Ecology and Environment
This domain tests population growth, species interactions, energy flow and matter cycling in ecosystems, succession, biomes, and human environmental impact. Items often use a graph or a short scenario requiring quantitative reasoning.
Populations
- Exponential growth produces a J-curve under unlimited resources; logistic growth produces an S-curve that levels at carrying capacity K.
- Density-dependent factors (competition, disease, predation) intensify with crowding; density-independent factors (fire, drought) act regardless of population size.
- r-selected species favor rapid reproduction; K-selected species invest in fewer, better-provisioned offspring.
Communities and Ecosystems
- Competition can drive competitive exclusion or niche partitioning; symbiosis includes mutualism, commensalism, and parasitism.
- Energy flows one way and roughly 10% passes between trophic levels; matter cycles through carbon, nitrogen, phosphorus, and water. Phosphorus has no atmospheric phase.
- Primary succession begins on bare rock with pioneers such as lichens; secondary succession starts where soil remains.
Worked Example
A grassland produces 20,000 kcal/m²/yr. At a 10% transfer rule, primary consumers assimilate 2,000, secondary consumers 200, and tertiary consumers 20 kcal/m²/yr. Reasoning: most energy is lost to respiration, heat, and waste at each step, so top predators are limited by the energy reaching them and need large territories. Note that a pyramid of numbers can invert while an energy pyramid cannot, and in aquatic systems a biomass pyramid can also invert when phytoplankton turnover is rapid.
Traps
- Saying energy cycles - it flows; nutrients cycle.
- Treating carrying capacity as permanently fixed.
- Confusing habitat (where an organism lives) with niche (its role and requirements).
- Confusing density-independent with density-dependent causes of mortality.