Changes in Population Size
Births and Deaths
Population Increasing and Decreasing
When a predator eats a prey, the prey's population will go down. Because of many of the prey dying, the predator's population increases, but will decrease when food become much scarcer. When most of the predators die, the prey will increases because of less threat of predator. When they go up the predator's population will increase and so on.
Not A Lot of Animals
As you can see, there are barely any wolves or salmon here. Only one. This shows that the population of the wolves and salmon have gone down.
Because of the lack of the wolves, salmon can reproduce now with barely any threat. Almost no wolves can kill many of these fish.
The Everlasting Circle
However, the lucky wolves who caught the salmon grew to where there are many wolves, but this will cause many salmon to die with such high threat. On the other hand, with almost no salmon, the wolves' population will go down, and repeat an everlasting circle unless a disease thrives in it.