Ecology and the Environment
Ecology questions ask you to build and read food webs, calculate energy transfer, explain nutrient cycles, and evaluate human impact using cause-and-effect chains.
Ecosystem structure
An ecosystem is the community of organisms plus the abiotic factors - light, temperature, water, soil, pH. Producers (plants, algae, some bacteria) fix energy; consumers eat; decomposers break down dead material and return mineral ions to the soil. A food chain shows energy flow; the arrow points from the organism being eaten to the eater. Food webs show feeding relationships more realistically.
Energy and nutrients
Only about 10 % of the energy at one trophic level passes to the next; the rest is lost in respiration, movement, heat and undigested waste. This is why food chains rarely exceed four or five levels and why pyramids of energy always narrow upwards. Carbon cycles through photosynthesis, respiration, feeding and decomposition; nitrogen is fixed by bacteria and returned by decomposers.
Interactions and human impact
Competition, predation, mutualism and parasitism shape populations. Human impacts to explain as sequences: eutrophication (fertiliser → nutrient enrichment → algal bloom → death and decomposition → oxygen depletion → fish kill); acid rain (sulphur dioxide → acidic rain → soil and water acidification); greenhouse effect (carbon dioxide and methane → heat retention → climate change); habitat loss and invasive species as drivers of biodiversity loss.
Worked example
Producers contain 10 000 kJ in a field. Reasoning with the 10 % rule: primary consumers receive 1 000 kJ, secondary consumers 100 kJ, tertiary consumers 10 kJ. So a top predator could support far fewer individuals, and the pyramid cannot be wide at the top. Always show the division - writing only "10 %" earns half the marks.
Traps
Drawing arrows in the wrong direction; confusing decomposers with detritivores; applying the 10 % rule to numbers rather than energy without saying so; mixing the greenhouse effect with ozone depletion; giving eutrophication steps out of order.