Producers, consumers and energy
Almost all energy in an ecosystem comes from the Sun. Producers (green plants and algae) capture light energy in photosynthesis and store it as chemical energy in food. Consumers obtain energy by eating other organisms.
Key idea
A food chain shows the transfer of energy. Each arrow means "is eaten by" and points in the direction of energy flow: producer → primary consumer (herbivore) → secondary consumer → tertiary consumer.
Food webs and trophic levels
Most organisms eat, and are eaten by, several species, so many food chains link into a food web. Each feeding stage is a trophic level. Decomposers (bacteria and fungi) break down dead material and return nutrients to the soil.
Worked example
A producer traps 10 000 kJ of light energy. Only about 10% passes to each next level, so the herbivore gains about 1000 kJ and the secondary consumer about 100 kJ. The rest is lost as heat from respiration, in movement, and in undigested waste.
Why chains are short
Because roughly 90% of energy is lost at each transfer, there is not enough energy to support many trophic levels, so food chains rarely exceed four or five links.
Remember
- Arrows show energy flow, not who eats whom the other way round.
- Only about 10% of energy transfers to the next level.
- Removing one species can affect many others in a web.
The loss of energy also explains ecological pyramids. A pyramid of numbers counts organisms at each level and is usually broad at the base, while a pyramid of biomass shows the dry mass at each level and is almost always widest at the producers, because energy and material decrease as they pass up the chain.