Ecology
Ecology
Ecology tests interactions among organisms and their environment: population growth, community interactions, energy flow, nutrient cycling, succession, and human impacts. Expect graph interpretation, food-web reasoning, and calculations involving population size or energy transfer.
Core concepts
- Populations: exponential growth occurs with unlimited resources; logistic growth slows as density approaches carrying capacity (K). Density-dependent factors include competition and disease; density-independent factors include fires and storms. Survivorship curves describe mortality patterns.
- Species interactions: competition (−/−), predation and parasitism (+/−), mutualism (+/+), and commensalism (+/0). A keystone species has a disproportionately large community effect; removing it can trigger trophic cascades.
- Energy and matter: energy flows through trophic levels and is lost as heat; roughly 10% is transferred between levels. Nutrients such as carbon, nitrogen, and phosphorus cycle through biotic and abiotic reservoirs.
- Succession: primary succession begins on bare substrate with no soil; secondary succession begins where soil remains after disturbance.
- Human impact: habitat fragmentation, invasive species, pollution, and climate change alter biodiversity and ecosystem services.
Worked example
If producers in a food web contain 20,000 kcal of energy, about how much energy reaches the tertiary consumers? Apply the 10% rule: primary consumers receive ~2,000 kcal, secondary consumers ~200 kcal, and tertiary consumers ~20 kcal. The reasoning is that most energy is used in metabolism or lost as heat at each transfer. A common error is to divide by 10 only once or to assume energy cycles; matter cycles, but energy flows and is eventually lost as heat.
Traps
Do not say energy cycles; nutrients cycle while energy flows. Not every introduced species becomes invasive. Carrying capacity can change with environmental conditions. Primary succession starts on bare rock or sand, not on soil left after a fire. And removing a predator does not always increase every lower trophic level-food webs have multiple pathways.