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 if they are to better compete against other species.
The most common resource that animals compete for is food, as this is where they get their energy from. Predation is a form of gaining food that involves one organism consuming another, this produces another struggle between the predator and the prey: predator looking to eat as much as possible and prey to avoid being eaten. This relationship is dynamic (constantly moving) and can be represented on a predator-prey graph as below.
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
This cyclical pattern is also seen between hosts and parasites, because in many ways, you can think about a parasite as a sort of predator.
Updated: 07 June 2012