Ecology studies how living things relate to one another and to their surroundings. Key terms build up in levels:
- Population — all organisms of one species in a habitat.
- Community — all the different populations living together in the same area.
- Ecosystem — a community together with its non-living (abiotic) environment, interacting as a unit.
Feeding relationships
Energy enters an ecosystem when producers (green plants) make food by photosynthesis. Consumers eat other organisms, and decomposers break down dead matter. A food chain shows the transfer of food energy from one organism to the next; a food web is many interconnected food chains.
Example
Grass → grasshopper → frog → snake is a food chain. The arrows point in the direction that energy flows — from the food to the feeder.
Energy flow
Energy flows in one direction along a food chain. At each trophic level only about 10% of the energy is passed on; the rest is lost as heat and in respiration and movement. Because of this loss, food chains are usually short and there are fewer organisms at higher levels — shown as a pyramid of energy or numbers.
Nutrient cycles
Unlike energy, nutrients are recycled. In the carbon cycle, carbon dioxide is fixed by photosynthesis and released by respiration, combustion and decomposition. In the nitrogen cycle, nitrogen-fixing bacteria and decomposers keep nitrogen moving between the air, soil and organisms. This is why a stable ecosystem depends on all its groups working together — remove the decomposers and dead matter would pile up while nutrients would no longer be returned to the soil for producers to reuse.
Key idea
Energy flows one way and is lost as heat; nutrients such as carbon and nitrogen are recycled again and again.
Remember
In a food chain the arrow means "is eaten by" — it points toward the organism that gains the energy.