Ecology, Environment and Human Biology: Ecosystems and Impacts
What this domain tests
This unit examines relationships between organisms and their environment and the effects of human activity. Questions often require interpreting food webs, calculating energy transfer, explaining population change, and evaluating pollution or conservation issues.
Core rules
- Energy enters most ecosystems as light and is transferred through trophic levels; only about 10% passes to the next level, so food chains are short and top carnivores are rare.
- Pyramids of energy are always broad-based; pyramids of numbers can be inverted, for example one tree supporting many insects.
- The carbon cycle depends on photosynthesis, respiration, combustion and decomposition; the nitrogen cycle depends on nitrogen fixation, nitrification, assimilation, ammonification and denitrification.
- Populations grow exponentially when resources are unlimited, but logistic growth slows as limiting factors produce a carrying capacity.
- Eutrophication follows nutrient enrichment: algal bloom, death and decomposition, oxygen depletion, and death of aerobic organisms.
- Conservation must address habitat, species and genetic diversity; sustainable management balances exploitation with regeneration.
Worked example
A grassland fixes 20,000 kJ m⁻² yr⁻¹. At 10% transfer, primary consumers receive 2,000 kJ, secondary consumers 200 kJ, and tertiary consumers 20 kJ. The loss at each step is mainly heat from respiration, plus uneaten material and excretion. This explains why a food chain with five or more levels cannot support a large predator population.
Traps
Do not say energy is 'destroyed'; it is lost as heat. The 10% figure is an approximation, not a law. Bioaccumulation is build-up in one organism; biomagnification is increase along a food chain. Do not confuse the natural greenhouse effect with its human enhancement.