What you'll learn
How organisms interact with each other and their environment (~10–15% of the exam).
Population ecology
- Exponential growth (J-curve) vs logistic growth (S-curve, leveling at carrying capacity K).
- Density-dependent (competition, disease) vs density-independent (weather) limits.
Community interactions
Competition, predation, and symbiosis: mutualism (+/+), commensalism (+/0), parasitism (+/−). Keystone species have outsized effects.
Energy flow
Producers → consumers → decomposers. Only ~10% of energy transfers between trophic levels, limiting food-chain length. Food webs show feeding relationships.
Biogeochemical cycles
Carbon, nitrogen, phosphorus, and water cycle through living and nonliving reservoirs. Human activity (burning fossil fuels, fertilizers) disrupts them.
Disruptions
Invasive species, habitat loss, and climate change reduce biodiversity; higher diversity generally means greater stability/resilience.
Exam tips
- Use the 10% rule to explain pyramid shapes and chain length.
- Connect human impacts to cycle disruption.
Common mistakes
- Confusing the symbiosis types.
- Assuming energy is recycled (it flows one way and is lost as heat; matter cycles).