B17-organising an ecosystem


Food chains and webs

All food chains start with a producer. This is a living thing which makes its own food. Green plants are producers because they use sunlight energy to produce their own food by photosynthesis.

The rest of the organisms in a food chain are consumers. A consumer is a living thing which relies on other living things for food. All animals are consumers.

A food chain is a diagram which shows simple feeding relationships. An example of a food chain is:

floating algae


water fleas



The arrows in a food chain show the direction of energy flow.

Most animals have more than one source of food and so they are a link in several food chains. When all the food chains are put together we get a food web. Here is an example of part of a food web.


If one species is lost from a food web all the others are affected. In the web above, if the mosquito larvae were removed the ducks will have less food and the floating algae population will increase. So the duck population is likely to decrease.

Predator – prey relationships

1)    Predators eat their prey, thereby reducing the population of prey.

2)    With fewer prey available the predators are in greater competition with each other for the prey that are left.  

3)    The predator population reduces as some individuals are unable to obtain enough prey for their survival.

4)    With fewer predators left, fewer prey are


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