Course 8.1Lesson 3 of 3
Green plants capture about one per cent of the sunlight that falls on them. At each step of a food chain only about ten per cent of the energy passes on, and the rest is lost, mostly as heat. So energy flows one way and food chains are short. Harmful chemicals that are not broken down build up along a chain: biological magnification.
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All the energy in an ecosystem comes from the Sun. Green plants capture it in photosynthesis and store it as chemical energy in food. They capture only about 1% of the sunlight that falls on their leaves.
The energy then moves along the food chain, from plants to herbivores and on to carnivores.
When a herbivore eats a plant, most of the energy in the plant does not end up in the herbivore's body.
On average, only about 10% of the energy at one trophic level is passed on to the next. This is the ten per cent law.
Set the energy of the producers below, and see how little reaches the top.
Energy moves from the Sun to plants, then to herbivores and to carnivores. The energy lost as heat at each step cannot be used again by plants. So the flow of energy is one way. It does not go round in a cycle.
Nutrients are different. Decomposers return them to the soil, and plants take them up again. Materials go round, but energy flows through.
Each step keeps only about a tenth of the energy. After three or four steps, so little is left that it cannot support another level of animals. That is why most food chains have only three or four trophic levels.
It is also why there are fewer animals at the top. A tiger at the end of a long chain gets only a tiny share of the energy that the grass first stored.
The program follows kJ stored by grass up to a deer and then a tiger.
Farmers spray pesticides on crops to kill pests. Some of these chemicals are not biodegradable: decomposers cannot break them down. Rain washes them into the soil and into ponds and rivers.
Plants and tiny water organisms take up the chemical. Animals that eat them take it in too. But the body cannot break it down or get rid of it, so it stays and builds up.
At each step up the chain, an animal eats many living things from the level below. It collects the chemical from all of them. So the chemical becomes more concentrated at each higher level. This is biological magnification.
That is why the use of such pesticides is now controlled, and why scientists test our food for them.
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