Learning outcomes:

4.4
explain the names given to different trophic levels to include producers, primary, secondary and tertiary consumers and decomposers
4.5
understand the concepts of food chains, food webs, pyramids of number, pyramids of biomass and pyramids of energy transfer - b - c
4.6
understand the transfer of substances and of energy along a food chain
4.7
explain why only about 10% of energy is transferred from one trophic level to the next.

Food chains are used to show the relationships between species in a habitat. E.g.
chain.GIF

Each level in a food chain is called a Trophic Level

Food chains can be built up into complex food webs. The difference between food chains and food webs is that food webs have branches, chains never do.

ponum.GIF
This shows the populations (to scale) of the species in the chain.
Sometimes a Pyramid of numbers can be inverted (i.e. have a tiny base). This occurs if there is a parasitic relationship in the food chain i.e. one tree, but many caterpillars eating the leaves!

To stop this a pyramid of biomass is more frequently used. This always has a pyramidal shape.

pobiom.GIF

Biomass – the mass of the organic material an organism is made from (i.e. dry it out totally and weigh it, water doesn’t count!) We can also represent the energy flow in a food chain using a Pyramid of Energy Transfer.

poetrans.GIF

This gives an indication of the huge amount of energy that is not passed on to the next trophic level. This is because at each level energy is wasted on;

- Respiration (most of it as waste heat)
- Undigested / egested food
- Used in movement

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