Predator-prey-relation is a term in ecology describing the change in numbers of predators and their prey over time:
The more prey there are, the more food can be used by the predator and their number increases. The number of prey decreases in turn. Food for the prey becomes scarce and the number of predators declines subsequently.
Then the prey can multiply again and the whole cycle starts anew.
This can be shown in the following diagram: