Ecology and Environment
Scope
Population interactions, population growth, ecosystem productivity, energy flow, nutrient cycling, succession, biodiversity and conservation are tested. Expect data interpretation and definitions with examples.
Core rules
- Population growth: exponential when resources are unlimited (dN/dt = rN); logistic when carrying capacity K limits growth (dN/dt = rN(K−N)/K).
- Interactions: mutualism (+,+), competition (−,−), predation and parasitism (+,−), commensalism (+,0), amensalism (−,0).
- Energy flow is unidirectional; the 10% law is a rough guide, not a universal constant.
- Gross primary productivity (GPP) is total carbon fixed; net primary productivity (NPP) = GPP − respiration.
- In situ conservation keeps species in their habitat (biosphere reserves); ex situ keeps them outside (zoos, seed banks).
- Nutrient cycles may be gaseous (carbon, nitrogen) or sedimentary (phosphorus).
Worked example
If producers fix 10,000 kJ m⁻² yr⁻¹ and each trophic level transfers 10%, how much energy reaches the tertiary consumer? Reasoning: primary consumer = 1,000 kJ; secondary = 100 kJ; tertiary = 10 kJ. The tertiary consumer is the fourth trophic level, so three transfers occur. Real transfer efficiency varies from about 5% to 20%.
Traps
- NPP is not the same as GPP; respiration is subtracted.
- A predator is not a parasite; parasites usually live on or in the host and do not immediately kill it.
- Sacred groves are in situ conservation, not ex situ.
- The 10% law is an approximation; do not treat it as a law of thermodynamics.