Ecology: Populations, Communities and Ecosystems
Levels of organisation
Understand organism, population, community, ecosystem and biosphere. Learn population growth models (exponential and logistic), carrying capacity, life tables and survivorship curves. Know species interactions - competition, predation, parasitism, mutualism, commensalism - and the competitive exclusion principle.
Community and ecosystem function
Master succession (primary and secondary), climax concepts, and the difference between gross and net primary productivity. Understand energy flow, trophic levels, the ten per cent rule, food webs, and biogeochemical cycles for carbon, nitrogen, phosphorus and water. Learn biodiversity measurement, species richness and evenness, and the concept of keystone species.
Traps
Examiners test whether a pyramid of numbers can be inverted (it can), and whether energy flow is cyclical (it is not; nutrients cycle). Another trap is confusing habitat with niche, or gross with net productivity. Watch for questions where succession is said to be reversible.
Rule of thumb
For any ecological question, identify the level of organisation, the interaction, and the flow of energy or matter. This locates the answer quickly.
Worked example
A grassland supports 10,000 kg of grass, 1,000 kg of herbivores and 100 kg of carnivores. This illustrates the ten per cent rule; the pyramid of numbers could still be inverted if many small herbivores feed on one large tree.