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Food chains, webs and pyramids
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Energy flows through ecosystems along food chains. Only a small part passes from one level to the next.
Trophic levels
Producers (plants) make food by photosynthesis.
Primary consumers (herbivores) eat producers. Secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
Decomposers (bacteria and fungi) break down dead material and waste.
A food chain shows one feeding pathway; a food web shows many linked chains.
Primary consumers (herbivores) eat producers. Secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
Decomposers (bacteria and fungi) break down dead material and waste.
A food chain shows one feeding pathway; a food web shows many linked chains.
Pyramids
Pyramid of numbers: the number of organisms at each level. It can be an odd shape (one oak tree feeds many caterpillars).
Pyramid of biomass: the dry mass of living material at each level. Usually a true pyramid.
Pyramid of energy transfer: the energy at each level. Always a true pyramid.
Pyramid of biomass: the dry mass of living material at each level. Usually a true pyramid.
Pyramid of energy transfer: the energy at each level. Always a true pyramid.
Why energy is lost
Only about 10% of the energy at one level is passed to the next.
The rest is lost: used in respiration and released as heat, lost in faeces (egestion) and excretion, and in parts that are not eaten, such as bones or roots.
This is why food chains rarely have more than four or five levels.
The rest is lost: used in respiration and released as heat, lost in faeces (egestion) and excretion, and in parts that are not eaten, such as bones or roots.
This is why food chains rarely have more than four or five levels.
Grass contains 20 000 kJ of energy. About how much reaches the rabbits that eat it, and the foxes that eat the rabbits?
- About 10% passes on each time.
- Rabbits: 10% of 20 000 = 2000 kJ.
- Foxes: 10% of 2000 = 200 kJ.
Answer: About 2000 kJ to rabbits and 200 kJ to foxes
Producers, primary, secondary and tertiary consumers, and decomposers. Food chains, food webs and pyramids of number, biomass and energy. About 10% of energy passes to the next level; the rest is lost in respiration (heat), waste and uneaten parts.
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