Food and Energy
Human Influence
Nutrient Cycles
Studying Ecosystems

Other Sections

Cell Biology
Human Biology
Plant Biology



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.

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:

  • If the prey population increases, there is more food for the predators so their numbers increase.
  • More prey are eaten, and so their numbers decrease.
  • Predator numbers fall because there is less food.
  • If the predator numbers are falling, the prey will become more numerous because they are not being eaten as much.
  • And the cycle continues
  • 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