In an environment there are many resources which all living things in it want. They must struggle to gain these resources and this is called competition. It is this that drives natural selection as organisms must constanly adapt so that they can compete against other species better.
The most common resource that animals compete for is food, as this is where they get their energy from. Predation is the relationship between the predators and prey; and this can be seen in a predator-prey graph
This graph shows what is called the predator - prey cycle where the numbers of predator and prey are closely related. And here is why the graph looks as it does:
Because of competition, organisms must adapt in order to survive in their habitat. Adaptation comes about through evolution
You can find out how some living things have adapted by look at adaptation