When scientists began to study plants and animals in their habitats some of the first things they wanted to know were which animals ate the plants and which animals ate other animals. The scientists found that they could put this information together in the form of a set of linked plants and animals which became known as a food chain. For example in a grassland, a rabbit eats grass and a fox eats rabbits. This information can be written as a food chain as follows:

grass > rabbit > fox.

Notice that the arrow in the food chain always points from the food to the feeder.

But what they also noticed was that more than one animal might depend on a common food source. In this case, foxes and people eat chickens and rabbits. So, in fact many food chains cross-link, making a much more complicated pattern or food web. Finding out what depends on what is very complicated, but if we are to understand how to look after not just ourselves, but also wildlife, it is vital thing to do.

Another example is:

algae (in water) > tadpole > heron.