The Earth's Biosphere and Ecosystems
Ecosystem structure
Understand biotic and abiotic components, producers, consumers, decomposers, trophic levels and food webs. Energy flows one way and is lost at each transfer (roughly 10% to the next level), while matter cycles. Know the carbon, nitrogen and phosphorus cycles, including the role of decomposers and nitrogen-fixing organisms.
Population and community dynamics
- Population growth: exponential versus logistic; carrying capacity; limiting factors (density-dependent and density-independent).
- Interactions: competition, predation, mutualism, parasitism, commensalism.
- Succession: primary (no soil, e.g. bare rock) versus secondary (soil intact after disturbance); pioneer to climax communities.
- Biodiversity: genetic, species and ecosystem levels; resilience and resistance to disturbance.
Common traps
- Drawing energy cycles - energy flows, matter cycles.
- Confusing a food chain with a food web, or placing decomposers at a single trophic level.
- Assuming carrying capacity is fixed; it shifts with conditions and technology.
- Calling any change 'succession' - it requires a directional community change over time.
- Confusing biodiversity with species richness alone; evenness matters.
Worked example: energy transfer
A grassland has 20 000 kJ m-2 yr-1 of producer energy. Using a 10% transfer efficiency, herbivores receive about 2 000 kJ, carnivores about 200 kJ. Rule of thumb - each step loses about 90%, so food chains are short and top predators need large areas.