Biological Diversity and Ecosystem Dynamics
What you must understand
You must understand how biodiversity is classified and measured, and how it is maintained by ecosystem processes. Key concepts include the three levels of biodiversity - genetic, species and ecosystem diversity - and classification using the binomial system and hierarchical taxa (domain to species). You should understand species concepts, adaptations, and the effect of selection pressures. In ecosystem dynamics, you need energy flow and trophic levels, the inefficiency of energy transfer (roughly ten per cent between levels), nutrient cycling, and the difference between photosynthesis and respiration in the carbon cycle. Symbiotic relationships, competition, predation and succession are also examinable, along with the causes and consequences of biodiversity loss.
Traps that appear in questions
- Energy versus matter. Energy flows and is lost as heat; nutrients cycle. A question asking about carbon usually wants cycling, not flow.
- Decomposers are not a trophic level in the usual sense. They act across all levels.
- Correlation in a food web. Removing a top predator can increase some populations and decrease others indirectly; trace the chain.
- Species richness versus evenness. Two communities can have equal richness but different diversity if evenness differs.
- Adaptation is not acclimatisation. An adaptation is heritable and selected over generations.
Rule of thumb
Use the ten per cent rule as a quick check: if producers fix 20 000 kJ, expect about 2 000 kJ at the primary consumer level and 200 kJ at the secondary consumer level. If a question supplies figures far outside this, re-examine whether the values are per unit area per year and whether the question is about gross or net productivity.