The Living World: Ecosystems and Biodiversity
Core concepts
Candidates must understand ecosystems as interacting biotic and abiotic components, and the flow of energy through trophic levels (producers, consumers, decomposers). Know that energy is lost at each transfer (roughly 10% passes to the next level) and that nutrients cycle while energy flows one way. Understand food webs, keystone species and the effects of removing a species.
Study population dynamics - carrying capacity, limiting factors, density-dependent and density-independent factors - and species interactions: competition, predation, mutualism, commensalism and parasitism. Grasp biodiversity at genetic, species and ecosystem levels, and the importance of measuring it (species richness and evenness). Understand threats to biodiversity (habitat loss, invasive species, climate change, overexploitation) and conservation strategies. Know the carbon, nitrogen and water cycles and human impacts on them.
Traps
- Energy flows and is lost as heat; nutrients cycle - do not confuse the two.
- Species richness counts species; evenness describes how balanced abundances are.
- A keystone species has disproportionate influence relative to its abundance.
- Density-dependent factors (disease, competition) intensify with population size; density-independent factors (fire, drought) do not.
Worked example
If producers fix 10,000 kJ, herbivores receive ~1,000 kJ and carnivores ~100 kJ. Rule of thumb: divide by ten per trophic level, so top predators support small populations - explaining why food chains are short.