Ecology and Environmental Biology
What this domain tests
Ecology and Environmental Biology covers energy flow, population dynamics, species interactions, nutrient cycles, succession, and human impacts. Exam questions often use data to test whether you can distinguish correlation from causation.
Core rules and concepts
- Energy enters most ecosystems as sunlight and flows through trophic levels; only a fraction is transferred because organisms use energy in respiration and lose heat. Nutrients cycle, but energy does not.
- Population growth can be exponential when resources are unlimited or logistic when carrying capacity limits growth. Density-dependent factors intensify with population size; density-independent factors do not.
- Species interactions include competition, predation, mutualism, commensalism, and parasitism. Invasive species can alter competition and food webs.
- Primary succession starts on bare substrate; secondary succession follows disturbance where soil remains. Biodiversity generally increases during succession but may decline in climax or degraded systems.
Worked example
Researchers mark 40 fish, release them, then later capture 50 fish and find 8 marked. Lincoln–Petersen estimate: N = (marked first × captured second) / marked recaptured = (40 × 50) / 8 = 250. Reasoning: the proportion marked in the second sample, 8/50, should estimate the proportion marked in the population, 40/N. Rearranging gives N = 250.
Traps
Do not apply the 10% rule to biomass or numbers as if it were exact; it is a rough energy guideline. Do not call a density-independent factor “density-dependent” merely because it kills many individuals. Remember that decomposition releases CO2 and nutrients, and that energy is lost at every trophic transfer.