Ecology
Levels and energy flow
Ecology spans organism, population, community, ecosystem, and biosphere. Energy flows one way through trophic levels (producers → primary consumers → secondary consumers → decomposers), with roughly 10% transferred and the rest lost as heat; biomass pyramids therefore narrow upward. Chemicals cycle (carbon, nitrogen, phosphorus, water) but energy does not. Decomposers recycle nutrients from all levels.
Populations
Exponential growth occurs with unlimited resources; logistic growth levels off at carrying capacity (K). Density-dependent factors (food, disease, predation) intensify with crowding; density-independent factors (weather, fire) do not. Species interactions: competition (often leading to competitive exclusion or niche partitioning), predation, parasitism, mutualism (+,+), commensalism (+,0).
Communities and ecosystems
Succession: primary begins on bare rock with pioneer species; secondary follows disturbance where soil remains. Biomes are shaped by temperature and precipitation. Biodiversity is highest in tropical rain forests and coral reefs; habitat loss is the leading threat.
Worked example
If producers fix 10,000 kcal, primary consumers receive about 1,000 kcal, secondary consumers about 100 kcal, and tertiary consumers about 10 kcal. Rule of thumb: divide by ten per trophic level and remember decomposers are not a trophic level.
Traps: confusing a food chain with a food web; assuming energy cycles; naming the top predator as most abundant (it is usually least abundant).