Answer:
In biology there's something called ecological relations
. These relations can be very different from each other, but the predator-prey one is pretty simple to understand, let's think about it.
Think of an island as an example, there are birds there (prey), and there are giant spiders (predators). So when the predator starts gathering resources from the birds, eating them, the birds population will decrease with time, after some time, the prey won't be available anymore, and the population of the spiders will too, decrease with time.
This is a cycle, because when the population of spiders are low, the birds will start increasing up again, and the whole process will occur all over again.
