Ecology
Ecology
This domain tests interactions among organisms and their environments. Questions cover population growth, carrying capacity, trophic levels, energy flow, biogeochemical cycles, symbiosis, succession, and human impact. You must interpret population data and explain why matter cycles while energy flows.
Core rules and concepts
- Exponential growth occurs with unlimited resources; logistic growth levels off at carrying capacity (K).
- Density-dependent factors include competition, disease, and predation; density-independent factors include fire, drought, and storms.
- Energy flows through trophic levels and is lost as heat; roughly 10% transfers between levels, so food chains are short.
- Matter cycles through biogeochemical cycles such as carbon, nitrogen, phosphorus, and water.
- Symbiosis includes mutualism, commensalism, and parasitism. Succession changes community composition over time.
- Biodiversity increases ecosystem resilience, but human activities can reduce it through habitat loss, pollution, and invasive species.
Worked example
A researcher captures and marks 40 fish, releases them, then later captures 50 fish and finds 10 marked. Using the mark-recapture estimate, N = (marked first x total second) / marked second = (40 x 50) / 10 = 200. The reasoning assumes the marked fish mix randomly, marking does not affect survival or capture, and the population is closed. If marked fish learn to avoid traps, the estimate becomes biased. This example shows how ecological abundance estimates depend on assumptions, not just arithmetic.
Traps that cost marks
- Saying energy cycles; energy flows and matter cycles.
- Treating the 10% rule as exact rather than an approximation.
- Assuming carrying capacity is fixed forever.
- Forgetting that parasitism is a form of symbiosis.