Ecosystem Dynamics
What you must understand
Ecosystems involve flows of energy and cycles of matter. Energy enters as sunlight, is captured by producers and transferred through trophic levels, but only about 10% passes to the next level; the rest is lost as heat through respiration. Matter (carbon, nitrogen, water) cycles and is reused.
Know how to interpret food chains and food webs, identify trophic levels, and explain the effect of removing or adding a species (top-down and bottom-up effects). Understand population dynamics: carrying capacity, limiting factors (density-dependent and density-independent), and predator–prey cycles.
Understand ecological relationships: competition, predation, mutualism, commensalism and parasitism. Also know the processes of primary and secondary succession, and the role of keystone species in maintaining community structure.
Traps in questions
- Energy flows and is lost; matter cycles and is conserved. Mixing these is a classic error.
- Arrows in food chains show the direction of energy flow (from organism eaten to eater), not "who eats whom" in reverse.
- The 10% rule is approximate; do not assume it is exact for calculations unless stated.
- Removing a predator can increase prey numbers, which then reduces producers - follow the chain.
Rule of thumb
For any disturbance question, trace the effect along the food chain, one link at a time, and state the direction (increase or decrease) at each step.
Worked example
If a predator is removed, prey numbers rise, grazing pressure on producers increases, and producer biomass falls. This can then reduce carrying capacity for the prey, causing their numbers to fall again.