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 make the predator population to decrease. The prey population then revert the tendency to decrease and begins to grow.

If variations in the size of the populations occur in an unexpected intensity, for example, due to ecological accidents killing many preys, 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 preys 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.

Population Ecology - Image Diversity: predator x prey curve