Ecology
Ecology tests how organisms interact with each other and with the environment, how energy and matter move through ecosystems, and how biodiversity is conserved. Numerical questions on populations and energy flow are common.
Core rules and concepts
- Population density can be estimated by mark-recapture. Exponential growth occurs under unlimited resources; logistic growth slows as carrying capacity is reached. Species interactions include mutualism, competition, parasitism, and predation.
- Ecosystem productivity: gross primary productivity is total organic matter fixed; net primary productivity is GPP minus respiration. Energy transfer between trophic levels is about 10%, so energy pyramids are upright.
- Nutrient cycles include carbon, nitrogen, and phosphorus. Biodiversity is highest in tropical regions; hotspots are conservation priorities. In situ conservation keeps species in habitats; ex situ conservation uses zoos, seed banks, and botanical gardens.
- Environmental issues include greenhouse warming, ozone depletion by CFCs, eutrophication, and biomagnification.
Worked example
If producers have a net primary productivity of 10,000 kJ, energy reaching primary consumers at 10% transfer is 1,000 kJ; secondary consumers receive 100 kJ; tertiary consumers receive 10 kJ. The reasoning applies the 10% law at each step, not once at the end.
Traps that cost marks
NPP is not the same as GPP; NPP = GPP - respiration. The 10% law refers to energy transferred, not energy lost as 90% at every level in a simple additive way. Ozone depletion is linked to CFCs, not CO₂. A biodiversity hotspot must have high endemism and severe habitat loss.