Ecology and Environmental Biology
This domain tests ecosystem structure, energy flow, nutrient cycling, population growth, species interactions and conservation. Numerical questions on trophic efficiency and population calculations are common.
Core rules
- Ecosystem components: producers, consumers, decomposers and abiotic factors. Energy flow is unidirectional; nutrients cycle.
- Trophic levels: the 10% law is an approximate ecological efficiency; pyramids of energy are always upright, while pyramids of numbers can be inverted.
- Biogeochemical cycles: carbon through photosynthesis, respiration and combustion; nitrogen through fixation, nitrification, assimilation and denitrification; phosphorus has no gaseous phase; water cycles by evaporation and precipitation.
- Population growth: exponential J-shaped growth when resources are unlimited; logistic S-shaped growth with carrying capacity K. r-selected species have many small offspring; K-selected species have few, well-parented offspring.
- Interactions: predation, competition with competitive exclusion, mutualism, commensalism and parasitism. A niche is the functional role of a species.
- Conservation: in situ measures include national parks and biosphere reserves; ex situ measures include zoos, seed banks and cryopreservation. Biodiversity hotspots have high endemism and threat.
Worked example
If producers fix 100,000 kcal, how much energy reaches tertiary consumers under the 10% law? Reasoning: primary consumers receive 10,000 kcal; secondary consumers 1,000 kcal; tertiary consumers 100 kcal. This is approximate, not a precise law, and does not imply that energy is lost only as heat.
Traps
Energy flows while nutrients cycle. The 10% law is not exact. A pyramid of numbers can be inverted. Biomagnification increases concentrations at higher trophic levels. Carrying capacity is not fixed forever.