Environmental Biology and Ecology
What this domain tests
This domain covers ecology at organism, population, community and ecosystem levels: population growth, interactions, limiting factors, carrying capacity, succession and biogeochemical cycles. The exam tests definitions, graphs and simple calculations.
Core rules and concepts
- Exponential growth occurs with unlimited resources; logistic growth slows as population approaches carrying capacity K.
- Density-dependent factors (food, disease, space) intensify with crowding; density-independent factors (fire, storm) act regardless of density.
- Species interactions include competition, predation, parasitism, mutualism and commensalism.
- Limiting factors control productivity; Liebig's law states that the scarcest resource limits growth.
- Biogeochemical cycles include carbon, nitrogen, phosphorus and water cycles; human activity alters their rates and pathways.
Worked example
A population of 50 organisms grows with intrinsic rate r = 0.2 yr⁻¹ in a habitat with K = 500. Logistic growth gives dN/dt = rN(K − N)/K = 0.2 × 50 × (500 − 50) ÷ 500 = 0.2 × 50 × 0.9 = 9 individuals yr⁻¹. Reason: growth is near its maximum at small N and falls to zero as N approaches K. This is why the same r produces different absolute growth at different population sizes.
Traps that cost marks
Do not use exponential growth when resources are limiting. Carrying capacity is not fixed forever; it can change with technology, habitat and climate. Mutualism benefits both species, while commensalism benefits one and leaves the other unaffected. A pyramid of biomass can be inverted in aquatic systems.