The graph shows a predator-prey relationship. As we can see, the number of snowshoe Hare increases when the amount of Lynx decreases.
When the number of Hares is big enough, the amount of Lynxes increases too because there is more food (Hares) available. After a period of time, the amount of Hares starts to decrease because they're being eaten, and in consequence the number of Lynxes starts to decrease too, starting a new cycle.
