The Living World: Ecosystems
What you must know
An ecosystem is the community of organisms plus the abiotic environment they exchange matter and energy with. Be able to trace energy flow (sunlight → producers → consumers → decomposers) and matter cycling (carbon, nitrogen, phosphorus, water). Energy is unidirectional and lost as heat at each trophic level; matter is recycled. Know the 10% rule: roughly 10% of energy transfers between trophic levels, so food chains rarely exceed four or five links and top predators are few. Understand primary productivity (GPP vs. NPP = GPP minus respiration), net primary productivity patterns (highest in tropical rainforests, estuaries, and wetlands; low in deserts and open ocean per unit area), and biogeochemical cycles including the roles of nitrogen fixation, nitrification, denitrification, and the phosphorus cycle's lack of an atmospheric phase.
Traps
- Confusing energy (flows, lost as heat) with matter (cycles).
- Assuming removing a top predator only affects its prey - expect trophic cascades.
- Mixing up GPP (total fixed) with NPP (net stored).
- Forgetting phosphorus has no gaseous reservoir, so it limits productivity in many freshwater systems.
Worked example / rule of thumb
If producers fix 20,000 kcal, herbivores assimilate about 2,000 kcal, and secondary consumers about 200 kcal. Rule: multiply by 0.10 per level up; divide when working downward.