The calculation assumes exactly 10%, which is an ecological average, not a law.
Traps that cost marks
Saying energy is recycled (matter is recycled; energy flows and is eventually lost as heat); placing decomposers at the end of every food chain as if they were a fourth trophic level in the grazing chain; reversing arrows; confusing a pyramid of numbers with a pyramid of energy; ignoring that a pyramid of numbers can be inverted, for example many insects on one tree.