Each species of animal just eats one range of foods. So, for example, people find some plants good to eat but not others. So we eat those and leave the others alone.
In this way the living world is made of species that eat each other. We call these food chains.
As an example, a tadpole grazes on pond weed and is in turn food for a dragonfly larva. The larva is food for a perch, while the perch is food for the heron.
