Ecosystems and Environmental Science
This domain tests how energy and matter move through ecosystems, what limits population growth, and how human activity alters these systems. Items ask you to trace a food web, apply the ten-percent rule, classify a symbiotic relationship, or identify the sequence of events in eutrophication or succession.
Structure and flow
- Levels of organisation: organism, population, community, ecosystem, biome, biosphere. Biotic factors are living; abiotic factors include light, temperature, water, pH and salinity.
- Producers fix energy by photosynthesis; consumers eat; decomposers break down dead matter and return nutrients to the soil.
- Arrows in a food chain point in the direction of energy flow - from the eaten to the eater.
- About 10% of energy passes to the next trophic level; the rest is lost mainly as heat through respiration. This limits food chains to roughly four or five links.
- Nutrient cycles (carbon, nitrogen, water) are closed loops; energy flow is one-way.
Populations and interactions
Exponential growth occurs with unlimited resources; logistic growth levels off at carrying capacity (K) as limiting factors bite. Symbiosis includes mutualism (both benefit), commensalism (one benefits, one unaffected) and parasitism (one benefits, host harmed). Competition and predation also regulate population size.
Worked example
Producers in a meadow fix 20,000 kJ m⁻² yr⁻¹. Estimate the energy reaching the third trophic level.
Reasoning: apply the ten-percent rule once per transfer. Herbivores receive about 2,000 kJ, primary carnivores about 200 kJ, and secondary carnivores (the third consumer level) about 20 kJ. The reasoning step that earns the marks is counting transfers correctly: three transfers means dividing by 10 three times, which is 10³, not 10². Because so little energy remains, top predators are few and are the most vulnerable to habitat loss.
Traps that cost marks
- Drawing food-chain arrows backwards.
- Counting decomposers as a consumer trophic level rather than a recycling pathway.
- Confusing an energy pyramid with a biomass or numbers pyramid.
- Missing a step in eutrophication: nutrient runoff → algal bloom → decomposition → oxygen depletion → fish kills.
- Confusing ozone depletion (chlorofluorocarbons) with the greenhouse effect (carbon dioxide, methane).
- Describing succession as needing soil already present when it is primary succession.