Ecosystems
Ecosystems tests structure and function, productivity, energy flow, decomposition, nutrient cycling and ecological succession.
Core rules
- An ecosystem has producers, consumers and decomposers, plus abiotic components.
- GPP is total photosynthesis; NPP = GPP minus respiration.
- Energy flow is unidirectional and follows the 10 percent law; energy pyramids are always upright.
- Decomposition involves fragmentation, leaching, catabolism, humification and mineralisation.
- Primary succession starts on bare substrate; secondary succession follows disturbance where soil remains.
Worked example
If producers capture 10,000 kJ of energy, about 1,000 kJ reaches primary consumers, 100 kJ reaches secondary consumers and 10 kJ reaches tertiary consumers under the 10 percent law. The reasoning is that most energy is lost as heat and used in metabolism at each transfer. This is why food chains rarely have more than four or five trophic levels.
Traps
- The energy pyramid cannot be inverted, but pyramids of number or biomass can be.
- NPP is not GPP; respiration must be subtracted.
- Decomposers are not the same as detritivores, though both act on dead matter.
- Phosphorus cycles mainly through sedimentary pathways, unlike carbon.