Ecology and the Environment
Ecology and the Environment tests population growth, species interactions, community structure, energy flow, nutrient cycling, and human impacts. Expect quantitative items such as mark-recapture estimates, logistic growth interpretation, and energy-transfer calculations, plus conceptual questions about succession and biodiversity.
Populations
- Exponential growth assumes unlimited resources and produces a J-shaped curve.
- Logistic growth includes carrying capacity K and produces an S-shaped curve; growth is fastest at K/2.
- Density-dependent factors include competition and disease; density-independent factors include weather and disturbance.
- Mark-recapture estimates population size as N ≈ (M × C)/R, where M is marked initially, C is the second sample, and R is recaptured marked individuals.
Communities
- Competition, predation, mutualism, parasitism, and commensalism structure communities.
- A species' niche includes its resource use and role; the competitive exclusion principle states that two species cannot occupy the same niche indefinitely.
- Keystone species have disproportionate effects relative to abundance.
- Primary succession begins on bare substrate; secondary succession follows disturbance with soil intact.
Ecosystems
- Energy flows through trophic levels, with roughly 10% transferred between levels; most is lost as heat.
- Carbon, nitrogen, and phosphorus cycle through biotic and abiotic reservoirs; phosphorus lacks a significant atmospheric reservoir.
- Primary productivity is the rate at which producers convert energy into biomass.
Worked example
A researcher marks 40 animals, releases them, and later captures 50 animals, of which 8 are marked. The estimate is N ≈ (40 × 50)/8 = 250. This assumes the marked animals mix randomly, are equally catchable, and the population is closed. If marked animals learn to avoid traps, the recapture rate falls and the estimate becomes too high.
Traps that cost marks
- Confusing energy pyramids with biomass or numbers pyramids.
- Treating the 10% rule as exact rather than approximate.
- Saying logistic growth is fastest at K; it is fastest at K/2.
- Ignoring mark-recapture assumptions.
- Calling any dominant competitor a keystone species.