Ecosystems, Soils and Biogeography
This domain covers ecosystem structure, energy and nutrient flow, soil formation and profiles, and the distribution of biomes. The exam tests whether you can trace matter and energy through a system and explain soil differences.
Core rules and concepts
- Ecosystems have producers, consumers and decomposers, plus abiotic stores. Food chains and webs show feeding relationships; only about 10% of energy passes to the next trophic level.
- Gross primary productivity (GPP) is total energy fixed by plants. Net primary productivity (NPP) = GPP − respiration. NPP is the energy available to consumers.
- Carbon and nitrogen cycle through atmosphere, biomass, soil and water. Decomposers release nutrients; temperature and moisture control decomposition rates.
- Soil forms from parent material under the influence of climate, organisms, relief and time. A typical profile has O, A, E, B and C horizons; texture depends on sand, silt and clay proportions.
- Biomes include tropical rainforest, savanna, desert and tundra. Succession is the sequence of communities over time; a climax community is the end point.
Worked example
A forest has GPP of 2200 g/m²/yr and plant respiration of 1500 g/m²/yr. NPP = 2200 − 1500 = 700 g/m²/yr. If herbivores assimilate 10%, they gain about 70 g/m²/yr. Reasoning: NPP is what remains after plants use energy for their own metabolism; the 10% rule then applies to transfer to the next level.
Traps
Do not confuse GPP with NPP. Do not assume every soil profile has identical horizons; horizons are labelled by position and properties, not fixed thickness. Do not treat decomposers as unimportant: they close nutrient cycles. In succession, do not assume the climax is always forest.