In your aquarium, you supply the food that keeps "the system" working. In nature, fish and other animals have to find their own food and that comes mainly in the form of other animals and plants.When there are hundreds of different species in a natural body of water, the realtionships can be quite complex.  Have a look at the "food web" below and see if you can work out what eats what.  Don't forget that animals at different stages of their life cycles can be be both predators and prey.

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Suggested activities:

You could make a poster of a food web from a river or lake near where you live.  First, develop a species list and then research what each one eats.

What are the main chemical and energy inputs to the food web above - extend the diagram to show these inputs.  What would happen if extra nutrients were added?

What happens in a flood and drought?  Wetlands, lakes and rivers - especially in Australia - are in a state of constant change during the year and between years.  How does this affect the food web?

How does the introduction of non-native fish species affect food webs - research the introduction of mosquito fish (Gambusia) into Australian waterways.  In hindsight, was this a good idea?  What have been the benefits and the costs?