Animals get energy and nutrients by eating other animals or plants.
For example, caterpillars eats leaves; small birds eat caterpillars; owls eat small birds; eagles eat owls.
These links between animals and plants are called food chains.
Nearly all food chains start with a green plant.
If one part of a food chain alters, the whole food chain is affected. For example, if a disease suddenly wiped out caterpillars, it would affect owls, eagles and many other animals.