Ecology and Environment: Interactions, Energy and Cycles
What you must understand
Ecology spans organism, population, community, ecosystem and biosphere. Population growth: exponential when resources are unlimited; logistic with a carrying capacity (K) when they are not. Density-dependent factors (competition, disease) intensify with crowding; density-independent factors (fire, drought) do not. Species interactions: competition, predation, parasitism, mutualism, commensalism - be able to name the sign of the effect on each partner.
Energy flow: sunlight → producers → consumers → decomposers. Only about 10% of energy passes to the next trophic level, so food chains are short and top predators are few. Nutrients (carbon, nitrogen, phosphorus, water) cycle, unlike energy, which flows one way and is lost as heat. Succession proceeds from pioneer species to a climax community; biodiversity supports resilience. Human impacts: habitat loss, pollution, eutrophication (nutrient enrichment causing algal blooms and oxygen depletion), climate change, invasive species.
Traps
Energy is not recycled - matter is. A pyramid of numbers can be inverted (one tree, many insects), so do not assume shape. "10% rule" is an approximation, not a law. Mutualism benefits both partners; commensalism benefits one and is neutral for the other - do not merge them. In eutrophication, the oxygen depletion comes from decomposition of dead algae, not from the algae themselves respiring the lake empty.
Worked example
If producers fix 10,000 kJ, primary consumers receive roughly 1,000 kJ, secondary consumers 100 kJ, tertiary 10 kJ. Rule: divide by ten per step; then ask what happens to the missing 90% - it is lost as heat and in waste.