Let's put it this way. Say you have a killer-whale and a penguin. Killer-whales are major predators to penguins. Now, say the killer-whale population increases. The penguins would be eaten more by the killer-whales, then causing a population decrease for the penguins. If the population decreases, they're won't be enough penguins, and they most likely will become extinct, as well as causing a population decrease for the killer-whales as well. Whereas, vis versa, they're were a killer-whale population decrease. The penguins would be less hunted, therefore, creating a population increase for the penguins.
