Ecology
Core concepts
Understand population ecology (exponential and logistic growth, carrying capacity, life tables, r- and K-selection), community ecology (competition, predation, mutualism, parasitism, niche differentiation, keystone species, trophic cascades), and ecosystem ecology (energy flow, trophic levels, the 10% rule, primary productivity, biogeochemical cycles). Be comfortable with species interactions and their population consequences, succession (primary versus secondary, pioneer and climax communities), and limiting factors. Know the difference between density-dependent and density-independent regulation, and the concept of ecological footprint and sustainability.
Common traps
Do not call the 10% rule a law - it is a rough efficiency estimate. Logistic growth slows as N approaches K; exponential growth assumes unlimited resources. A keystone species has disproportionate influence relative to abundance; a dominant species has high abundance. Secondary succession occurs where soil already exists. Energy flows one way, but nutrients cycle.
Worked example
Rule of thumb: for a food chain of four levels with 10,000 kcal at the producer level, only about 10 kcal reach the tertiary consumer. Always multiply by 0.1 per level. If a question gives energy at the top and asks for producers, divide by the same factor.