Whenever a predator population increases at the first moment the prey population tends to decrease. At a second moment the decrease of the prey population and the bigger population density of predators cause the predator population to decrease. The prey population then reverts the tendency to decrease and begins to grow.
If variations in the size of populations occur in an unexpected intensity (different from the usual intensity of the ecological interaction) for example, due to ecological accidents killing many prey, the prey-predator equilibrium is disturbed and both species can be harmed. The existence of the predator sometimes is fundamental for the survival of the prey population, since the absence of predatism favors the proliferation of the prey and, in some cases, when the excessive proliferation creates a population size over the sustenance capacity of the ecosystem, environmental degradation occurs and the entire prey population is destroyed.