Competition
Above is a picture of a snake getting ready to eat a mouse. Let's say in a forrest, the snake population relies on mice. If the snakes start living and their population keeps growing, there will be less mice for them to eat. As the mice population decreases, the snakes have to fight to get their food. Some may find a different food source. For the ones that dont't, sone of them will die. As the snake population begins to decrease, the mice populations will start to increase again and the snakes will have their food back.
For more fun, here is a link to a game: http://education.nationalgeographic.com/education/activity/ecological-relationships/?ar_a=1
For more fun, here is a link to a game: http://education.nationalgeographic.com/education/activity/ecological-relationships/?ar_a=1