Ecology and Environmental Science
What you must know
Levels of organisation: organism, population, community, ecosystem, biome, biosphere, and the distinction between habitat and niche. Energy flow: producers, consumers and decomposers; food chains and webs; roughly 10 per cent transfer between trophic levels, giving pyramids of energy (always upright), numbers and biomass (which can be inverted). Nutrient cycling: carbon (photosynthesis, respiration, combustion, decomposition) and nitrogen (fixation by bacteria, nitrification, assimilation, denitrification). Population dynamics: carrying capacity and limiting factors, predator-prey cycles. Interactions: competition, predation, mutualism and parasitism. Succession: primary and secondary, ending in a climax community. Human impact: enhanced greenhouse effect, acid rain, eutrophication, bioaccumulation, deforestation, and conservation through sustainable management.
Traps and a worked example
Traps: placing decomposers as the final link of a food chain (they are not a trophic level); forgetting that energy is lost as heat through respiration, so energy pyramids are never inverted; muddling the sequence of eutrophication; assuming field correlations prove cause.
Worked example: if producers fix 10 000 kJ per square metre per year, about 1 000 kJ reaches primary consumers, 100 kJ the secondary consumers and 10 kJ the tertiary consumers. Rule of thumb: divide by ten at each step, and always state that the lost energy is mainly heat released by respiration rather than energy that has been used up.