Ecology and Environmental Biology
This domain tests energy flow, nutrient cycling, population dynamics, sampling and human impact. Expect food chains, pyramids, mark-recapture and quadrat calculations, succession and conservation questions.
Energy and nutrients
Energy flows one way through ecosystems and is lost as heat at each trophic level; only about 10% is transferred. Pyramids of energy are always upright, but pyramids of numbers can be inverted. Carbon and nitrogen cycle through living organisms and the environment; decomposers release nutrients. Nitrogen fixation, nitrification and denitrification are key bacterial processes.
Populations and succession
Exponential growth occurs with unlimited resources; logistic growth levels off at carrying capacity. Density-dependent factors include competition and disease; density-independent factors include fire and drought. Succession begins with pioneer species and proceeds to a climax community, changing biodiversity.
Sampling
Quadrats estimate plant abundance or distribution. Mark-recapture estimates animal population size using the Lincoln index: N = (M × C) ÷ R, where M is marked, C is the second sample and R is marked recaptures.
Worked example
Forty snails are marked and released. A second sample of 50 contains 8 marked snails. N = (40 × 50) ÷ 8 = 250. This assumes marked and unmarked animals mix randomly, the population is closed and marking does not affect survival or capture.
Traps
Energy is not recycled; nutrients are. The pyramid of numbers is not always upright. Mark-recapture fails if marked animals are more or less likely to be caught. Do not confuse a habitat with a niche.
Human impact
Pollution, habitat loss, overexploitation and introduced species reduce biodiversity. Conservation uses protected areas, corridors, sustainable harvesting and ex situ measures.