Ecology, Ecosystems, Nutrient Cycles and Human Impact
What this domain tests
Ecology questions ask you to define terms precisely, explain energy flow and nutrient cycling, and evaluate human impact on ecosystems.
Core concepts
- Habitat is where an organism lives; population is all organisms of one species in an area; community is all the populations present; ecosystem includes the community plus abiotic factors.
- A niche is an organism's role, including its habitat, food and interactions.
- Food chains show energy flow; only about 10% passes to the next trophic level, so chains are short and top predators are few.
- Decomposers return nutrients to the soil. Carbon cycles through photosynthesis, respiration, combustion and decomposition.
- Nitrogen cycle: fixation by Rhizobium and lightning, nitrification by nitrifying bacteria, assimilation by plants, denitrification releasing N₂.
Human impact
- Eutrophication: nitrate and phosphate run-off causes an algal bloom, then decomposition depletes oxygen and fish die.
- Acid rain from sulfur dioxide damages soils and lakes; greenhouse gases raise temperature; CFCs deplete ozone.
- Conservation and biodiversity management aim to protect habitats and species.
Worked example
40 woodlice were marked and released. A later sample of 50 contained 8 marked. Estimated population = (40 × 50) ÷ 8 = 250. Reasoning: the proportion marked in the second sample should equal the proportion marked in the whole population, assuming random mixing, no births, deaths, immigration or emigration, and that marking does not affect survival.
Traps
Using 'ecosystem' for 'habitat'; ignoring the assumptions of capture–recapture; confusing nitrification with nitrogen fixation; describing eutrophication without the oxygen-depletion step; assuming a pyramid of numbers cannot be inverted.