Different species compete to survive and breed. The size of a predator population depends on the size of the prey population, and the reverse is true as well.
Competition
Habitats have limited amounts of the resources needed by living organisms. Organisms must compete with others in order to get enough of these resources to survive. If they are unsuccessful and cannot move to another habitat, they will die.
Animals
Some of the resources that animals compete for:
food
water
space
Animals may also compete for mates so that they can reproduce.
Plants
Remember that plants make their own food using photosynthesis, so they do not compete for food. Here are some of the things that plants do compete for:
light
water
space
mineral salts
Human beings
Human beings are very successful organisms. We compete with animals for food resources, and we compete with both animals and plants for space and water.
Predators and prey
Predators are animals that eat other animals. Prey are the animals that get eaten. The size of the predator population and prey population depend on each other.
If the prey population grows, predator numbers will respond to the increased food supply by increasing as well. But the growing predator population will eventually reduce the food supply to the point where it can no longer be sustained.
http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway_pre_2011/environment/2_compete_or_die1.shtml
